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Fix _bt_readnextpage half-dead leftmost page assert.

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Backwards scans that reach a half-dead leftmost leaf page step over the
page without calling _bt_readpage.  This happens whenever the most
recent _bt_lock_and_validate_left call returned a half-dead leftmost
page.  An assertion within _bt_readnextpage failed to consider this,
causing it to fail spuriously.

To fix, make the assertion accept the case where currPage doesn't match
lastcurrblkno as long as blkno is P_NONE (it's always P_NONE when the
last leaf page examined by _bt_readnextpage is either the leftmost or
rightmost page).

Oversight in commit caca6d8d27.

Author: Peter Geoghegan <pg@bowt.ie>
Backpatch-through: 18
This commit is contained in:
Peter Geoghegan
2026-10-15 12:17:18 -04:00
parent 8a59a526de
commit 7a28f1d028
1 changed files with 7 additions and 2 deletions
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9
src/backend/access/nbtree/nbtsearch.c
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@@ -2387,8 +2387,13 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
(ScanDirectionIsForward(dir) ?
!so->currPos.moreRight : !so->currPos.moreLeft))
{
/* most recent _bt_readpage call (for lastcurrblkno) ended scan */
Assert(so->currPos.currPage == lastcurrblkno && !seized);
/*
* most recent _bt_readpage call (for lastcurrblkno) ended scan,
* or lastcurrblkno is a half-dead leftmost page (not read by
* _bt_readpage, so so->currPos doesn't describe it)
*/
Assert(so->currPos.currPage == lastcurrblkno || blkno == P_NONE);
Assert(!seized);
BTScanPosInvalidate(so->currPos);
_bt_parallel_done(scan); /* iff !so->needPrimScan */
return false;