While testing this feature I found a crash on PG head with parallel create index using pgbanch tables.
-- GUCs under postgres.conf
max_parallel_maintenance_workers = 16
max_parallel_workers = 16
max_parallel_workers_per_gather = 8
maintenance_work_mem = 8GB
max_wal_size = 4GB
./pgbench -i -s 500 -d postgres
postgres=# create index pgb_acc_idx3 on pgbench_accounts(aid, abalance,filler);
WARNING: terminating connection because of crash of another server process
DETAIL: The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
HINT: In a moment you should be able to reconnect to the database and repeat your command.
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!>
That makes it look like perhaps one of the worker backends crashed. Did you get a message in the logfile that might indicate the nature of the crash? Something with PANIC or TRAP, perhaps?
