[PATCH] BUG FIX: Core dump could happen when VACUUM FULL in standalone mode

Hi hackers,

I found that when running command vaccum full in stand-alone mode there will be a core dump.
The core stack looks like this:
------------------------------------
backend> vacuum full
TRAP: FailedAssertion("IsUnderPostmaster", File: "dsm.c", Line: 439)
./postgres(ExceptionalCondition+0xac)[0x55c664c36913]
./postgres(dsm_create+0x3c)[0x55c664a79fee]
./postgres(GetSessionDsmHandle+0xdc)[0x55c6645f8296]
./postgres(InitializeParallelDSM+0xf9)[0x55c6646c59ca]
./postgres(+0x16bdef)[0x55c664692def]
./postgres(+0x16951e)[0x55c66469051e]
./postgres(btbuild+0xcc)[0x55c6646903ec]
./postgres(index_build+0x322)[0x55c664719749]
./postgres(reindex_index+0x2ee)[0x55c66471a765]
./postgres(reindex_relation+0x1e5)[0x55c66471acca]
./postgres(finish_heap_swap+0x118)[0x55c6647c8db1]
./postgres(+0x2a0848)[0x55c6647c7848]
./postgres(cluster_rel+0x34e)[0x55c6647c727f]
./postgres(+0x3414cf)[0x55c6648684cf]
./postgres(vacuum+0x4f3)[0x55c664866591]
./postgres(ExecVacuum+0x736)[0x55c66486609b]
./postgres(standard_ProcessUtility+0x840)[0x55c664ab74fc]
./postgres(ProcessUtility+0x131)[0x55c664ab6cb5]
./postgres(+0x58ea69)[0x55c664ab5a69]
./postgres(+0x58ec95)[0x55c664ab5c95]
./postgres(PortalRun+0x307)[0x55c664ab5184]
./postgres(+0x587ef6)[0x55c664aaeef6]
./postgres(PostgresMain+0x819)[0x55c664ab3271]
./postgres(main+0x2e1)[0x55c6648f9df5]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1)[0x7f65376192e1]
./postgres(_start+0x2a)[0x55c6645df86a]
Aborted (core dumped)
------------------------------------

I think that when there is a btree index in a table, vacuum full tries to rebuild the btree index in a parallel way.

This will launch several workers and each of then will try to apply a dynamic shared memory segment, which is not allowed in stand-alone mode.

I think it is better not to use btree index build in parallel in stand-alone mode. My patch is attached below.

Best Regards!
Yulin PEI