FreeBSD/Linux Kernel Cross Reference
sys/vfs/hammer/hammer_btree.c

     /*
      * Copyright (c) 2007-2008 The DragonFly Project.  All rights reserved.
      * 
      * This code is derived from software contributed to The DragonFly Project
      * by Matthew Dillon <dillon@backplane.com>
      * 
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
     * 
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in
     *    the documentation and/or other materials provided with the
     *    distribution.
     * 3. Neither the name of The DragonFly Project nor the names of its
     *    contributors may be used to endorse or promote products derived
     *    from this software without specific, prior written permission.
     * 
     * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
     * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
     * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
     * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
     * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
     * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
     * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
     * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
     * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
     * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     */
    
    /*
     * HAMMER B-Tree index
     *
     * HAMMER implements a modified B+Tree.  In documentation this will
     * simply be refered to as the HAMMER B-Tree.  Basically a HAMMER B-Tree
     * looks like a B+Tree (A B-Tree which stores its records only at the leafs
     * of the tree), but adds two additional boundary elements which describe
     * the left-most and right-most element a node is able to represent.  In
     * otherwords, we have boundary elements at the two ends of a B-Tree node
     * instead of sub-tree pointers.
     *
     * A B-Tree internal node looks like this:
     *
     *      B N N N N N N B   <-- boundary and internal elements
     *       S S S S S S S    <-- subtree pointers
     *
     * A B-Tree leaf node basically looks like this:
     *
     *      L L L L L L L L   <-- leaf elemenets
     *
     * The radix for an internal node is 1 less then a leaf but we get a
     * number of significant benefits for our troubles.
     *
     * The big benefit to using a B-Tree containing boundary information
     * is that it is possible to cache pointers into the middle of the tree
     * and not have to start searches, insertions, OR deletions at the root
     * node.   In particular, searches are able to progress in a definitive
     * direction from any point in the tree without revisting nodes.  This
     * greatly improves the efficiency of many operations, most especially
     * record appends.
     *
     * B-Trees also make the stacking of trees fairly straightforward.
     *
     * INSERTIONS:  A search performed with the intention of doing
     * an insert will guarantee that the terminal leaf node is not full by
     * splitting full nodes.  Splits occur top-down during the dive down the
     * B-Tree.
     *
     * DELETIONS: A deletion makes no attempt to proactively balance the
     * tree and will recursively remove nodes that become empty.  If a
     * deadlock occurs a deletion may not be able to remove an empty leaf.
     * Deletions never allow internal nodes to become empty (that would blow
     * up the boundaries).
     */
    #include "hammer.h"
    #include <sys/buf.h>
    #include <sys/buf2.h>
    
    static int btree_search(hammer_cursor_t cursor, int flags);
    static int btree_split_internal(hammer_cursor_t cursor);
    static int btree_split_leaf(hammer_cursor_t cursor);
    static int btree_remove(hammer_cursor_t cursor);
    static int btree_node_is_full(hammer_node_ondisk_t node);
    static int hammer_btree_mirror_propagate(hammer_cursor_t cursor,        
                            hammer_tid_t mirror_tid);
    static void hammer_make_separator(hammer_base_elm_t key1,
                            hammer_base_elm_t key2, hammer_base_elm_t dest);
    static void hammer_cursor_mirror_filter(hammer_cursor_t cursor);
    
    /*
     * Iterate records after a search.  The cursor is iterated forwards past
     * the current record until a record matching the key-range requirements
     * is found.  ENOENT is returned if the iteration goes past the ending
     * key. 
     *
    * The iteration is inclusive of key_beg and can be inclusive or exclusive
    * of key_end depending on whether HAMMER_CURSOR_END_INCLUSIVE is set.
    *
    * When doing an as-of search (cursor->asof != 0), key_beg.create_tid
    * may be modified by B-Tree functions.
    *
    * cursor->key_beg may or may not be modified by this function during
    * the iteration.  XXX future - in case of an inverted lock we may have
    * to reinitiate the lookup and set key_beg to properly pick up where we
    * left off.
    *
    * If HAMMER_CURSOR_ITERATE_CHECK is set it is possible that the cursor
    * was reverse indexed due to being moved to a parent while unlocked,
    * and something else might have inserted an element outside the iteration
    * range.  When this case occurs the iterator just keeps iterating until
    * it gets back into the iteration range (instead of asserting).
    *
    * NOTE!  EDEADLK *CANNOT* be returned by this procedure.
    */
   int
   hammer_btree_iterate(hammer_cursor_t cursor)
   {
           hammer_node_ondisk_t node;
           hammer_btree_elm_t elm;
           hammer_mount_t hmp;
           int error = 0;
           int r;
           int s;
   
           /*
            * Skip past the current record
            */
           hmp = cursor->trans->hmp;
           node = cursor->node->ondisk;
           if (node == NULL)
                   return(ENOENT);
           if (cursor->index < node->count && 
               (cursor->flags & HAMMER_CURSOR_ATEDISK)) {
                   ++cursor->index;
           }
   
           /*
            * HAMMER can wind up being cpu-bound.
            */
           if (++hmp->check_yield > hammer_yield_check) {
                   hmp->check_yield = 0;
                   lwkt_user_yield();
           }
   
   
           /*
            * Loop until an element is found or we are done.
            */
           for (;;) {
                   /*
                    * We iterate up the tree and then index over one element
                    * while we are at the last element in the current node.
                    *
                    * If we are at the root of the filesystem, cursor_up
                    * returns ENOENT.
                    *
                    * XXX this could be optimized by storing the information in
                    * the parent reference.
                    *
                    * XXX we can lose the node lock temporarily, this could mess
                    * up our scan.
                    */
                   ++hammer_stats_btree_iterations;
                   hammer_flusher_clean_loose_ios(hmp);
   
                   if (cursor->index == node->count) {
                           if (hammer_debug_btree) {
                                   kprintf("BRACKETU %016llx[%d] -> %016llx[%d] (td=%p)\n",
                                           (long long)cursor->node->node_offset,
                                           cursor->index,
                                           (long long)(cursor->parent ? cursor->parent->node_offset : -1),
                                           cursor->parent_index,
                                           curthread);
                           }
                           KKASSERT(cursor->parent == NULL || cursor->parent->ondisk->elms[cursor->parent_index].internal.subtree_offset == cursor->node->node_offset);
                           error = hammer_cursor_up(cursor);
                           if (error)
                                   break;
                           /* reload stale pointer */
                           node = cursor->node->ondisk;
                           KKASSERT(cursor->index != node->count);
   
                           /*
                            * If we are reblocking we want to return internal
                            * nodes.  Note that the internal node will be
                            * returned multiple times, on each upward recursion
                            * from its children.  The caller selects which
                            * revisit it cares about (usually first or last only).
                            */
                           if (cursor->flags & HAMMER_CURSOR_REBLOCKING) {
                                   cursor->flags |= HAMMER_CURSOR_ATEDISK;
                                   return(0);
                           }
                           ++cursor->index;
                           continue;
                   }
   
                   /*
                    * Check internal or leaf element.  Determine if the record
                    * at the cursor has gone beyond the end of our range.
                    *
                    * We recurse down through internal nodes.
                    */
                   if (node->type == HAMMER_BTREE_TYPE_INTERNAL) {
                           elm = &node->elms[cursor->index];
   
                           r = hammer_btree_cmp(&cursor->key_end, &elm[0].base);
                           s = hammer_btree_cmp(&cursor->key_beg, &elm[1].base);
                           if (hammer_debug_btree) {
                                   kprintf("BRACKETL %016llx[%d] %016llx %02x %016llx lo=%02x %d (td=%p)\n",
                                           (long long)cursor->node->node_offset,
                                           cursor->index,
                                           (long long)elm[0].internal.base.obj_id,
                                           elm[0].internal.base.rec_type,
                                           (long long)elm[0].internal.base.key,
                                           elm[0].internal.base.localization,
                                           r,
                                           curthread
                                   );
                                   kprintf("BRACKETR %016llx[%d] %016llx %02x %016llx lo=%02x %d\n",
                                           (long long)cursor->node->node_offset,
                                           cursor->index + 1,
                                           (long long)elm[1].internal.base.obj_id,
                                           elm[1].internal.base.rec_type,
                                           (long long)elm[1].internal.base.key,
                                           elm[1].internal.base.localization,
                                           s
                                   );
                           }
   
                           if (r < 0) {
                                   error = ENOENT;
                                   break;
                           }
                           if (r == 0 && (cursor->flags &
                                          HAMMER_CURSOR_END_INCLUSIVE) == 0) {
                                   error = ENOENT;
                                   break;
                           }
   
                           /*
                            * Better not be zero
                            */
                           KKASSERT(elm->internal.subtree_offset != 0);
   
                           if (s <= 0) {
                                   /*
                                    * If running the mirror filter see if we
                                    * can skip one or more entire sub-trees.
                                    * If we can we return the internal node
                                    * and the caller processes the skipped
                                    * range (see mirror_read).
                                    */
                                   if (cursor->flags &
                                       HAMMER_CURSOR_MIRROR_FILTERED) {
                                           if (elm->internal.mirror_tid <
                                               cursor->cmirror->mirror_tid) {
                                                   hammer_cursor_mirror_filter(cursor);
                                                   return(0);
                                           }
                                   }
                           } else {
                                   /*
                                    * Normally it would be impossible for the
                                    * cursor to have gotten back-indexed,
                                    * but it can happen if a node is deleted
                                    * and the cursor is moved to its parent
                                    * internal node.  ITERATE_CHECK will be set.
                                    */
                                   KKASSERT(cursor->flags &
                                            HAMMER_CURSOR_ITERATE_CHECK);
                                   kprintf("hammer_btree_iterate: "
                                           "DEBUG: Caught parent seek "
                                           "in internal iteration\n");
                           }
   
                           error = hammer_cursor_down(cursor);
                           if (error)
                                   break;
                           KKASSERT(cursor->index == 0);
                           /* reload stale pointer */
                           node = cursor->node->ondisk;
                           continue;
                   } else {
                           elm = &node->elms[cursor->index];
                           r = hammer_btree_cmp(&cursor->key_end, &elm->base);
                           if (hammer_debug_btree) {
                                   kprintf("ELEMENT  %016llx:%d %c %016llx %02x %016llx lo=%02x %d\n",
                                           (long long)cursor->node->node_offset,
                                           cursor->index,
                                           (elm[0].leaf.base.btype ?
                                            elm[0].leaf.base.btype : '?'),
                                           (long long)elm[0].leaf.base.obj_id,
                                           elm[0].leaf.base.rec_type,
                                           (long long)elm[0].leaf.base.key,
                                           elm[0].leaf.base.localization,
                                           r
                                   );
                           }
                           if (r < 0) {
                                   error = ENOENT;
                                   break;
                           }
   
                           /*
                            * We support both end-inclusive and
                            * end-exclusive searches.
                            */
                           if (r == 0 &&
                              (cursor->flags & HAMMER_CURSOR_END_INCLUSIVE) == 0) {
                                   error = ENOENT;
                                   break;
                           }
   
                           /*
                            * If ITERATE_CHECK is set an unlocked cursor may
                            * have been moved to a parent and the iterate can
                            * happen upon elements that are not in the requested
                            * range.
                            */
                           if (cursor->flags & HAMMER_CURSOR_ITERATE_CHECK) {
                                   s = hammer_btree_cmp(&cursor->key_beg,
                                                        &elm->base);
                                   if (s > 0) {
                                           kprintf("hammer_btree_iterate: "
                                                   "DEBUG: Caught parent seek "
                                                   "in leaf iteration\n");
                                           ++cursor->index;
                                           continue;
                                   }
                           }
                           cursor->flags &= ~HAMMER_CURSOR_ITERATE_CHECK;
   
                           /*
                            * Return the element
                            */
                           switch(elm->leaf.base.btype) {
                           case HAMMER_BTREE_TYPE_RECORD:
                                   if ((cursor->flags & HAMMER_CURSOR_ASOF) &&
                                       hammer_btree_chkts(cursor->asof, &elm->base)) {
                                           ++cursor->index;
                                           continue;
                                   }
                                   error = 0;
                                   break;
                           default:
                                   error = EINVAL;
                                   break;
                           }
                           if (error)
                                   break;
                   }
                   /*
                    * node pointer invalid after loop
                    */
   
                   /*
                    * Return entry
                    */
                   if (hammer_debug_btree) {
                           int i = cursor->index;
                           hammer_btree_elm_t elm = &cursor->node->ondisk->elms[i];
                           kprintf("ITERATE  %p:%d %016llx %02x %016llx lo=%02x\n",
                                   cursor->node, i,
                                   (long long)elm->internal.base.obj_id,
                                   elm->internal.base.rec_type,
                                   (long long)elm->internal.base.key,
                                   elm->internal.base.localization
                           );
                   }
                   return(0);
           }
           return(error);
   }
   
   /*
    * We hit an internal element that we could skip as part of a mirroring
    * scan.  Calculate the entire range being skipped.
    *
    * It is important to include any gaps between the parent's left_bound
    * and the node's left_bound, and same goes for the right side.
    */
   static void
   hammer_cursor_mirror_filter(hammer_cursor_t cursor)
   {
           struct hammer_cmirror *cmirror;
           hammer_node_ondisk_t ondisk;
           hammer_btree_elm_t elm;
   
           ondisk = cursor->node->ondisk;
           cmirror = cursor->cmirror;
   
