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commit    8ce29bb4f0df0806cf1995594b6390e9a9997665
Author:   Amit Kapila <akapila@postgresql.org>
Date:     Fri Jul 27 05:23:00 2018 +0000
First released in: 12.0, 10.5, 11.0

    Fix the buffer release order for parallel index scans.
    
    During parallel index scans, if the current page to be read is deleted, we
    skip it and try to get the next page for a scan without releasing the buffer
    lock on the current page.  To get the next page, sometimes it needs to wait
    for another process to complete its scan and advance it to the next page.
    Now, it is quite possible that the master backend has errored out before
    advancing the scan and issued a termination signal for all workers.  The
    workers failed to notice the termination request during wait because the
    interrupts are held due to buffer lock on the previous page.  This lead to
    all workers being stuck.
    
    The fix is to release the buffer lock on current page before trying to get
    the next page.  We are already doing same in backward scans, but missed
    it for forward scans.
    
    Reported-by: Victor Yegorov
    Bug: 15290
    Diagnosed-by: Thomas Munro and Amit Kapila
    Author: Amit Kapila
    Reviewed-by: Thomas Munro
    Tested-By: Thomas Munro and Victor Yegorov
    Backpatch-through: 10 where parallel index scans were introduced
    Discussion: https://postgr.es/m/153228422922.1395.1746424054206154747@wrigleys.postgresql.org


 src/backend/access/nbtree/nbtsearch.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 0bcfa10b864..6831bc8c032 100644
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -1497,17 +1497,19 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
 			/* nope, keep going */
 			if (scan->parallel_scan != NULL)
 			{
+				_bt_relbuf(rel, so->currPos.buf);
 				status = _bt_parallel_seize(scan, &blkno);
 				if (!status)
 				{
-					_bt_relbuf(rel, so->currPos.buf);
 					BTScanPosInvalidate(so->currPos);
 					return false;
 				}
 			}
 			else
+			{
 				blkno = opaque->btpo_next;
-			_bt_relbuf(rel, so->currPos.buf);
+				_bt_relbuf(rel, so->currPos.buf);
+			}
 		}
 	}
 	else


[parent: 39d51fe87dae]