Correct an old logic error in btree page splitting: when considering a split

exactly at the point where we need to insert a new item, the calculation used
the wrong size for the "high key" of the new left page.  This could lead to
choosing an unworkable split, resulting in "PANIC: failed to add item to the
left sibling" (or "right sibling") failure.  Although this bug has been there
a long time, it's very difficult to trigger a failure before 8.2, since there
was generally a lot of free space on both sides of a chosen split.  In 8.2,
where the user-selected fill factor determines how much free space the code
tries to leave, an unworkable split is much more likely.  Report by Joe
Conway, diagnosis and fix by Heikki Linnakangas.

1 files changed, 7 insertions, 2 deletions

diff --git a/src/backend/access/nbtree/nbtinsert.c b/src/backend/access/nbtree/nbtinsert.c
index c528460b622..beb543f0939 100644
--- a/src/backend/access/nbtree/nbtinsert.c
+++ b/src/backend/access/nbtree/nbtinsert.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.146 2006/11/11 01:14:18 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.146.2.1 2007/01/27 20:53:36 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1153,7 +1153,12 @@ _bt_findsplitloc(Relation rel,
 			/* need to try it both ways! */
 			_bt_checksplitloc(&state, offnum, leftfree, rightfree,
 							  true, itemsz);
-			/* here we are contemplating newitem as first on right */
+			/*
+			 * Here we are contemplating newitem as first on right.  In this
+			 * case it, not the current item, will become the high key of the
+			 * left page, and so we have to correct the allowance made above.
+			 */
+			leftfree += (int) itemsz - (int) newitemsz;
 			_bt_checksplitloc(&state, offnum, leftfree, rightfree,
 							  false, newitemsz);
 		}