           /*
            * Calculate the skipped range
            */
           elm = &ondisk->elms[cursor->index];
           if (cursor->index == 0)
                   cmirror->skip_beg = *cursor->left_bound;
           else
                   cmirror->skip_beg = elm->internal.base;
           while (cursor->index < ondisk->count) {
                   if (elm->internal.mirror_tid >= cmirror->mirror_tid)
                           break;
                   ++cursor->index;
                   ++elm;
           }
           if (cursor->index == ondisk->count)
                   cmirror->skip_end = *cursor->right_bound;
           else
                   cmirror->skip_end = elm->internal.base;
   
           /*
            * clip the returned result.
            */
           if (hammer_btree_cmp(&cmirror->skip_beg, &cursor->key_beg) < 0)
                   cmirror->skip_beg = cursor->key_beg;
           if (hammer_btree_cmp(&cmirror->skip_end, &cursor->key_end) > 0)
                   cmirror->skip_end = cursor->key_end;
   }
   
   /*
    * Iterate in the reverse direction.  This is used by the pruning code to
    * avoid overlapping records.
    */
   int
   hammer_btree_iterate_reverse(hammer_cursor_t cursor)
   {
           hammer_node_ondisk_t node;
           hammer_btree_elm_t elm;
           hammer_mount_t hmp;
           int error = 0;
           int r;
           int s;
   
           /* mirror filtering not supported for reverse iteration */
           KKASSERT ((cursor->flags & HAMMER_CURSOR_MIRROR_FILTERED) == 0);
   
           /*
            * Skip past the current record.  For various reasons the cursor
            * may end up set to -1 or set to point at the end of the current
            * node.  These cases must be addressed.
            */
           node = cursor->node->ondisk;
           if (node == NULL)
                   return(ENOENT);
           if (cursor->index != -1 && 
               (cursor->flags & HAMMER_CURSOR_ATEDISK)) {
                   --cursor->index;
           }
           if (cursor->index == cursor->node->ondisk->count)
                   --cursor->index;
   
           /*
            * HAMMER can wind up being cpu-bound.
            */
           hmp = cursor->trans->hmp;
           if (++hmp->check_yield > hammer_yield_check) {
                   hmp->check_yield = 0;
                   lwkt_user_yield();
           }
   
           /*
            * Loop until an element is found or we are done.
            */
           for (;;) {
                   ++hammer_stats_btree_iterations;
                   hammer_flusher_clean_loose_ios(hmp);
   
                   /*
                    * We iterate up the tree and then index over one element
                    * while we are at the last element in the current node.
                    */
                   if (cursor->index == -1) {
                           error = hammer_cursor_up(cursor);
                           if (error) {
                                   cursor->index = 0; /* sanity */
                                   break;
                           }
                           /* reload stale pointer */
                           node = cursor->node->ondisk;
                           KKASSERT(cursor->index != node->count);
                           --cursor->index;
                           continue;
                   }
   
                   /*
                    * Check internal or leaf element.  Determine if the record
                    * at the cursor has gone beyond the end of our range.
                    *
                    * We recurse down through internal nodes. 
                    */
                   KKASSERT(cursor->index != node->count);
                   if (node->type == HAMMER_BTREE_TYPE_INTERNAL) {
                           elm = &node->elms[cursor->index];
                           r = hammer_btree_cmp(&cursor->key_end, &elm[0].base);
                           s = hammer_btree_cmp(&cursor->key_beg, &elm[1].base);
                           if (hammer_debug_btree) {
                                   kprintf("BRACKETL %016llx[%d] %016llx %02x %016llx lo=%02x %d\n",
                                           (long long)cursor->node->node_offset,
                                           cursor->index,
                                           (long long)elm[0].internal.base.obj_id,
                                           elm[0].internal.base.rec_type,
                                           (long long)elm[0].internal.base.key,
                                           elm[0].internal.base.localization,
                                           r
                                   );
                                   kprintf("BRACKETR %016llx[%d] %016llx %02x %016llx lo=%02x %d\n",
                                           (long long)cursor->node->node_offset,
                                           cursor->index + 1,
                                           (long long)elm[1].internal.base.obj_id,
                                           elm[1].internal.base.rec_type,
                                           (long long)elm[1].internal.base.key,
                                           elm[1].internal.base.localization,
                                           s
                                   );
                           }
   
                           if (s >= 0) {
                                   error = ENOENT;
                                   break;
                           }
   
                           /*
                            * It shouldn't be possible to be seeked past key_end,
                            * even if the cursor got moved to a parent.
                            */
                           KKASSERT(r >= 0);
   
                           /*
                            * Better not be zero
                            */
                           KKASSERT(elm->internal.subtree_offset != 0);
   
                           error = hammer_cursor_down(cursor);
                           if (error)
                                   break;
                           KKASSERT(cursor->index == 0);
                           /* reload stale pointer */
                           node = cursor->node->ondisk;
   
                           /* this can assign -1 if the leaf was empty */
                           cursor->index = node->count - 1;
                           continue;
                   } else {
                           elm = &node->elms[cursor->index];
                           s = hammer_btree_cmp(&cursor->key_beg, &elm->base);
                           if (hammer_debug_btree) {
                                   kprintf("ELEMENT  %016llx:%d %c %016llx %02x %016llx lo=%02x %d\n",
                                           (long long)cursor->node->node_offset,
                                           cursor->index,
                                           (elm[0].leaf.base.btype ?
                                            elm[0].leaf.base.btype : '?'),
                                           (long long)elm[0].leaf.base.obj_id,
                                           elm[0].leaf.base.rec_type,
                                           (long long)elm[0].leaf.base.key,
                                           elm[0].leaf.base.localization,
                                           s
                                   );
                           }
                           if (s > 0) {
                                   error = ENOENT;
                                   break;
                           }
   
                           /*
                            * It shouldn't be possible to be seeked past key_end,
                            * even if the cursor got moved to a parent.
                            */
                           cursor->flags &= ~HAMMER_CURSOR_ITERATE_CHECK;
   
                           /*
                            * Return the element
                            */
                           switch(elm->leaf.base.btype) {
                           case HAMMER_BTREE_TYPE_RECORD:
                                   if ((cursor->flags & HAMMER_CURSOR_ASOF) &&
                                       hammer_btree_chkts(cursor->asof, &elm->base)) {
                                           --cursor->index;
                                           continue;
                                   }
                                   error = 0;
                                   break;
                           default:
                                   error = EINVAL;
                                   break;
                           }
                           if (error)
                                   break;
                   }
                   /*
                    * node pointer invalid after loop
                    */
   
                   /*
                    * Return entry
                    */
                   if (hammer_debug_btree) {
                           int i = cursor->index;
                           hammer_btree_elm_t elm = &cursor->node->ondisk->elms[i];
                           kprintf("ITERATE  %p:%d %016llx %02x %016llx lo=%02x\n",
                                   cursor->node, i,
                                   (long long)elm->internal.base.obj_id,
                                   elm->internal.base.rec_type,
                                   (long long)elm->internal.base.key,
                                   elm->internal.base.localization
                           );
                   }
                   return(0);
           }
           return(error);
   }
   
   /*
    * Lookup cursor->key_beg.  0 is returned on success, ENOENT if the entry
    * could not be found, EDEADLK if inserting and a retry is needed, and a
    * fatal error otherwise.  When retrying, the caller must terminate the
    * cursor and reinitialize it.  EDEADLK cannot be returned if not inserting.
    * 
    * The cursor is suitably positioned for a deletion on success, and suitably
    * positioned for an insertion on ENOENT if HAMMER_CURSOR_INSERT was
    * specified.
    *
    * The cursor may begin anywhere, the search will traverse the tree in
    * either direction to locate the requested element.
    *
    * Most of the logic implementing historical searches is handled here.  We
    * do an initial lookup with create_tid set to the asof TID.  Due to the
    * way records are laid out, a backwards iteration may be required if
    * ENOENT is returned to locate the historical record.  Here's the
    * problem:
    *
    * create_tid:    10      15       20
    *                   LEAF1   LEAF2
    * records:         (11)        (18)
    *
    * Lets say we want to do a lookup AS-OF timestamp 17.  We will traverse
    * LEAF2 but the only record in LEAF2 has a create_tid of 18, which is
    * not visible and thus causes ENOENT to be returned.  We really need
    * to check record 11 in LEAF1.  If it also fails then the search fails
    * (e.g. it might represent the range 11-16 and thus still not match our
    * AS-OF timestamp of 17).  Note that LEAF1 could be empty, requiring
    * further iterations.
    *
    * If this case occurs btree_search() will set HAMMER_CURSOR_CREATE_CHECK
    * and the cursor->create_check TID if an iteration might be needed.
    * In the above example create_check would be set to 14.
    */
   int
   hammer_btree_lookup(hammer_cursor_t cursor)
   {
           int error;
   
           cursor->flags &= ~HAMMER_CURSOR_ITERATE_CHECK;
           KKASSERT ((cursor->flags & HAMMER_CURSOR_INSERT) == 0 ||
                     cursor->trans->sync_lock_refs > 0);
           ++hammer_stats_btree_lookups;
           if (cursor->flags & HAMMER_CURSOR_ASOF) {
                   KKASSERT((cursor->flags & HAMMER_CURSOR_INSERT) == 0);
                   cursor->key_beg.create_tid = cursor->asof;
                   for (;;) {
                           cursor->flags &= ~HAMMER_CURSOR_CREATE_CHECK;
                           error = btree_search(cursor, 0);
                           if (error != ENOENT ||
                               (cursor->flags & HAMMER_CURSOR_CREATE_CHECK) == 0) {
                                   /*
                                    * Stop if no error.
                                    * Stop if error other then ENOENT.
                                    * Stop if ENOENT and not special case.
                                    */
                                   break;
                           }
                           if (hammer_debug_btree) {
                                   kprintf("CREATE_CHECK %016llx\n",
                                           (long long)cursor->create_check);
                           }
                           cursor->key_beg.create_tid = cursor->create_check;
                           /* loop */
                   }
           } else {
                   error = btree_search(cursor, 0);
           }
           if (error == 0)
                   error = hammer_btree_extract(cursor, cursor->flags);
           return(error);
   }
   
   /*
    * Execute the logic required to start an iteration.  The first record
    * located within the specified range is returned and iteration control
    * flags are adjusted for successive hammer_btree_iterate() calls.
    *
    * Set ATEDISK so a low-level caller can call btree_first/btree_iterate
    * in a loop without worrying about it.  Higher-level merged searches will
    * adjust the flag appropriately.
    */
   int
   hammer_btree_first(hammer_cursor_t cursor)
   {
           int error;
   
           error = hammer_btree_lookup(cursor);
           if (error == ENOENT) {
                   cursor->flags &= ~HAMMER_CURSOR_ATEDISK;
                   error = hammer_btree_iterate(cursor);
           }
           cursor->flags |= HAMMER_CURSOR_ATEDISK;
           return(error);
   }
   
   /*
    * Similarly but for an iteration in the reverse direction.
    *
    * Set ATEDISK when iterating backwards to skip the current entry,
    * which after an ENOENT lookup will be pointing beyond our end point.
    *
    * Set ATEDISK so a low-level caller can call btree_last/btree_iterate_reverse
    * in a loop without worrying about it.  Higher-level merged searches will
    * adjust the flag appropriately.
    */
   int
   hammer_btree_last(hammer_cursor_t cursor)
   {
           struct hammer_base_elm save;
           int error;
   
           save = cursor->key_beg;
           cursor->key_beg = cursor->key_end;
           error = hammer_btree_lookup(cursor);
           cursor->key_beg = save;
           if (error == ENOENT ||
               (cursor->flags & HAMMER_CURSOR_END_INCLUSIVE) == 0) {
                   cursor->flags |= HAMMER_CURSOR_ATEDISK;
                   error = hammer_btree_iterate_reverse(cursor);
           }
           cursor->flags |= HAMMER_CURSOR_ATEDISK;
           return(error);
   }
   
   /*
    * Extract the record and/or data associated with the cursor's current
    * position.  Any prior record or data stored in the cursor is replaced.
    * The cursor must be positioned at a leaf node.
    *
    * NOTE: All extractions occur at the leaf of the B-Tree.
    */
   int
   hammer_btree_extract(hammer_cursor_t cursor, int flags)
   {
           hammer_node_ondisk_t node;
           hammer_btree_elm_t elm;
           hammer_off_t data_off;
           hammer_mount_t hmp;
           int32_t data_len;
           int error;
   
           /*
            * The case where the data reference resolves to the same buffer
            * as the record reference must be handled.
            */
           node = cursor->node->ondisk;
           elm = &node->elms[cursor->index];
           cursor->data = NULL;
           hmp = cursor->node->hmp;
   
           /*
            * There is nothing to extract for an internal element.
            */
           if (node->type == HAMMER_BTREE_TYPE_INTERNAL)
                   return(EINVAL);
   
           /*
            * Only record types have data.
            */
           KKASSERT(node->type == HAMMER_BTREE_TYPE_LEAF);
           cursor->leaf = &elm->leaf;
   
           if ((flags & HAMMER_CURSOR_GET_DATA) == 0)
                   return(0);
           if (elm->leaf.base.btype != HAMMER_BTREE_TYPE_RECORD)
                   return(0);
           data_off = elm->leaf.data_offset;
           data_len = elm->leaf.data_len;
           if (data_off == 0)
                   return(0);
   
           /*
            * Load the data
            */
           KKASSERT(data_len >= 0 && data_len <= HAMMER_XBUFSIZE);
           cursor->data = hammer_bread_ext(hmp, data_off, data_len,
                                           &error, &cursor->data_buffer);
   
