Assert consistency of currPage that ended scan.

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 b5ee4e52.

diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index ebb6c108367e158b6f3bda192e2d732ba97cc68d..2786a8564f2e8a15a2707666727b006b076b5826 100644 (file)
--- 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;
                }