Handle lack of DSM slots in parallel btree build, take 2.

Commit 74618e77 added a new check intended to fix a bug, but put
it in the wrong place so that parallel btree build was always
disabled.  Do the check after we've actually tried to create
a DSM segment.  Back-patch to 11, like the earlier commit.

Reviewed-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-WzmDABkJzrNnvf%2BOULK-_A_j9gkYg_Dz-H62jzNv4eKQTw%40mail.gmail.com

diff --git a/src/backend/access/nbtree/nbtsort.c b/src/backend/access/nbtree/nbtsort.c
index bc00799b964b91437ef932bd3ddd6c9c781e9283..dab41ea298aa334463d5a64b7d8cdf46adef292d 100644 (file)
--- a/src/backend/access/nbtree/nbtsort.c
+++ b/src/backend/access/nbtree/nbtsort.c
@@ -1257,14 +1257,6 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
        pcxt = CreateParallelContext("postgres", "_bt_parallel_build_main",
                                                                 request, true);
 
-       /* If no DSM segment was available, back out (do serial build) */
-       if (pcxt->seg == NULL)
-       {
-               DestroyParallelContext(pcxt);
-               ExitParallelMode();
-               return;
-       }
-
        scantuplesortstates = leaderparticipates ? request + 1 : request;
 
        /*
@@ -1308,6 +1300,16 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
        /* Everyone's had a chance to ask for space, so now create the DSM */
        InitializeParallelDSM(pcxt);
 
+       /* If no DSM segment was available, back out (do serial build) */
+       if (pcxt->seg == NULL)
+       {
+               if (IsMVCCSnapshot(snapshot))
+                       UnregisterSnapshot(snapshot);
+               DestroyParallelContext(pcxt);
+               ExitParallelMode();
+               return;
+       }
+
        /* Store shared build state, for which we reserved space */
        btshared = (BTShared *) shm_toc_allocate(pcxt->toc, estbtshared);
        /* Initialize immutable state */