           /*
            * Mark the data buffer as not being meta-data if it isn't
            * meta-data (sometimes bulk data is accessed via a volume
            * block device).
            */
           if (error == 0) {
                   switch(elm->leaf.base.rec_type) {
                   case HAMMER_RECTYPE_DATA:
                   case HAMMER_RECTYPE_DB:
                           if ((data_off & HAMMER_ZONE_LARGE_DATA) == 0)
                                   break;
                           if (hammer_double_buffer == 0 ||
                               (cursor->flags & HAMMER_CURSOR_NOSWAPCACHE)) {
                                   hammer_io_notmeta(cursor->data_buffer);
                           }
                           break;
                   default:
                           break;
                   }
           }
   
           /*
            * Deal with CRC errors on the extracted data.
            */
           if (error == 0 &&
               hammer_crc_test_leaf(cursor->data, &elm->leaf) == 0) {
                   kprintf("CRC DATA @ %016llx/%d FAILED\n",
                           (long long)elm->leaf.data_offset, elm->leaf.data_len);
                   if (hammer_debug_critical)
                           Debugger("CRC FAILED: DATA");
                   if (cursor->trans->flags & HAMMER_TRANSF_CRCDOM)
                           error = EDOM;   /* less critical (mirroring) */
                   else
                           error = EIO;    /* critical */
           }
           return(error);
   }
   
   
   /*
    * Insert a leaf element into the B-Tree at the current cursor position.
    * The cursor is positioned such that the element at and beyond the cursor
    * are shifted to make room for the new record.
    *
    * The caller must call hammer_btree_lookup() with the HAMMER_CURSOR_INSERT
    * flag set and that call must return ENOENT before this function can be
    * called.
    *
    * The caller may depend on the cursor's exclusive lock after return to
    * interlock frontend visibility (see HAMMER_RECF_CONVERT_DELETE).
    *
    * ENOSPC is returned if there is no room to insert a new record.
    */
   int
   hammer_btree_insert(hammer_cursor_t cursor, hammer_btree_leaf_elm_t elm,
                       int *doprop)
   {
           hammer_node_ondisk_t node;
           int i;
           int error;
   
           *doprop = 0;
           if ((error = hammer_cursor_upgrade_node(cursor)) != 0)
                   return(error);
           ++hammer_stats_btree_inserts;
   
           /*
            * Insert the element at the leaf node and update the count in the
            * parent.  It is possible for parent to be NULL, indicating that
            * the filesystem's ROOT B-Tree node is a leaf itself, which is
            * possible.  The root inode can never be deleted so the leaf should
            * never be empty.
            *
            * Remember that the right-hand boundary is not included in the
            * count.
            */
           hammer_modify_node_all(cursor->trans, cursor->node);
           node = cursor->node->ondisk;
           i = cursor->index;
           KKASSERT(elm->base.btype != 0);
           KKASSERT(node->type == HAMMER_BTREE_TYPE_LEAF);
           KKASSERT(node->count < HAMMER_BTREE_LEAF_ELMS);
           if (i != node->count) {
                   bcopy(&node->elms[i], &node->elms[i+1],
                         (node->count - i) * sizeof(*elm));
           }
           node->elms[i].leaf = *elm;
           ++node->count;
           hammer_cursor_inserted_element(cursor->node, i);
   
           /*
            * Update the leaf node's aggregate mirror_tid for mirroring
            * support.
            */
           if (node->mirror_tid < elm->base.delete_tid) {
                   node->mirror_tid = elm->base.delete_tid;
                   *doprop = 1;
           }
           if (node->mirror_tid < elm->base.create_tid) {
                   node->mirror_tid = elm->base.create_tid;
                   *doprop = 1;
           }
           hammer_modify_node_done(cursor->node);
   
           /*
            * Debugging sanity checks.
            */
           KKASSERT(hammer_btree_cmp(cursor->left_bound, &elm->base) <= 0);
           KKASSERT(hammer_btree_cmp(cursor->right_bound, &elm->base) > 0);
           if (i) {
                   KKASSERT(hammer_btree_cmp(&node->elms[i-1].leaf.base, &elm->base) < 0);
           }
           if (i != node->count - 1)
                   KKASSERT(hammer_btree_cmp(&node->elms[i+1].leaf.base, &elm->base) > 0);
   
           return(0);
   }
   
   /*
    * Delete a record from the B-Tree at the current cursor position.
    * The cursor is positioned such that the current element is the one
    * to be deleted.
    *
    * On return the cursor will be positioned after the deleted element and
    * MAY point to an internal node.  It will be suitable for the continuation
    * of an iteration but not for an insertion or deletion.
    *
    * Deletions will attempt to partially rebalance the B-Tree in an upward
    * direction, but will terminate rather then deadlock.  Empty internal nodes
    * are never allowed by a deletion which deadlocks may end up giving us an
    * empty leaf.  The pruner will clean up and rebalance the tree.
    *
    * This function can return EDEADLK, requiring the caller to retry the
    * operation after clearing the deadlock.
    */
   int
   hammer_btree_delete(hammer_cursor_t cursor)
   {
           hammer_node_ondisk_t ondisk;
           hammer_node_t node;
           hammer_node_t parent __debugvar;
           int error;
           int i;
   
           KKASSERT (cursor->trans->sync_lock_refs > 0);
           if ((error = hammer_cursor_upgrade(cursor)) != 0)
                   return(error);
           ++hammer_stats_btree_deletes;
   
           /*
            * Delete the element from the leaf node. 
            *
            * Remember that leaf nodes do not have boundaries.
            */
           node = cursor->node;
           ondisk = node->ondisk;
           i = cursor->index;
   
           KKASSERT(ondisk->type == HAMMER_BTREE_TYPE_LEAF);
           KKASSERT(i >= 0 && i < ondisk->count);
           hammer_modify_node_all(cursor->trans, node);
           if (i + 1 != ondisk->count) {
                   bcopy(&ondisk->elms[i+1], &ondisk->elms[i],
                         (ondisk->count - i - 1) * sizeof(ondisk->elms[0]));
           }
           --ondisk->count;
           hammer_modify_node_done(node);
           hammer_cursor_deleted_element(node, i);
   
           /*
            * Validate local parent
            */
           if (ondisk->parent) {
                   parent = cursor->parent;
   
                   KKASSERT(parent != NULL);
                   KKASSERT(parent->node_offset == ondisk->parent);
           }
   
           /*
            * If the leaf becomes empty it must be detached from the parent,
            * potentially recursing through to the filesystem root.
            *
            * This may reposition the cursor at one of the parent's of the
            * current node.
            *
            * Ignore deadlock errors, that simply means that btree_remove
            * was unable to recurse and had to leave us with an empty leaf. 
            */
           KKASSERT(cursor->index <= ondisk->count);
           if (ondisk->count == 0) {
                   error = btree_remove(cursor);
                   if (error == EDEADLK)
                           error = 0;
           } else {
                   error = 0;
           }
           KKASSERT(cursor->parent == NULL ||
                    cursor->parent_index < cursor->parent->ondisk->count);
           return(error);
   }
   
   /*
   * PRIMAY B-TREE SEARCH SUPPORT PROCEDURE
   *
   * Search the filesystem B-Tree for cursor->key_beg, return the matching node.
   *
   * The search can begin ANYWHERE in the B-Tree.  As a first step the search
   * iterates up the tree as necessary to properly position itself prior to
   * actually doing the sarch.
   * 
   * INSERTIONS: The search will split full nodes and leaves on its way down
   * and guarentee that the leaf it ends up on is not full.  If we run out
   * of space the search continues to the leaf (to position the cursor for
   * the spike), but ENOSPC is returned.
   *
   * The search is only guarenteed to end up on a leaf if an error code of 0
   * is returned, or if inserting and an error code of ENOENT is returned.
   * Otherwise it can stop at an internal node.  On success a search returns
   * a leaf node.
   *
   * COMPLEXITY WARNING!  This is the core B-Tree search code for the entire
   * filesystem, and it is not simple code.  Please note the following facts:
   *
   * - Internal node recursions have a boundary on the left AND right.  The
   *   right boundary is non-inclusive.  The create_tid is a generic part
   *   of the key for internal nodes.
   *
   * - Leaf nodes contain terminal elements only now.
   *
   * - Filesystem lookups typically set HAMMER_CURSOR_ASOF, indicating a
   *   historical search.  ASOF and INSERT are mutually exclusive.  When
   *   doing an as-of lookup btree_search() checks for a right-edge boundary
   *   case.  If while recursing down the left-edge differs from the key
   *   by ONLY its create_tid, HAMMER_CURSOR_CREATE_CHECK is set along
   *   with cursor->create_check.  This is used by btree_lookup() to iterate.
   *   The iteration backwards because as-of searches can wind up going
   *   down the wrong branch of the B-Tree.
   */
  static 
  int
  btree_search(hammer_cursor_t cursor, int flags)
  {
          hammer_node_ondisk_t node;
          hammer_btree_elm_t elm;
          int error;
          int enospc = 0;
          int i;
          int r;
          int s;
  
          flags |= cursor->flags;
          ++hammer_stats_btree_searches;
  
          if (hammer_debug_btree) {
                  kprintf("SEARCH   %016llx[%d] %016llx %02x key=%016llx cre=%016llx lo=%02x (td = %p)\n",
                          (long long)cursor->node->node_offset,
                          cursor->index,
                          (long long)cursor->key_beg.obj_id,
                          cursor->key_beg.rec_type,
                          (long long)cursor->key_beg.key,
                          (long long)cursor->key_beg.create_tid,
                          cursor->key_beg.localization, 
                          curthread
                  );
                  if (cursor->parent)
                      kprintf("SEARCHP %016llx[%d] (%016llx/%016llx %016llx/%016llx) (%p/%p %p/%p)\n",
                          (long long)cursor->parent->node_offset,
                          cursor->parent_index,
                          (long long)cursor->left_bound->obj_id,
                          (long long)cursor->parent->ondisk->elms[cursor->parent_index].internal.base.obj_id,
                          (long long)cursor->right_bound->obj_id,
                          (long long)cursor->parent->ondisk->elms[cursor->parent_index+1].internal.base.obj_id,
                          cursor->left_bound,
                          &cursor->parent->ondisk->elms[cursor->parent_index],
                          cursor->right_bound,
                          &cursor->parent->ondisk->elms[cursor->parent_index+1]
                      );
          }
  
          /*
           * Move our cursor up the tree until we find a node whos range covers
           * the key we are trying to locate.
           *
           * The left bound is inclusive, the right bound is non-inclusive.
           * It is ok to cursor up too far.
           */
          for (;;) {
                  r = hammer_btree_cmp(&cursor->key_beg, cursor->left_bound);
                  s = hammer_btree_cmp(&cursor->key_beg, cursor->right_bound);
                  if (r >= 0 && s < 0)
                          break;
                  KKASSERT(cursor->parent);
                  ++hammer_stats_btree_iterations;
                  error = hammer_cursor_up(cursor);
                  if (error)
                          goto done;
          }
  
          /*
           * The delete-checks below are based on node, not parent.  Set the
           * initial delete-check based on the parent.
           */
          if (r == 1) {
                  KKASSERT(cursor->left_bound->create_tid != 1);
                  cursor->create_check = cursor->left_bound->create_tid - 1;
                  cursor->flags |= HAMMER_CURSOR_CREATE_CHECK;
          }
  
          /*
           * We better have ended up with a node somewhere.
           */
          KKASSERT(cursor->node != NULL);
  
          /*
           * If we are inserting we can't start at a full node if the parent
           * is also full (because there is no way to split the node),
           * continue running up the tree until the requirement is satisfied
           * or we hit the root of the filesystem.
           *
           * (If inserting we aren't doing an as-of search so we don't have
           *  to worry about create_check).
           */
          while ((flags & HAMMER_CURSOR_INSERT) && enospc == 0) {
                  if (cursor->node->ondisk->type == HAMMER_BTREE_TYPE_INTERNAL) {
                          if (btree_node_is_full(cursor->node->ondisk) == 0)
                                  break;
                  } else {
                          if (btree_node_is_full(cursor->node->ondisk) ==0)
                                  break;
                  }
                  if (cursor->node->ondisk->parent == 0 ||
                      cursor->parent->ondisk->count != HAMMER_BTREE_INT_ELMS) {
                          break;
                  }
                  ++hammer_stats_btree_iterations;
                  error = hammer_cursor_up(cursor);
                  /* node may have become stale */
                  if (error)
                          goto done;
          }
  
          /*
           * Push down through internal nodes to locate the requested key.
           */
          node = cursor->node->ondisk;
          while (node->type == HAMMER_BTREE_TYPE_INTERNAL) {
                  /*
                   * Scan the node to find the subtree index to push down into.
                   * We go one-past, then back-up.
                   *
                   * We must proactively remove deleted elements which may
                   * have been left over from a deadlocked btree_remove().
                   *
                   * The left and right boundaries are included in the loop
                   * in order to detect edge cases.
                   *
                   * If the separator only differs by create_tid (r == 1)
                   * and we are doing an as-of search, we may end up going
                   * down a branch to the left of the one containing the
                   * desired key.  This requires numerous special cases.
                   */
                  ++hammer_stats_btree_iterations;
                  if (hammer_debug_btree) {
                          kprintf("SEARCH-I %016llx count=%d\n",
                                  (long long)cursor->node->node_offset,
                                  node->count);
                  }
  
