Assert consistency of currPage that ended scan.

petergeoghegan · petergeoghegan · commit caca6d8d276c · 2024-11-08T16:34:41.000-05:00
When _bt_readnextpage is called with our nbtree parallel scan already seized (i.e. when it is directly called by _bt_first), we never expect a prior call to _bt_readpage for lastcurrblkno to already indicate that the scan should end -- the _bt_first caller's blkno must always be read. After all, the "prior" _bt_readpage call (the call for lastcurrblkno) probably took place in some other backend (and it might not even have finished by the time our backend reaches _bt_first/_bt_readnextpage). Add a documenting assertion to the path where _bt_readnextpage ends the parallel scan based on information about lastcurrblkno from so->currPos. Assert that the most recent _bt_readpage call that set so->currPos is in fact lastcurrblkno's _bt_readpage call. Follow-up to bugfix commit b5ee4e5.
diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
@@ -2230,8 +2230,9 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
 			 !so->currPos.moreRight : !so->currPos.moreLeft))
 		{
 			/* most recent _bt_readpage call (for lastcurrblkno) ended scan */
+			Assert(so->currPos.currPage == lastcurrblkno && !seized);
 			BTScanPosInvalidate(so->currPos);
-			_bt_parallel_done(scan);
+			_bt_parallel_done(scan);	/* iff !so->needPrimScan */
 			return false;
 		}
 
