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 247f7d462525325998a73e6681bbb912388e9907..1a94b58f8beee3f70f63ac448dcd6e4c615028ff 100644 (file)
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -6375,8 +6375,11 @@ 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 */