                  /*
                   * Try to shortcut the search before dropping into the
                   * linear loop.  Locate the first node where r <= 1.
                   */
                  i = hammer_btree_search_node(&cursor->key_beg, node);
                  while (i <= node->count) {
                          ++hammer_stats_btree_elements;
                          elm = &node->elms[i];
                          r = hammer_btree_cmp(&cursor->key_beg, &elm->base);
                          if (hammer_debug_btree > 2) {
                                  kprintf(" IELM %p %d r=%d\n",
                                          &node->elms[i], i, r);
                          }
                          if (r < 0)
                                  break;
                          if (r == 1) {
                                  KKASSERT(elm->base.create_tid != 1);
                                  cursor->create_check = elm->base.create_tid - 1;
                                  cursor->flags |= HAMMER_CURSOR_CREATE_CHECK;
                          }
                          ++i;
                  }
                  if (hammer_debug_btree) {
                          kprintf("SEARCH-I preI=%d/%d r=%d\n",
                                  i, node->count, r);
                  }
  
                  /*
                   * These cases occur when the parent's idea of the boundary
                   * is wider then the child's idea of the boundary, and
                   * require special handling.  If not inserting we can
                   * terminate the search early for these cases but the
                   * child's boundaries cannot be unconditionally modified.
                   */
                  if (i == 0) {
                          /*
                           * If i == 0 the search terminated to the LEFT of the
                           * left_boundary but to the RIGHT of the parent's left
                           * boundary.
                           */
                          u_int8_t save;
  
                          elm = &node->elms[0];
  
                          /*
                           * If we aren't inserting we can stop here.
                           */
                          if ((flags & (HAMMER_CURSOR_INSERT |
                                        HAMMER_CURSOR_PRUNING)) == 0) {
                                  cursor->index = 0;
                                  return(ENOENT);
                          }
  
                          /*
                           * Correct a left-hand boundary mismatch.
                           *
                           * We can only do this if we can upgrade the lock,
                           * and synchronized as a background cursor (i.e.
                           * inserting or pruning).
                           *
                           * WARNING: We can only do this if inserting, i.e.
                           * we are running on the backend.
                           */
                          if ((error = hammer_cursor_upgrade(cursor)) != 0)
                                  return(error);
                          KKASSERT(cursor->flags & HAMMER_CURSOR_BACKEND);
                          hammer_modify_node_field(cursor->trans, cursor->node,
                                                   elms[0]);
                          save = node->elms[0].base.btype;
                          node->elms[0].base = *cursor->left_bound;
                          node->elms[0].base.btype = save;
                          hammer_modify_node_done(cursor->node);
                  } else if (i == node->count + 1) {
                          /*
                           * If i == node->count + 1 the search terminated to
                           * the RIGHT of the right boundary but to the LEFT
                           * of the parent's right boundary.  If we aren't
                           * inserting we can stop here.
                           *
                           * Note that the last element in this case is
                           * elms[i-2] prior to adjustments to 'i'.
                           */
                          --i;
                          if ((flags & (HAMMER_CURSOR_INSERT |
                                        HAMMER_CURSOR_PRUNING)) == 0) {
                                  cursor->index = i;
                                  return (ENOENT);
                          }
  
                          /*
                           * Correct a right-hand boundary mismatch.
                           * (actual push-down record is i-2 prior to
                           * adjustments to i).
                           *
                           * We can only do this if we can upgrade the lock,
                           * and synchronized as a background cursor (i.e.
                           * inserting or pruning).
                           *
                           * WARNING: We can only do this if inserting, i.e.
                           * we are running on the backend.
                           */
                          if ((error = hammer_cursor_upgrade(cursor)) != 0)
                                  return(error);
                          elm = &node->elms[i];
                          KKASSERT(cursor->flags & HAMMER_CURSOR_BACKEND);
                          hammer_modify_node(cursor->trans, cursor->node,
                                             &elm->base, sizeof(elm->base));
                          elm->base = *cursor->right_bound;
                          hammer_modify_node_done(cursor->node);
                          --i;
                  } else {
                          /*
                           * The push-down index is now i - 1.  If we had
                           * terminated on the right boundary this will point
                           * us at the last element.
                           */
                          --i;
                  }
                  cursor->index = i;
                  elm = &node->elms[i];
  
                  if (hammer_debug_btree) {
                          kprintf("RESULT-I %016llx[%d] %016llx %02x "
                                  "key=%016llx cre=%016llx lo=%02x\n",
                                  (long long)cursor->node->node_offset,
                                  i,
                                  (long long)elm->internal.base.obj_id,
                                  elm->internal.base.rec_type,
                                  (long long)elm->internal.base.key,
                                  (long long)elm->internal.base.create_tid,
                                  elm->internal.base.localization
                          );
                  }
  
                  /*
                   * We better have a valid subtree offset.
                   */
                  KKASSERT(elm->internal.subtree_offset != 0);
  
                  /*
                   * Handle insertion and deletion requirements.
                   *
                   * If inserting split full nodes.  The split code will
                   * adjust cursor->node and cursor->index if the current
                   * index winds up in the new node.
                   *
                   * If inserting and a left or right edge case was detected,
                   * we cannot correct the left or right boundary and must
                   * prepend and append an empty leaf node in order to make
                   * the boundary correction.
                   *
                   * If we run out of space we set enospc and continue on
                   * to a leaf to provide the spike code with a good point
                   * of entry.
                   */
                  if ((flags & HAMMER_CURSOR_INSERT) && enospc == 0) {
                          if (btree_node_is_full(node)) {
                                  error = btree_split_internal(cursor);
                                  if (error) {
                                          if (error != ENOSPC)
                                                  goto done;
                                          enospc = 1;
                                  }
                                  /*
                                   * reload stale pointers
                                   */
                                  i = cursor->index;
                                  node = cursor->node->ondisk;
                          }
                  }
  
                  /*
                   * Push down (push into new node, existing node becomes
                   * the parent) and continue the search.
                   */
                  error = hammer_cursor_down(cursor);
                  /* node may have become stale */
                  if (error)
                          goto done;
                  node = cursor->node->ondisk;
          }
  
          /*
           * We are at a leaf, do a linear search of the key array.
           *
           * On success the index is set to the matching element and 0
           * is returned.
           *
           * On failure the index is set to the insertion point and ENOENT
           * is returned.
           *
           * Boundaries are not stored in leaf nodes, so the index can wind
           * up to the left of element 0 (index == 0) or past the end of
           * the array (index == node->count).  It is also possible that the
           * leaf might be empty.
           */
          ++hammer_stats_btree_iterations;
          KKASSERT (node->type == HAMMER_BTREE_TYPE_LEAF);
          KKASSERT(node->count <= HAMMER_BTREE_LEAF_ELMS);
          if (hammer_debug_btree) {
                  kprintf("SEARCH-L %016llx count=%d\n",
                          (long long)cursor->node->node_offset,
                          node->count);
          }
  
          /*
           * Try to shortcut the search before dropping into the
           * linear loop.  Locate the first node where r <= 1.
           */
          i = hammer_btree_search_node(&cursor->key_beg, node);
          while (i < node->count) {
                  ++hammer_stats_btree_elements;
                  elm = &node->elms[i];
  
                  r = hammer_btree_cmp(&cursor->key_beg, &elm->leaf.base);
  
                  if (hammer_debug_btree > 1)
                          kprintf("  ELM %p %d r=%d\n", &node->elms[i], i, r);
  
                  /*
                   * We are at a record element.  Stop if we've flipped past
                   * key_beg, not counting the create_tid test.  Allow the
                   * r == 1 case (key_beg > element but differs only by its
                   * create_tid) to fall through to the AS-OF check.
                   */
                  KKASSERT (elm->leaf.base.btype == HAMMER_BTREE_TYPE_RECORD);
  
                  if (r < 0)
                          goto failed;
                  if (r > 1) {
                          ++i;
                          continue;
                  }
  
                  /*
                   * Check our as-of timestamp against the element.
                   */
                  if (flags & HAMMER_CURSOR_ASOF) {
                          if (hammer_btree_chkts(cursor->asof,
                                                 &node->elms[i].base) != 0) {
                                  ++i;
                                  continue;
                          }
                          /* success */
                  } else {
                          if (r > 0) {    /* can only be +1 */
                                  ++i;
                                  continue;
                          }
                          /* success */
                  }
                  cursor->index = i;
                  error = 0;
                  if (hammer_debug_btree) {
                          kprintf("RESULT-L %016llx[%d] (SUCCESS)\n",
                                  (long long)cursor->node->node_offset, i);
                  }
                  goto done;
          }
  
          /*
           * The search of the leaf node failed.  i is the insertion point.
           */
  failed:
          if (hammer_debug_btree) {
                  kprintf("RESULT-L %016llx[%d] (FAILED)\n",
                          (long long)cursor->node->node_offset, i);
          }
  
          /*
           * No exact match was found, i is now at the insertion point.
           *
           * If inserting split a full leaf before returning.  This
           * may have the side effect of adjusting cursor->node and
           * cursor->index.
           */
          cursor->index = i;
          if ((flags & HAMMER_CURSOR_INSERT) && enospc == 0 &&
               btree_node_is_full(node)) {
                  error = btree_split_leaf(cursor);
                  if (error) {
                          if (error != ENOSPC)
                                  goto done;
                          enospc = 1;
                  }
                  /*
                   * reload stale pointers
                   */
                  /* NOT USED
                  i = cursor->index;
                  node = &cursor->node->internal;
                  */
          }
  
          /*
           * We reached a leaf but did not find the key we were looking for.
           * If this is an insert we will be properly positioned for an insert
           * (ENOENT) or spike (ENOSPC) operation.
           */
          error = enospc ? ENOSPC : ENOENT;
  done:
          return(error);
  }
  
  /*
   * Heuristical search for the first element whos comparison is <= 1.  May
   * return an index whos compare result is > 1 but may only return an index
   * whos compare result is <= 1 if it is the first element with that result.
   */
  int
  hammer_btree_search_node(hammer_base_elm_t elm, hammer_node_ondisk_t node)
  {
          int b;
          int s;
          int i;
          int r;
  
          /*
           * Don't bother if the node does not have very many elements
           */
          b = 0;
          s = node->count;
          while (s - b > 4) {
                  i = b + (s - b) / 2;
                  ++hammer_stats_btree_elements;
                  r = hammer_btree_cmp(elm, &node->elms[i].leaf.base);
                  if (r <= 1) {
                          s = i;
                  } else {
                          b = i;
                  }
          }
          return(b);
  }
  
  
  /************************************************************************
   *                         SPLITTING AND MERGING                        *
   ************************************************************************
   *
   * These routines do all the dirty work required to split and merge nodes.
   */
  
  /*
   * Split an internal node into two nodes and move the separator at the split
   * point to the parent.
   *
   * (cursor->node, cursor->index) indicates the element the caller intends
   * to push into.  We will adjust node and index if that element winds
   * up in the split node.
   *
   * If we are at the root of the filesystem a new root must be created with
   * two elements, one pointing to the original root and one pointing to the
   * newly allocated split node.
   */
  static
  int
  btree_split_internal(hammer_cursor_t cursor)
  {
          hammer_node_ondisk_t ondisk;
          hammer_node_t node;
          hammer_node_t parent;
          hammer_node_t new_node;
          hammer_btree_elm_t elm;
          hammer_btree_elm_t parent_elm;
          struct hammer_node_lock lockroot;
          hammer_mount_t hmp = cursor->trans->hmp;
          int parent_index;
          int made_root;
          int split;
          int error;
          int i;
          const int esize = sizeof(*elm);
  
          hammer_node_lock_init(&lockroot, cursor->node);
          error = hammer_btree_lock_children(cursor, 1, &lockroot, NULL);
          if (error)
                  goto done;
          if ((error = hammer_cursor_upgrade(cursor)) != 0)
                  goto done;
          ++hammer_stats_btree_splits;
  
          /* 
           * Calculate the split point.  If the insertion point is at the
           * end of the leaf we adjust the split point significantly to the
           * right to try to optimize node fill and flag it.  If we hit
           * that same leaf again our heuristic failed and we don't try
           * to optimize node fill (it could lead to a degenerate case).
           */
          node = cursor->node;
          ondisk = node->ondisk;
          KKASSERT(ondisk->count > 4);
          if (cursor->index == ondisk->count &&
              (node->flags & HAMMER_NODE_NONLINEAR) == 0) {
                  split = (ondisk->count + 1) * 3 / 4;
                  node->flags |= HAMMER_NODE_NONLINEAR;
          } else {
                  /*
                   * We are splitting but elms[split] will be promoted to
                   * the parent, leaving the right hand node with one less
                   * element.  If the insertion point will be on the
                   * left-hand side adjust the split point to give the
                   * right hand side one additional node.
                   */
                  split = (ondisk->count + 1) / 2;
                  if (cursor->index <= split)
                          --split;
          }
  
