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 0b941ae12ca441b5e75a3b6fb9a8561f21f8923c..edc4a82b02f718d97b644adc52e6411dbabe80ea 100644 (file)
--- a/src/backend/access/nbtree/nbtsort.c
+++ b/src/backend/access/nbtree/nbtsort.c
@@ -1353,14 +1353,6 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
    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;
 
    /*
@@ -1404,6 +1396,16 @@ _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 */