Handle lack of DSM slots in parallel btree build.

macdice · macdice · commit 74618e77b43c · 2020-01-31T10:25:34.000+13:00
If no DSM slots are available, a ParallelContext can still be created, but its seg pointer is NULL. Teach parallel btree build to cope with that by falling back to a regular non-parallel build, to avoid crashing with a segmentation fault. Back-patch to 11, where parallel CREATE INDEX landed. Reported-by: Nicola Contu Reviewed-by: Peter Geoghegan Discussion: https://postgr.es/m/CA%2BhUKGJgJEBnkuODBVomyK3MWFvDBbMVj%3Dgdt6DnRPU-5sQ6UQ%40mail.gmail.com
diff --git a/src/backend/access/nbtree/nbtsort.c b/src/backend/access/nbtree/nbtsort.c
@@ -1331,6 +1331,15 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	Assert(request > 0);
 	pcxt = CreateParallelContext("postgres", "_bt_parallel_build_main",
 								 request);
+
+	/* If no DSM segment was available, back out (do serial build) */
+	if (pcxt->seg == NULL)
+	{
+		DestroyParallelContext(pcxt);
+		ExitParallelMode();
+		return;
+	}
+
 	scantuplesortstates = leaderparticipates ? request + 1 : request;
 
 	/*