          /*
           * If we are at the root of the filesystem, create a new root node
           * with 1 element and split normally.  Avoid making major
           * modifications until we know the whole operation will work.
           */
          if (ondisk->parent == 0) {
                  parent = hammer_alloc_btree(cursor->trans, 0, &error);
                  if (parent == NULL)
                          goto done;
                  hammer_lock_ex(&parent->lock);
                  hammer_modify_node_noundo(cursor->trans, parent);
                  ondisk = parent->ondisk;
                  ondisk->count = 1;
                  ondisk->parent = 0;
                  ondisk->mirror_tid = node->ondisk->mirror_tid;
                  ondisk->type = HAMMER_BTREE_TYPE_INTERNAL;
                  ondisk->elms[0].base = hmp->root_btree_beg;
                  ondisk->elms[0].base.btype = node->ondisk->type;
                  ondisk->elms[0].internal.subtree_offset = node->node_offset;
                  ondisk->elms[1].base = hmp->root_btree_end;
                  hammer_modify_node_done(parent);
                  /* ondisk->elms[1].base.btype - not used */
                  made_root = 1;
                  parent_index = 0;       /* index of current node in parent */
          } else {
                  made_root = 0;
                  parent = cursor->parent;
                  parent_index = cursor->parent_index;
          }
  
          /*
           * Split node into new_node at the split point.
           *
           *  B O O O P N N B     <-- P = node->elms[split] (index 4)
           *   0 1 2 3 4 5 6      <-- subtree indices
           *
           *       x x P x x
           *        s S S s  
           *         /   \
           *  B O O O B    B N N B        <--- inner boundary points are 'P'
           *   0 1 2 3      4 5 6  
           */
          new_node = hammer_alloc_btree(cursor->trans, 0, &error);
          if (new_node == NULL) {
                  if (made_root) {
                          hammer_unlock(&parent->lock);
                          hammer_delete_node(cursor->trans, parent);
                          hammer_rel_node(parent);
                  }
                  goto done;
          }
          hammer_lock_ex(&new_node->lock);
  
          /*
           * Create the new node.  P becomes the left-hand boundary in the
           * new node.  Copy the right-hand boundary as well.
           *
           * elm is the new separator.
           */
          hammer_modify_node_noundo(cursor->trans, new_node);
          hammer_modify_node_all(cursor->trans, node);
          ondisk = node->ondisk;
          elm = &ondisk->elms[split];
          bcopy(elm, &new_node->ondisk->elms[0],
                (ondisk->count - split + 1) * esize);
          new_node->ondisk->count = ondisk->count - split;
          new_node->ondisk->parent = parent->node_offset;
          new_node->ondisk->type = HAMMER_BTREE_TYPE_INTERNAL;
          new_node->ondisk->mirror_tid = ondisk->mirror_tid;
          KKASSERT(ondisk->type == new_node->ondisk->type);
          hammer_cursor_split_node(node, new_node, split);
  
          /*
           * Cleanup the original node.  Elm (P) becomes the new boundary,
           * its subtree_offset was moved to the new node.  If we had created
           * a new root its parent pointer may have changed.
           */
          elm->internal.subtree_offset = 0;
          ondisk->count = split;
  
          /*
           * Insert the separator into the parent, fixup the parent's
           * reference to the original node, and reference the new node.
           * The separator is P.
           *
           * Remember that base.count does not include the right-hand boundary.
           */
          hammer_modify_node_all(cursor->trans, parent);
          ondisk = parent->ondisk;
          KKASSERT(ondisk->count != HAMMER_BTREE_INT_ELMS);
          parent_elm = &ondisk->elms[parent_index+1];
          bcopy(parent_elm, parent_elm + 1,
                (ondisk->count - parent_index) * esize);
          parent_elm->internal.base = elm->base;  /* separator P */
          parent_elm->internal.base.btype = new_node->ondisk->type;
          parent_elm->internal.subtree_offset = new_node->node_offset;
          parent_elm->internal.mirror_tid = new_node->ondisk->mirror_tid;
          ++ondisk->count;
          hammer_modify_node_done(parent);
          hammer_cursor_inserted_element(parent, parent_index + 1);
  
          /*
           * The children of new_node need their parent pointer set to new_node.
           * The children have already been locked by
           * hammer_btree_lock_children().
           */
          for (i = 0; i < new_node->ondisk->count; ++i) {
                  elm = &new_node->ondisk->elms[i];
                  error = btree_set_parent(cursor->trans, new_node, elm);
                  if (error) {
                          panic("btree_split_internal: btree-fixup problem");
                  }
          }
          hammer_modify_node_done(new_node);
  
          /*
           * The filesystem's root B-Tree pointer may have to be updated.
           */
          if (made_root) {
                  hammer_volume_t volume;
  
                  volume = hammer_get_root_volume(hmp, &error);
                  KKASSERT(error == 0);
  
                  hammer_modify_volume_field(cursor->trans, volume,
                                             vol0_btree_root);
                  volume->ondisk->vol0_btree_root = parent->node_offset;
                  hammer_modify_volume_done(volume);
                  node->ondisk->parent = parent->node_offset;
                  if (cursor->parent) {
                          hammer_unlock(&cursor->parent->lock);
                          hammer_rel_node(cursor->parent);
                  }
                  cursor->parent = parent;        /* lock'd and ref'd */
                  hammer_rel_volume(volume, 0);
          }
          hammer_modify_node_done(node);
  
          /*
           * Ok, now adjust the cursor depending on which element the original
           * index was pointing at.  If we are >= the split point the push node
           * is now in the new node.
           *
           * NOTE: If we are at the split point itself we cannot stay with the
           * original node because the push index will point at the right-hand
           * boundary, which is illegal.
           *
           * NOTE: The cursor's parent or parent_index must be adjusted for
           * the case where a new parent (new root) was created, and the case
           * where the cursor is now pointing at the split node.
           */
          if (cursor->index >= split) {
                  cursor->parent_index = parent_index + 1;
                  cursor->index -= split;
                  hammer_unlock(&cursor->node->lock);
                  hammer_rel_node(cursor->node);
                  cursor->node = new_node;        /* locked and ref'd */
          } else {
                  cursor->parent_index = parent_index;
                  hammer_unlock(&new_node->lock);
                  hammer_rel_node(new_node);
          }
  
          /*
           * Fixup left and right bounds
           */
          parent_elm = &parent->ondisk->elms[cursor->parent_index];
          cursor->left_bound = &parent_elm[0].internal.base;
          cursor->right_bound = &parent_elm[1].internal.base;
          KKASSERT(hammer_btree_cmp(cursor->left_bound,
                   &cursor->node->ondisk->elms[0].internal.base) <= 0);
          KKASSERT(hammer_btree_cmp(cursor->right_bound,
                   &cursor->node->ondisk->elms[cursor->node->ondisk->count].internal.base) >= 0);
  
  done:
          hammer_btree_unlock_children(cursor->trans->hmp, &lockroot, NULL);
          hammer_cursor_downgrade(cursor);
          return (error);
  }
  
  /*
   * Same as the above, but splits a full leaf node.
   *
   * This function
   */
  static
  int
  btree_split_leaf(hammer_cursor_t cursor)
  {
          hammer_node_ondisk_t ondisk;
          hammer_node_t parent;
          hammer_node_t leaf;
          hammer_mount_t hmp;
          hammer_node_t new_leaf;
          hammer_btree_elm_t elm;
          hammer_btree_elm_t parent_elm;
          hammer_base_elm_t mid_boundary;
          int parent_index;
          int made_root;
          int split;
          int error;
          const size_t esize = sizeof(*elm);
  
          if ((error = hammer_cursor_upgrade(cursor)) != 0)
                  return(error);
          ++hammer_stats_btree_splits;
  
          KKASSERT(hammer_btree_cmp(cursor->left_bound,
                   &cursor->node->ondisk->elms[0].leaf.base) <= 0);
          KKASSERT(hammer_btree_cmp(cursor->right_bound,
                   &cursor->node->ondisk->elms[cursor->node->ondisk->count-1].leaf.base) > 0);
  
          /* 
           * Calculate the split point.  If the insertion point is at the
           * end of the leaf we adjust the split point significantly to the
           * right to try to optimize node fill and flag it.  If we hit
           * that same leaf again our heuristic failed and we don't try
           * to optimize node fill (it could lead to a degenerate case).
           *
           * Spikes are made up of two leaf elements which cannot be
           * safely split.
           */
          leaf = cursor->node;
          ondisk = leaf->ondisk;
          KKASSERT(ondisk->count > 4);
          if (cursor->index == ondisk->count &&
              (leaf->flags & HAMMER_NODE_NONLINEAR) == 0) {
                  split = (ondisk->count + 1) * 3 / 4;
                  leaf->flags |= HAMMER_NODE_NONLINEAR;
          } else {
                  split = (ondisk->count + 1) / 2;
          }
  
  #if 0
          /*
           * If the insertion point is at the split point shift the
           * split point left so we don't have to worry about
           */
          if (cursor->index == split)
                  --split;
  #endif
          KKASSERT(split > 0 && split < ondisk->count);
  
          error = 0;
          hmp = leaf->hmp;
  
          elm = &ondisk->elms[split];
  
          KKASSERT(hammer_btree_cmp(cursor->left_bound, &elm[-1].leaf.base) <= 0);
          KKASSERT(hammer_btree_cmp(cursor->left_bound, &elm->leaf.base) <= 0);
          KKASSERT(hammer_btree_cmp(cursor->right_bound, &elm->leaf.base) > 0);
          KKASSERT(hammer_btree_cmp(cursor->right_bound, &elm[1].leaf.base) > 0);
  
          /*
           * If we are at the root of the tree, create a new root node with
           * 1 element and split normally.  Avoid making major modifications
           * until we know the whole operation will work.
           */
          if (ondisk->parent == 0) {
                  parent = hammer_alloc_btree(cursor->trans, 0, &error);
                  if (parent == NULL)
                          goto done;
                  hammer_lock_ex(&parent->lock);
                  hammer_modify_node_noundo(cursor->trans, parent);
                  ondisk = parent->ondisk;
                  ondisk->count = 1;
                  ondisk->parent = 0;
                  ondisk->mirror_tid = leaf->ondisk->mirror_tid;
                  ondisk->type = HAMMER_BTREE_TYPE_INTERNAL;
                  ondisk->elms[0].base = hmp->root_btree_beg;
                  ondisk->elms[0].base.btype = leaf->ondisk->type;
                  ondisk->elms[0].internal.subtree_offset = leaf->node_offset;
                  ondisk->elms[1].base = hmp->root_btree_end;
                  /* ondisk->elms[1].base.btype = not used */
                  hammer_modify_node_done(parent);
                  made_root = 1;
                  parent_index = 0;       /* insertion point in parent */
          } else {
                  made_root = 0;
                  parent = cursor->parent;
                  parent_index = cursor->parent_index;
          }
  
          /*
           * Split leaf into new_leaf at the split point.  Select a separator
           * value in-between the two leafs but with a bent towards the right
           * leaf since comparisons use an 'elm >= separator' inequality.
           *
           *  L L L L L L L L
           *
           *       x x P x x
           *        s S S s  
           *         /   \
           *  L L L L     L L L L
           */
          new_leaf = hammer_alloc_btree(cursor->trans, 0, &error);
          if (new_leaf == NULL) {
                  if (made_root) {
                          hammer_unlock(&parent->lock);
                          hammer_delete_node(cursor->trans, parent);
                          hammer_rel_node(parent);
                  }
                  goto done;
          }
          hammer_lock_ex(&new_leaf->lock);
  
          /*
           * Create the new node and copy the leaf elements from the split 
           * point on to the new node.
           */
          hammer_modify_node_all(cursor->trans, leaf);
          hammer_modify_node_noundo(cursor->trans, new_leaf);
          ondisk = leaf->ondisk;
          elm = &ondisk->elms[split];
          bcopy(elm, &new_leaf->ondisk->elms[0], (ondisk->count - split) * esize);
          new_leaf->ondisk->count = ondisk->count - split;
          new_leaf->ondisk->parent = parent->node_offset;
          new_leaf->ondisk->type = HAMMER_BTREE_TYPE_LEAF;
          new_leaf->ondisk->mirror_tid = ondisk->mirror_tid;
          KKASSERT(ondisk->type == new_leaf->ondisk->type);
          hammer_modify_node_done(new_leaf);
          hammer_cursor_split_node(leaf, new_leaf, split);
  
          /*
           * Cleanup the original node.  Because this is a leaf node and
           * leaf nodes do not have a right-hand boundary, there
           * aren't any special edge cases to clean up.  We just fixup the
           * count.
           */
          ondisk->count = split;
  
          /*
           * Insert the separator into the parent, fixup the parent's
           * reference to the original node, and reference the new node.
           * The separator is P.
           *
           * Remember that base.count does not include the right-hand boundary.
           * We are copying parent_index+1 to parent_index+2, not +0 to +1.
           */
          hammer_modify_node_all(cursor->trans, parent);
          ondisk = parent->ondisk;
          KKASSERT(split != 0);
          KKASSERT(ondisk->count != HAMMER_BTREE_INT_ELMS);
          parent_elm = &ondisk->elms[parent_index+1];
          bcopy(parent_elm, parent_elm + 1,
                (ondisk->count - parent_index) * esize);
  
          hammer_make_separator(&elm[-1].base, &elm[0].base, &parent_elm->base);
          parent_elm->internal.base.btype = new_leaf->ondisk->type;
          parent_elm->internal.subtree_offset = new_leaf->node_offset;
          parent_elm->internal.mirror_tid = new_leaf->ondisk->mirror_tid;
          mid_boundary = &parent_elm->base;
          ++ondisk->count;
          hammer_modify_node_done(parent);
          hammer_cursor_inserted_element(parent, parent_index + 1);
  
