From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Thu, 24 Jun 2021 08:19:03 +0000 (+0300)
Subject: Another fix to relmapper race condition.
X-Git-Tag: REL9_6_23~56
X-Git-Url: http://git.postgresql.org/gitweb/?a=commitdiff_plain;h=5956795cb5befdc7b5dab3e1e781b74fbc3590e8;p=postgresql.git

Another fix to relmapper race condition.

In previous commit, I missed that relmap_redo() was also not acquiring the
RelationMappingLock. Thanks to Thomas Munro for pointing that out.

Backpatch-through: 9.6, like previous commit.
Discussion: https://www.postgresql.org/message-id/CA%2BhUKGLev%3DPpOSaL3WRZgOvgk217et%2BbxeJcRr4eR-NttP1F6Q%40mail.gmail.com
---

diff --git a/src/backend/utils/cache/relmapper.c b/src/backend/utils/cache/relmapper.c
index b150118b280..346112aba32 100644
--- a/src/backend/utils/cache/relmapper.c
+++ b/src/backend/utils/cache/relmapper.c
@@ -941,12 +941,13 @@ relmap_redo(XLogReaderState *record)
 		 * preserve files, either.
 		 *
 		 * There shouldn't be anyone else updating relmaps during WAL replay,
-		 * so we don't bother to take the RelationMappingLock.  We would need
-		 * to do so if load_relmap_file needed to interlock against writers.
+		 * but grab the lock to interlock against load_relmap_file().
 		 */
+		LWLockAcquire(RelationMappingLock, LW_EXCLUSIVE);
 		write_relmap_file((xlrec->dbid == InvalidOid), &newmap,
 						  false, true, false,
 						  xlrec->dbid, xlrec->tsid, dbpath);
+		LWLockRelease(RelationMappingLock);
 
 		pfree(dbpath);
 	}