Fix parallel index scan hang with deleted or half-dead pages.

The previous coding forgot to release the scan before seizing
it again, leading to a lockup.

Report by Patrick Hemmer.  Diagnosis by Thomas Munro.  Patch by
Amit Kapila.

Discussion: http://postgr.es/m/CAEepm=2xZUcOGP9V0O_G0=2P2wwXwPrkF=upWTCJSisUxMnuSg@mail.gmail.com

diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 558113bd13a859411f3d81824153ff0dd2b20238..12d3f081b6181aed3bf71b80f26a32df0ed30d69 100644 (file)
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -1486,6 +1486,11 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
                                if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque)))
                                        break;
                        }
+                       else if (scan->parallel_scan != NULL)
+                       {
+                               /* allow next page be processed by parallel worker */
+                               _bt_parallel_release(scan, opaque->btpo_next);
+                       }
 
                        /* nope, keep going */
                        if (scan->parallel_scan != NULL)
@@ -1581,6 +1586,11 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
                                if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page)))
                                        break;
                        }
+                       else if (scan->parallel_scan != NULL)
+                       {
+                               /* allow next page be processed by parallel worker */
+                               _bt_parallel_release(scan, BufferGetBlockNumber(so->currPos.buf));
+                       }
 
                        /*
                         * For parallel scans, get the last page scanned as it is quite