          /*
           * The filesystem's root B-Tree pointer may have to be updated.
           */
          if (made_root) {
                  hammer_volume_t volume;
  
                  volume = hammer_get_root_volume(hmp, &error);
                  KKASSERT(error == 0);
  
                  hammer_modify_volume_field(cursor->trans, volume,
                                             vol0_btree_root);
                  volume->ondisk->vol0_btree_root = parent->node_offset;
                  hammer_modify_volume_done(volume);
                  leaf->ondisk->parent = parent->node_offset;
                  if (cursor->parent) {
                          hammer_unlock(&cursor->parent->lock);
                          hammer_rel_node(cursor->parent);
                  }
                  cursor->parent = parent;        /* lock'd and ref'd */
                  hammer_rel_volume(volume, 0);
          }
          hammer_modify_node_done(leaf);
  
          /*
           * Ok, now adjust the cursor depending on which element the original
           * index was pointing at.  If we are >= the split point the push node
           * is now in the new node.
           *
           * NOTE: If we are at the split point itself we need to select the
           * old or new node based on where key_beg's insertion point will be.
           * If we pick the wrong side the inserted element will wind up in
           * the wrong leaf node and outside that node's bounds.
           */
          if (cursor->index > split ||
              (cursor->index == split &&
               hammer_btree_cmp(&cursor->key_beg, mid_boundary) >= 0)) {
                  cursor->parent_index = parent_index + 1;
                  cursor->index -= split;
                  hammer_unlock(&cursor->node->lock);
                  hammer_rel_node(cursor->node);
                  cursor->node = new_leaf;
          } else {
                  cursor->parent_index = parent_index;
                  hammer_unlock(&new_leaf->lock);
                  hammer_rel_node(new_leaf);
          }
  
          /*
           * Fixup left and right bounds
           */
          parent_elm = &parent->ondisk->elms[cursor->parent_index];
          cursor->left_bound = &parent_elm[0].internal.base;
          cursor->right_bound = &parent_elm[1].internal.base;
  
          /*
           * Assert that the bounds are correct.
           */
          KKASSERT(hammer_btree_cmp(cursor->left_bound,
                   &cursor->node->ondisk->elms[0].leaf.base) <= 0);
          KKASSERT(hammer_btree_cmp(cursor->right_bound,
                   &cursor->node->ondisk->elms[cursor->node->ondisk->count-1].leaf.base) > 0);
          KKASSERT(hammer_btree_cmp(cursor->left_bound, &cursor->key_beg) <= 0);
          KKASSERT(hammer_btree_cmp(cursor->right_bound, &cursor->key_beg) > 0);
  
  done:
          hammer_cursor_downgrade(cursor);
          return (error);
  }
  
  #if 0
  
  /*
   * Recursively correct the right-hand boundary's create_tid to (tid) as
   * long as the rest of the key matches.  We have to recurse upward in
   * the tree as well as down the left side of each parent's right node.
   *
   * Return EDEADLK if we were only partially successful, forcing the caller
   * to try again.  The original cursor is not modified.  This routine can
   * also fail with EDEADLK if it is forced to throw away a portion of its
   * record history.
   *
   * The caller must pass a downgraded cursor to us (otherwise we can't dup it).
   */
  struct hammer_rhb {
          TAILQ_ENTRY(hammer_rhb) entry;
          hammer_node_t   node;
          int             index;
  };
  
  TAILQ_HEAD(hammer_rhb_list, hammer_rhb);
  
  int
  hammer_btree_correct_rhb(hammer_cursor_t cursor, hammer_tid_t tid)
  {
          struct hammer_mount *hmp;
          struct hammer_rhb_list rhb_list;
          hammer_base_elm_t elm;
          hammer_node_t orig_node;
          struct hammer_rhb *rhb;
          int orig_index;
          int error;
  
          TAILQ_INIT(&rhb_list);
          hmp = cursor->trans->hmp;
  
          /*
           * Save our position so we can restore it on return.  This also
           * gives us a stable 'elm'.
           */
          orig_node = cursor->node;
          hammer_ref_node(orig_node);
          hammer_lock_sh(&orig_node->lock);
          orig_index = cursor->index;
          elm = &orig_node->ondisk->elms[orig_index].base;
  
          /*
           * Now build a list of parents going up, allocating a rhb
           * structure for each one.
           */
          while (cursor->parent) {
                  /*
                   * Stop if we no longer have any right-bounds to fix up
                   */
                  if (elm->obj_id != cursor->right_bound->obj_id ||
                      elm->rec_type != cursor->right_bound->rec_type ||
                      elm->key != cursor->right_bound->key) {
                          break;
                  }
  
                  /*
                   * Stop if the right-hand bound's create_tid does not
                   * need to be corrected.
                   */
                  if (cursor->right_bound->create_tid >= tid)
                          break;
  
                  rhb = kmalloc(sizeof(*rhb), hmp->m_misc, M_WAITOK|M_ZERO);
                  rhb->node = cursor->parent;
                  rhb->index = cursor->parent_index;
                  hammer_ref_node(rhb->node);
                  hammer_lock_sh(&rhb->node->lock);
                  TAILQ_INSERT_HEAD(&rhb_list, rhb, entry);
  
                  hammer_cursor_up(cursor);
          }
  
          /*
           * now safely adjust the right hand bound for each rhb.  This may
           * also require taking the right side of the tree and iterating down
           * ITS left side.
           */
          error = 0;
          while (error == 0 && (rhb = TAILQ_FIRST(&rhb_list)) != NULL) {
                  error = hammer_cursor_seek(cursor, rhb->node, rhb->index);
                  if (error)
                          break;
                  TAILQ_REMOVE(&rhb_list, rhb, entry);
                  hammer_unlock(&rhb->node->lock);
                  hammer_rel_node(rhb->node);
                  kfree(rhb, hmp->m_misc);
  
                  switch (cursor->node->ondisk->type) {
                  case HAMMER_BTREE_TYPE_INTERNAL:
                          /*
                           * Right-boundary for parent at internal node
                           * is one element to the right of the element whos
                           * right boundary needs adjusting.  We must then
                           * traverse down the left side correcting any left
                           * bounds (which may now be too far to the left).
                           */
                          ++cursor->index;
                          error = hammer_btree_correct_lhb(cursor, tid);
                          break;
                  default:
                          panic("hammer_btree_correct_rhb(): Bad node type");
                          error = EINVAL;
                          break;
                  }
          }
  
          /*
           * Cleanup
           */
          while ((rhb = TAILQ_FIRST(&rhb_list)) != NULL) {
                  TAILQ_REMOVE(&rhb_list, rhb, entry);
                  hammer_unlock(&rhb->node->lock);
                  hammer_rel_node(rhb->node);
                  kfree(rhb, hmp->m_misc);
          }
          error = hammer_cursor_seek(cursor, orig_node, orig_index);
          hammer_unlock(&orig_node->lock);
          hammer_rel_node(orig_node);
          return (error);
  }
  
  /*
   * Similar to rhb (in fact, rhb calls lhb), but corrects the left hand
   * bound going downward starting at the current cursor position.
   *
   * This function does not restore the cursor after use.
   */
  int
  hammer_btree_correct_lhb(hammer_cursor_t cursor, hammer_tid_t tid)
  {
          struct hammer_rhb_list rhb_list;
          hammer_base_elm_t elm;
          hammer_base_elm_t cmp;
          struct hammer_rhb *rhb;
          struct hammer_mount *hmp;
          int error;
  
          TAILQ_INIT(&rhb_list);
          hmp = cursor->trans->hmp;
  
          cmp = &cursor->node->ondisk->elms[cursor->index].base;
  
          /*
           * Record the node and traverse down the left-hand side for all
           * matching records needing a boundary correction.
           */
          error = 0;
          for (;;) {
                  rhb = kmalloc(sizeof(*rhb), hmp->m_misc, M_WAITOK|M_ZERO);
                  rhb->node = cursor->node;
                  rhb->index = cursor->index;
                  hammer_ref_node(rhb->node);
                  hammer_lock_sh(&rhb->node->lock);
                  TAILQ_INSERT_HEAD(&rhb_list, rhb, entry);
  
                  if (cursor->node->ondisk->type == HAMMER_BTREE_TYPE_INTERNAL) {
                          /*
                           * Nothing to traverse down if we are at the right
                           * boundary of an internal node.
                           */
                          if (cursor->index == cursor->node->ondisk->count)
                                  break;
                  } else {
                          elm = &cursor->node->ondisk->elms[cursor->index].base;
                          if (elm->btype == HAMMER_BTREE_TYPE_RECORD)
                                  break;
                          panic("Illegal leaf record type %02x", elm->btype);
                  }
                  error = hammer_cursor_down(cursor);
                  if (error)
                          break;
  
                  elm = &cursor->node->ondisk->elms[cursor->index].base;
                  if (elm->obj_id != cmp->obj_id ||
                      elm->rec_type != cmp->rec_type ||
                      elm->key != cmp->key) {
                          break;
                  }
                  if (elm->create_tid >= tid)
                          break;
  
          }
  
          /*
           * Now we can safely adjust the left-hand boundary from the bottom-up.
           * The last element we remove from the list is the caller's right hand
           * boundary, which must also be adjusted.
           */
          while (error == 0 && (rhb = TAILQ_FIRST(&rhb_list)) != NULL) {
                  error = hammer_cursor_seek(cursor, rhb->node, rhb->index);
                  if (error)
                          break;
                  TAILQ_REMOVE(&rhb_list, rhb, entry);
                  hammer_unlock(&rhb->node->lock);
                  hammer_rel_node(rhb->node);
                  kfree(rhb, hmp->m_misc);
  
                  elm = &cursor->node->ondisk->elms[cursor->index].base;
                  if (cursor->node->ondisk->type == HAMMER_BTREE_TYPE_INTERNAL) {
                          hammer_modify_node(cursor->trans, cursor->node,
                                             &elm->create_tid,
                                             sizeof(elm->create_tid));
                          elm->create_tid = tid;
                          hammer_modify_node_done(cursor->node);
                  } else {
                          panic("hammer_btree_correct_lhb(): Bad element type");
                  }
          }
  
          /*
           * Cleanup
           */
          while ((rhb = TAILQ_FIRST(&rhb_list)) != NULL) {
                  TAILQ_REMOVE(&rhb_list, rhb, entry);
                  hammer_unlock(&rhb->node->lock);
                  hammer_rel_node(rhb->node);
                  kfree(rhb, hmp->m_misc);
          }
          return (error);
  }
  
  #endif
  
  /*
   * Attempt to remove the locked, empty or want-to-be-empty B-Tree node at
   * (cursor->node).  Returns 0 on success, EDEADLK if we could not complete
   * the operation due to a deadlock, or some other error.
   *
   * This routine is initially called with an empty leaf and may be
   * recursively called with single-element internal nodes.
   *
   * It should also be noted that when removing empty leaves we must be sure
   * to test and update mirror_tid because another thread may have deadlocked
   * against us (or someone) trying to propagate it up and cannot retry once
   * the node has been deleted.
   *
   * On return the cursor may end up pointing to an internal node, suitable
   * for further iteration but not for an immediate insertion or deletion.
   */
  static int
  btree_remove(hammer_cursor_t cursor)
  {
          hammer_node_ondisk_t ondisk;
          hammer_btree_elm_t elm;
          hammer_node_t node;
          hammer_node_t parent;
          const int esize = sizeof(*elm);
          int error;
  
          node = cursor->node;
  
          /*
           * When deleting the root of the filesystem convert it to
           * an empty leaf node.  Internal nodes cannot be empty.
           */
          ondisk = node->ondisk;
          if (ondisk->parent == 0) {
                  KKASSERT(cursor->parent == NULL);
                  hammer_modify_node_all(cursor->trans, node);
                  KKASSERT(ondisk == node->ondisk);
                  ondisk->type = HAMMER_BTREE_TYPE_LEAF;
                  ondisk->count = 0;
                  hammer_modify_node_done(node);
                  cursor->index = 0;
                  return(0);
          }
  
          parent = cursor->parent;
  
          /*
           * Attempt to remove the parent's reference to the child.  If the
           * parent would become empty we have to recurse.  If we fail we 
           * leave the parent pointing to an empty leaf node.
           *
           * We have to recurse successfully before we can delete the internal
           * node as it is illegal to have empty internal nodes.  Even though
           * the operation may be aborted we must still fixup any unlocked
           * cursors as if we had deleted the element prior to recursing
           * (by calling hammer_cursor_deleted_element()) so those cursors
           * are properly forced up the chain by the recursion.
           */
          if (parent->ondisk->count == 1) {
                  /*
                   * This special cursor_up_locked() call leaves the original
                   * node exclusively locked and referenced, leaves the
                   * original parent locked (as the new node), and locks the
                   * new parent.  It can return EDEADLK.
                   *
                   * We cannot call hammer_cursor_removed_node() until we are
                   * actually able to remove the node.  If we did then tracked
                   * cursors in the middle of iterations could be repointed
                   * to a parent node.  If this occurs they could end up
                   * scanning newly inserted records into the node (that could
                   * not be deleted) when they push down again.
                   *
                   * Due to the way the recursion works the final parent is left
                   * in cursor->parent after the recursion returns.  Each
                   * layer on the way back up is thus able to call
                   * hammer_cursor_removed_node() and 'jump' the node up to
                   * the (same) final parent.
                   *
                   * NOTE!  The local variable 'parent' is invalid after we
                   *        call hammer_cursor_up_locked().
                   */
                  error = hammer_cursor_up_locked(cursor);
                  parent = NULL;
  
                  if (error == 0) {
                          hammer_cursor_deleted_element(cursor->node, 0);
                          error = btree_remove(cursor);
                          if (error == 0) {
                                  KKASSERT(node != cursor->node);
                                  hammer_cursor_removed_node(
                                          node, cursor->node,
                                          cursor->index);
                                  hammer_modify_node_all(cursor->trans, node);
                                  ondisk = node->ondisk;
                                  ondisk->type = HAMMER_BTREE_TYPE_DELETED;
                                  ondisk->count = 0;
                                  hammer_modify_node_done(node);
                                  hammer_flush_node(node, 0);
                                  hammer_delete_node(cursor->trans, node);
                          } else {
                                  /*
                                   * Defer parent removal because we could not
                                   * get the lock, just let the leaf remain
                                   * empty.
                                   */
                                  /**/
                          }
                          hammer_unlock(&node->lock);
                          hammer_rel_node(node);
                  } else {
                          /*
                           * Defer parent removal because we could not
                           * get the lock, just let the leaf remain
                           * empty.
                           */
                          /**/
                  }
          } else {
                  KKASSERT(parent->ondisk->count > 1);
  
                  hammer_modify_node_all(cursor->trans, parent);
                  ondisk = parent->ondisk;
                  KKASSERT(ondisk->type == HAMMER_BTREE_TYPE_INTERNAL);
  
                  elm = &ondisk->elms[cursor->parent_index];
                  KKASSERT(elm->internal.subtree_offset == node->node_offset);
                  KKASSERT(ondisk->count > 0);
  
