Prevent parallel index build in a standalone backend.

This can't work if there's no postmaster, and indeed the code got an
assertion failure trying.  There should be a check on IsUnderPostmaster
gating the use of parallelism, as the planner has for ordinary
parallel queries.

Commit 40d964ec9 got this right, so follow its model of checking
IsUnderPostmaster at the same place where we check for
max_parallel_maintenance_workers == 0.  In general, new code
implementing parallel utility operations should do the same.

Report and patch by Yulin Pei, cosmetically adjusted by me.
Back-patch to v11 where this code came in.

Discussion: https://postgr.es/m/HK0PR01MB22747D839F77142D7E76A45DF4F50@HK0PR01MB2274.apcprd01.prod.exchangelabs.com

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 401299e542043e5b4c6b7721ec850da74f4f8021..10ecc2976e8d957e15ca41d0db914040e377ef9a 100644 (file)
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -6254,8 +6254,11 @@ plan_create_index_workers(Oid tableOid, Oid indexOid)
    double      reltuples;
    double      allvisfrac;
 
-   /* Return immediately when parallelism disabled */
-   if (max_parallel_maintenance_workers == 0)
+   /*
+    * We don't allow performing parallel operation in standalone backend or
+    * when parallelism is disabled.
+    */
+   if (!IsUnderPostmaster || max_parallel_maintenance_workers == 0)
        return 0;
 
    /* Set up largely-dummy planner state */