                  /*
                   * We must retain the highest mirror_tid.  The deleted
                   * range is now encompassed by the element to the left.
                   * If we are already at the left edge the new left edge
                   * inherits mirror_tid.
                   *
                   * Note that bounds of the parent to our parent may create
                   * a gap to the left of our left-most node or to the right
                   * of our right-most node.  The gap is silently included
                   * in the mirror_tid's area of effect from the point of view
                   * of the scan.
                   */
                  if (cursor->parent_index) {
                          if (elm[-1].internal.mirror_tid <
                              elm[0].internal.mirror_tid) {
                                  elm[-1].internal.mirror_tid =
                                      elm[0].internal.mirror_tid;
                          }
                  } else {
                          if (elm[1].internal.mirror_tid <
                              elm[0].internal.mirror_tid) {
                                  elm[1].internal.mirror_tid =
                                      elm[0].internal.mirror_tid;
                          }
                  }
  
                  /*
                   * Delete the subtree reference in the parent.  Include
                   * boundary element at end.
                   */
                  bcopy(&elm[1], &elm[0],
                        (ondisk->count - cursor->parent_index) * esize);
                  --ondisk->count;
                  hammer_modify_node_done(parent);
                  hammer_cursor_removed_node(node, parent, cursor->parent_index);
                  hammer_cursor_deleted_element(parent, cursor->parent_index);
                  hammer_flush_node(node, 0);
                  hammer_delete_node(cursor->trans, node);
  
                  /*
                   * cursor->node is invalid, cursor up to make the cursor
                   * valid again.  We have to flag the condition in case
                   * another thread wiggles an insertion in during an
                   * iteration.
                   */
                  cursor->flags |= HAMMER_CURSOR_ITERATE_CHECK;
                  error = hammer_cursor_up(cursor);
          }
          return (error);
  }
  
  /*
   * Propagate cursor->trans->tid up the B-Tree starting at the current
   * cursor position using pseudofs info gleaned from the passed inode.
   *
   * The passed inode has no relationship to the cursor position other
   * then being in the same pseudofs as the insertion or deletion we
   * are propagating the mirror_tid for.
   *
   * WARNING!  Because we push and pop the passed cursor, it may be
   *           modified by other B-Tree operations while it is unlocked
   *           and things like the node & leaf pointers, and indexes might
   *           change.
   */
  void
  hammer_btree_do_propagation(hammer_cursor_t cursor,
                              hammer_pseudofs_inmem_t pfsm,
                              hammer_btree_leaf_elm_t leaf)
  {
          hammer_cursor_t ncursor;
          hammer_tid_t mirror_tid;
          int error __debugvar;
  
          /*
           * We do not propagate a mirror_tid if the filesystem was mounted
           * in no-mirror mode.
           */
          if (cursor->trans->hmp->master_id < 0)
                  return;
  
          /*
           * This is a bit of a hack because we cannot deadlock or return
           * EDEADLK here.  The related operation has already completed and
           * we must propagate the mirror_tid now regardless.
           *
           * Generate a new cursor which inherits the original's locks and
           * unlock the original.  Use the new cursor to propagate the
           * mirror_tid.  Then clean up the new cursor and reacquire locks
           * on the original.
           *
           * hammer_dup_cursor() cannot dup locks.  The dup inherits the
           * original's locks and the original is tracked and must be
           * re-locked.
           */
          mirror_tid = cursor->node->ondisk->mirror_tid;
          KKASSERT(mirror_tid != 0);
          ncursor = hammer_push_cursor(cursor);
          error = hammer_btree_mirror_propagate(ncursor, mirror_tid);
          KKASSERT(error == 0);
          hammer_pop_cursor(cursor, ncursor);
          /* WARNING: cursor's leaf pointer may change after pop */
  }
  
  
  /*
   * Propagate a mirror TID update upwards through the B-Tree to the root.
   *
   * A locked internal node must be passed in.  The node will remain locked
   * on return.
   *
   * This function syncs mirror_tid at the specified internal node's element,
   * adjusts the node's aggregation mirror_tid, and then recurses upwards.
   */
  static int
  hammer_btree_mirror_propagate(hammer_cursor_t cursor, hammer_tid_t mirror_tid)
  {
          hammer_btree_internal_elm_t elm;
          hammer_node_t node;
          int error;
  
          for (;;) {
                  error = hammer_cursor_up(cursor);
                  if (error == 0)
                          error = hammer_cursor_upgrade(cursor);
  
                  /*
                   * We can ignore HAMMER_CURSOR_ITERATE_CHECK, the
                   * cursor will still be properly positioned for
                   * mirror propagation, just not for iterations.
                   */
                  while (error == EDEADLK) {
                          hammer_recover_cursor(cursor);
                          error = hammer_cursor_upgrade(cursor);
                  }
                  if (error)
                          break;
  
                  /*
                   * If the cursor deadlocked it could end up at a leaf
                   * after we lost the lock.
                   */
                  node = cursor->node;
                  if (node->ondisk->type != HAMMER_BTREE_TYPE_INTERNAL)
                          continue;
  
                  /*
                   * Adjust the node's element
                   */
                  elm = &node->ondisk->elms[cursor->index].internal;
                  if (elm->mirror_tid >= mirror_tid)
                          break;
                  hammer_modify_node(cursor->trans, node, &elm->mirror_tid,
                                     sizeof(elm->mirror_tid));
                  elm->mirror_tid = mirror_tid;
                  hammer_modify_node_done(node);
                  if (hammer_debug_general & 0x0002) {
                          kprintf("mirror_propagate: propagate "
                                  "%016llx @%016llx:%d\n",
                                  (long long)mirror_tid,
                                  (long long)node->node_offset,
                                  cursor->index);
                  }
  
  
                  /*
                   * Adjust the node's mirror_tid aggregator
                   */
                  if (node->ondisk->mirror_tid >= mirror_tid)
                          return(0);
                  hammer_modify_node_field(cursor->trans, node, mirror_tid);
                  node->ondisk->mirror_tid = mirror_tid;
                  hammer_modify_node_done(node);
                  if (hammer_debug_general & 0x0002) {
                          kprintf("mirror_propagate: propagate "
                                  "%016llx @%016llx\n",
                                  (long long)mirror_tid,
                                  (long long)node->node_offset);
                  }
          }
          if (error == ENOENT)
                  error = 0;
          return(error);
  }
  
  hammer_node_t
  hammer_btree_get_parent(hammer_transaction_t trans, hammer_node_t node,
                          int *parent_indexp, int *errorp, int try_exclusive)
  {
          hammer_node_t parent;
          hammer_btree_elm_t elm;
          int i;
  
          /*
           * Get the node
           */
          parent = hammer_get_node(trans, node->ondisk->parent, 0, errorp);
          if (*errorp) {
                  KKASSERT(parent == NULL);
                  return(NULL);
          }
          KKASSERT ((parent->flags & HAMMER_NODE_DELETED) == 0);
  
          /*
           * Lock the node
           */
          if (try_exclusive) {
                  if (hammer_lock_ex_try(&parent->lock)) {
                          hammer_rel_node(parent);
                          *errorp = EDEADLK;
                          return(NULL);
                  }
          } else {
                  hammer_lock_sh(&parent->lock);
          }
  
          /*
           * Figure out which element in the parent is pointing to the
           * child.
           */
          if (node->ondisk->count) {
                  i = hammer_btree_search_node(&node->ondisk->elms[0].base,
                                               parent->ondisk);
          } else {
                  i = 0;
          }
          while (i < parent->ondisk->count) {
                  elm = &parent->ondisk->elms[i];
                  if (elm->internal.subtree_offset == node->node_offset)
                          break;
                  ++i;
          }
          if (i == parent->ondisk->count) {
                  hammer_unlock(&parent->lock);
                  panic("Bad B-Tree link: parent %p node %p", parent, node);
          }
          *parent_indexp = i;
          KKASSERT(*errorp == 0);
          return(parent);
  }
  
  /*
   * The element (elm) has been moved to a new internal node (node).
   *
   * If the element represents a pointer to an internal node that node's
   * parent must be adjusted to the element's new location.
   *
   * XXX deadlock potential here with our exclusive locks
   */
  int
  btree_set_parent(hammer_transaction_t trans, hammer_node_t node,
                   hammer_btree_elm_t elm)
  {
          hammer_node_t child;
          int error;
  
          error = 0;
  
          switch(elm->base.btype) {
          case HAMMER_BTREE_TYPE_INTERNAL:
          case HAMMER_BTREE_TYPE_LEAF:
                  child = hammer_get_node(trans, elm->internal.subtree_offset,
                                          0, &error);
                  if (error == 0) {
                          hammer_modify_node_field(trans, child, parent);
                          child->ondisk->parent = node->node_offset;
                          hammer_modify_node_done(child);
                          hammer_rel_node(child);
                  }
                  break;
          default:
                  break;
          }
          return(error);
  }
  
  /*
   * Initialize the root of a recursive B-Tree node lock list structure.
   */
  void
  hammer_node_lock_init(hammer_node_lock_t parent, hammer_node_t node)
  {
          TAILQ_INIT(&parent->list);
          parent->parent = NULL;
          parent->node = node;
          parent->index = -1;
          parent->count = node->ondisk->count;
          parent->copy = NULL;
          parent->flags = 0;
  }
  
  /*
   * Initialize a cache of hammer_node_lock's including space allocated
   * for node copies.
   *
   * This is used by the rebalancing code to preallocate the copy space
   * for ~4096 B-Tree nodes (16MB of data) prior to acquiring any HAMMER
   * locks, otherwise we can blow out the pageout daemon's emergency
   * reserve and deadlock it.
   *
   * NOTE: HAMMER_NODE_LOCK_LCACHE is not set on items cached in the lcache.
   *       The flag is set when the item is pulled off the cache for use.
   */
  void
  hammer_btree_lcache_init(hammer_mount_t hmp, hammer_node_lock_t lcache,
                           int depth)
  {
          hammer_node_lock_t item;
          int count;
  
          for (count = 1; depth; --depth)
                  count *= HAMMER_BTREE_LEAF_ELMS;
          bzero(lcache, sizeof(*lcache));
          TAILQ_INIT(&lcache->list);
          while (count) {
                  item = kmalloc(sizeof(*item), hmp->m_misc, M_WAITOK|M_ZERO);
                  item->copy = kmalloc(sizeof(*item->copy),
                                       hmp->m_misc, M_WAITOK);
                  TAILQ_INIT(&item->list);
                  TAILQ_INSERT_TAIL(&lcache->list, item, entry);
                  --count;
          }
  }
  
  void
  hammer_btree_lcache_free(hammer_mount_t hmp, hammer_node_lock_t lcache)
  {
          hammer_node_lock_t item;
  
          while ((item = TAILQ_FIRST(&lcache->list)) != NULL) {
                  TAILQ_REMOVE(&lcache->list, item, entry);
                  KKASSERT(item->copy);
                  KKASSERT(TAILQ_EMPTY(&item->list));
                  kfree(item->copy, hmp->m_misc);
                  kfree(item, hmp->m_misc);
          }
          KKASSERT(lcache->copy == NULL);
  }
  
  /*
   * Exclusively lock all the children of node.  This is used by the split
   * code to prevent anyone from accessing the children of a cursor node
   * while we fix-up its parent offset.
   *
   * If we don't lock the children we can really mess up cursors which block
   * trying to cursor-up into our node.
   *
   * On failure EDEADLK (or some other error) is returned.  If a deadlock
   * error is returned the cursor is adjusted to block on termination.
   *
   * The caller is responsible for managing parent->node, the root's node
   * is usually aliased from a cursor.
   */
  int
  hammer_btree_lock_children(hammer_cursor_t cursor, int depth,
                             hammer_node_lock_t parent,
                             hammer_node_lock_t lcache)
  {
          hammer_node_t node;
          hammer_node_lock_t item;
          hammer_node_ondisk_t ondisk;
          hammer_btree_elm_t elm;
          hammer_node_t child;
          struct hammer_mount *hmp;
          int error;
          int i;
  
          node = parent->node;
          ondisk = node->ondisk;
          error = 0;
          hmp = cursor->trans->hmp;
  
          /*
           * We really do not want to block on I/O with exclusive locks held,
           * pre-get the children before trying to lock the mess.  This is
           * only done one-level deep for now.
           */
          for (i = 0; i < ondisk->count; ++i) {
                  ++hammer_stats_btree_elements;
                  elm = &ondisk->elms[i];
                  if (elm->base.btype != HAMMER_BTREE_TYPE_LEAF &&
                      elm->base.btype != HAMMER_BTREE_TYPE_INTERNAL) {
                          continue;
                  }
                  child = hammer_get_node(cursor->trans,
                                          elm->internal.subtree_offset,
                                          0, &error);
                  if (child)
                          hammer_rel_node(child);
          }
  
          /*
           * Do it for real
           */
          for (i = 0; error == 0 && i < ondisk->count; ++i) {
                  ++hammer_stats_btree_elements;
                  elm = &ondisk->elms[i];
  
                  switch(elm->base.btype) {
                  case HAMMER_BTREE_TYPE_INTERNAL:
                  case HAMMER_BTREE_TYPE_LEAF:
                          KKASSERT(elm->internal.subtree_offset != 0);
                          child = hammer_get_node(cursor->trans,
                                                  elm->internal.subtree_offset,
                                                  0, &error);
                          break;
                  default:
                          child = NULL;
                          break;
                  }
                  if (child) {
                          if (hammer_lock_ex_try(&child->lock) != 0) {
                                  if (cursor->deadlk_node == NULL) {
                                          cursor->deadlk_node = child;
                                          hammer_ref_node(cursor->deadlk_node);
                                  }
                                  error = EDEADLK;
                                  hammer_rel_node(child);
                          } else {
                                  if (lcache) {
                                          item = TAILQ_FIRST(&lcache->list);
                                          KKASSERT(item != NULL);
                                          item->flags |= HAMMER_NODE_LOCK_LCACHE;
                                          TAILQ_REMOVE(&lcache->list,
                                                       item, entry);
                                  } else {
                                          item = kmalloc(sizeof(*item),
                                                         hmp->m_misc,
                                                         M_WAITOK|M_ZERO);
                                          TAILQ_INIT(&item->list);
                                  }
  
                                  TAILQ_INSERT_TAIL(&parent->list, item, entry);
                                  item->parent = parent;
                                  item->node = child;
                                  item->index = i;
                                  item->count = child->ondisk->count;
  
                                  /*
                                   * Recurse (used by the rebalancing code)
                                   */
                                  if (depth > 1 && elm->base.btype == HAMMER_BTREE_TYPE_INTERNAL) {
                                          error = hammer_btree_lock_children(
                                                          cursor,
                                                          depth - 1,
                                                          item,
                                                          lcache);
                                  }
                          }
                  }
          }
          if (error)
                  hammer_btree_unlock_children(hmp, parent, lcache);
          return(error);
  }
  
  /*
   * Create an in-memory copy of all B-Tree nodes listed, recursively,
   * including the parent.
   */
  void
  hammer_btree_lock_copy(hammer_cursor_t cursor, hammer_node_lock_t parent)
  {
          hammer_mount_t hmp = cursor->trans->hmp;
          hammer_node_lock_t item;
  
          if (parent->copy == NULL) {
                  KKASSERT((parent->flags & HAMMER_NODE_LOCK_LCACHE) == 0);
                  parent->copy = kmalloc(sizeof(*parent->copy),
                                         hmp->m_misc, M_WAITOK);
          }
          KKASSERT((parent->flags & HAMMER_NODE_LOCK_UPDATED) == 0);
          *parent->copy = *parent->node->ondisk;
          TAILQ_FOREACH(item, &parent->list, entry) {
                  hammer_btree_lock_copy(cursor, item);
          }
  }
  
  /*
   * Recursively sync modified copies to the media.
   */
  int
  hammer_btree_sync_copy(hammer_cursor_t cursor, hammer_node_lock_t parent)
  {
          hammer_node_lock_t item;
          int count = 0;
  
          if (parent->flags & HAMMER_NODE_LOCK_UPDATED) {
                  ++count;
                  hammer_modify_node_all(cursor->trans, parent->node);
                  *parent->node->ondisk = *parent->copy;
                  hammer_modify_node_done(parent->node);
                  if (parent->copy->type == HAMMER_BTREE_TYPE_DELETED) {
                          hammer_flush_node(parent->node, 0);
                          hammer_delete_node(cursor->trans, parent->node);
                  }
          }
          TAILQ_FOREACH(item, &parent->list, entry) {
                  count += hammer_btree_sync_copy(cursor, item);
          }
          return(count);
  }
  
  /*
   * Release previously obtained node locks.  The caller is responsible for
   * cleaning up parent->node itself (its usually just aliased from a cursor),
   * but this function will take care of the copies.
   *
   * NOTE: The root node is not placed in the lcache and node->copy is not
   *       deallocated when lcache != NULL.
   */
  void
  hammer_btree_unlock_children(hammer_mount_t hmp, hammer_node_lock_t parent,
                               hammer_node_lock_t lcache)
  {
          hammer_node_lock_t item;
          hammer_node_ondisk_t copy;
  
          while ((item = TAILQ_FIRST(&parent->list)) != NULL) {
                  TAILQ_REMOVE(&parent->list, item, entry);
                  hammer_btree_unlock_children(hmp, item, lcache);
                  hammer_unlock(&item->node->lock);
                  hammer_rel_node(item->node);
                  if (lcache) {
                          /*
                           * NOTE: When placing the item back in the lcache
                           *       the flag is cleared by the bzero().
                           *       Remaining fields are cleared as a safety
                           *       measure.
                           */
                          KKASSERT(item->flags & HAMMER_NODE_LOCK_LCACHE);
                          KKASSERT(TAILQ_EMPTY(&item->list));
                          copy = item->copy;
                          bzero(item, sizeof(*item));
                          TAILQ_INIT(&item->list);
                          item->copy = copy;
                          if (copy)
                                  bzero(copy, sizeof(*copy));
                          TAILQ_INSERT_TAIL(&lcache->list, item, entry);
                  } else {
                          kfree(item, hmp->m_misc);
                  }
          }
          if (parent->copy && (parent->flags & HAMMER_NODE_LOCK_LCACHE) == 0) {
                  kfree(parent->copy, hmp->m_misc);
                  parent->copy = NULL;    /* safety */
          }
  }
  
  /************************************************************************
   *                         MISCELLANIOUS SUPPORT                        *
   ************************************************************************/
  
  /*
   * Compare two B-Tree elements, return -N, 0, or +N (e.g. similar to strcmp).
   *
   * Note that for this particular function a return value of -1, 0, or +1
   * can denote a match if create_tid is otherwise discounted.  A create_tid
   * of zero is considered to be 'infinity' in comparisons.
   *
   * See also hammer_rec_rb_compare() and hammer_rec_cmp() in hammer_object.c.
   */
  int
  hammer_btree_cmp(hammer_base_elm_t key1, hammer_base_elm_t key2)
  {
          if (key1->localization < key2->localization)
                  return(-5);
          if (key1->localization > key2->localization)
                  return(5);
  
          if (key1->obj_id < key2->obj_id)
                  return(-4);
          if (key1->obj_id > key2->obj_id)
                  return(4);
  
          if (key1->rec_type < key2->rec_type)
                  return(-3);
          if (key1->rec_type > key2->rec_type)
                  return(3);
  
          if (key1->key < key2->key)
                  return(-2);
          if (key1->key > key2->key)
                  return(2);
  
          /*
           * A create_tid of zero indicates a record which is undeletable
           * and must be considered to have a value of positive infinity.
           */
          if (key1->create_tid == 0) {
                  if (key2->create_tid == 0)
                          return(0);
                  return(1);
          }
          if (key2->create_tid == 0)
                  return(-1);
          if (key1->create_tid < key2->create_tid)
                  return(-1);
          if (key1->create_tid > key2->create_tid)
                  return(1);
          return(0);
  }
  
  /*
   * Test a timestamp against an element to determine whether the
   * element is visible.  A timestamp of 0 means 'infinity'.
   */
  int
  hammer_btree_chkts(hammer_tid_t asof, hammer_base_elm_t base)
  {
          if (asof == 0) {
                  if (base->delete_tid)
                          return(1);
                  return(0);
          }
          if (asof < base->create_tid)
                  return(-1);
          if (base->delete_tid && asof >= base->delete_tid)
                  return(1);
          return(0);
  }
  
  /*
   * Create a separator half way inbetween key1 and key2.  For fields just
   * one unit apart, the separator will match key2.  key1 is on the left-hand
   * side and key2 is on the right-hand side.
   *
   * key2 must be >= the separator.  It is ok for the separator to match key2.
   *
   * NOTE: Even if key1 does not match key2, the separator may wind up matching
   * key2.
   *
   * NOTE: It might be beneficial to just scrap this whole mess and just
   * set the separator to key2.
   */
  #define MAKE_SEPARATOR(key1, key2, dest, field) \
          dest->field = key1->field + ((key2->field - key1->field + 1) >> 1);
  
  static void
  hammer_make_separator(hammer_base_elm_t key1, hammer_base_elm_t key2,
                        hammer_base_elm_t dest)
  {
          bzero(dest, sizeof(*dest));
  
          dest->rec_type = key2->rec_type;
          dest->key = key2->key;
          dest->obj_id = key2->obj_id;
          dest->create_tid = key2->create_tid;
  
          MAKE_SEPARATOR(key1, key2, dest, localization);
          if (key1->localization == key2->localization) {
                  MAKE_SEPARATOR(key1, key2, dest, obj_id);
                  if (key1->obj_id == key2->obj_id) {
                          MAKE_SEPARATOR(key1, key2, dest, rec_type);
                          if (key1->rec_type == key2->rec_type) {
                                  MAKE_SEPARATOR(key1, key2, dest, key);
                                  /*
                                   * Don't bother creating a separator for
                                   * create_tid, which also conveniently avoids
                                   * having to handle the create_tid == 0
                                   * (infinity) case.  Just leave create_tid
                                   * set to key2.
                                   *
                                   * Worst case, dest matches key2 exactly,
                                   * which is acceptable.
                                   */
                          }
                  }
          }
  }
  
  #undef MAKE_SEPARATOR
  
  /*
   * Return whether a generic internal or leaf node is full
   */
  static int
  btree_node_is_full(hammer_node_ondisk_t node)
  {
          switch(node->type) {
          case HAMMER_BTREE_TYPE_INTERNAL:
                  if (node->count == HAMMER_BTREE_INT_ELMS)
                          return(1);
                  break;
          case HAMMER_BTREE_TYPE_LEAF:
                  if (node->count == HAMMER_BTREE_LEAF_ELMS)
                          return(1);
                  break;
          default:
                  panic("illegal btree subtype");
          }
          return(0);
  }
  
  #if 0
  static int
  btree_max_elements(u_int8_t type)
  {
          if (type == HAMMER_BTREE_TYPE_LEAF)
                  return(HAMMER_BTREE_LEAF_ELMS);
          if (type == HAMMER_BTREE_TYPE_INTERNAL)
                  return(HAMMER_BTREE_INT_ELMS);
          panic("btree_max_elements: bad type %d", type);
  }
  #endif
  
  void
  hammer_print_btree_node(hammer_node_ondisk_t ondisk)
  {
          hammer_btree_elm_t elm;
          int i;
  
          kprintf("node %p count=%d parent=%016llx type=%c\n",
                  ondisk, ondisk->count,
                  (long long)ondisk->parent, ondisk->type);
  
          /*
           * Dump both boundary elements if an internal node
           */
          if (ondisk->type == HAMMER_BTREE_TYPE_INTERNAL) {
                  for (i = 0; i <= ondisk->count; ++i) {
                          elm = &ondisk->elms[i];
                          hammer_print_btree_elm(elm, ondisk->type, i);
                  }
          } else {
                  for (i = 0; i < ondisk->count; ++i) {
                          elm = &ondisk->elms[i];
                          hammer_print_btree_elm(elm, ondisk->type, i);
                  }
          }
  }
  
  void
  hammer_print_btree_elm(hammer_btree_elm_t elm, u_int8_t type, int i)
  {
          kprintf("  %2d", i);
          kprintf("\tobj_id       = %016llx\n", (long long)elm->base.obj_id);
          kprintf("\tkey          = %016llx\n", (long long)elm->base.key);
          kprintf("\tcreate_tid   = %016llx\n", (long long)elm->base.create_tid);
          kprintf("\tdelete_tid   = %016llx\n", (long long)elm->base.delete_tid);
          kprintf("\trec_type     = %04x\n", elm->base.rec_type);
          kprintf("\tobj_type     = %02x\n", elm->base.obj_type);
          kprintf("\tbtype        = %02x (%c)\n",
                  elm->base.btype,
                  (elm->base.btype ? elm->base.btype : '?'));
          kprintf("\tlocalization = %02x\n", elm->base.localization);
  
          switch(type) {
          case HAMMER_BTREE_TYPE_INTERNAL:
                  kprintf("\tsubtree_off  = %016llx\n",
                          (long long)elm->internal.subtree_offset);
                  break;
          case HAMMER_BTREE_TYPE_RECORD:
                  kprintf("\tdata_offset  = %016llx\n",
                          (long long)elm->leaf.data_offset);
                  kprintf("\tdata_len     = %08x\n", elm->leaf.data_len);
                  kprintf("\tdata_crc     = %08x\n", elm->leaf.data_crc);
                  break;
          }
  }

Cache object: 04de8ef20302a5ff1596fc8a4b133403