Re: BUG #15036: Un-killable queries Hanging in BgWorkerShutdown

Sorry for the delay on this.

>
> You can get a backtrace from a running program with by connecting to
> it with gdb -p PID, then bt for the backtrace. You might need to
> install the symbols package if you only see addresses (on debianoid
> systems postgresql-10-dbgsym, not sure what it's called on RHELish
> systems).
>
>
> https://wiki.postgresql.org/wiki/Getting_a_stack_trace_of_a_running_PostgreSQL_backend_on_Linux/BSD
>
> here's a backtrace from one of the running pids:
```
#0 0x00007f699037c9f3 in __epoll_wait_nocancel () at
../sysdeps/unix/syscall-template.S:84
#1 0x00005639b9d28c61 in WaitEventSetWaitBlock (nevents=1,
occurred_events=0x7ffd43678d90, cur_timeout=-1, set=0x5639bbef83c8) at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/storage/ipc/latch.c:1048
#2 WaitEventSetWait (set=set(at)entry=0x5639bbef83c8, timeout=timeout(at)entry=-1,
occurred_events=occurred_events(at)entry=0x7ffd43678d90, nevents=nevents(at)entry=1,
wait_event_info=wait_event_info(at)entry=134217728)
at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/storage/ipc/latch.c:1000
#3 0x00005639b9d290d4 in WaitLatchOrSocket (latch=0x7f697e506f54,
wakeEvents=wakeEvents(at)entry=17, sock=sock(at)entry=-1, timeout=-1,
timeout(at)entry=0, wait_event_info=wait_event_info(at)entry=134217728)
at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/storage/ipc/latch.c:385
#4 0x00005639b9d29185 in WaitLatch (latch=<optimized out>,
wakeEvents=wakeEvents(at)entry=17, timeout=timeout(at)entry=0,
wait_event_info=wait_event_info(at)entry=134217728)
at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/storage/ipc/latch.c:339
#5 0x00005639b9cccf1b in WaitForBackgroundWorkerShutdown
(handle=0x5639bbe8aaf0) at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/postmaster/bgworker.c:1154
#6 0x00005639b9af36fd in WaitForParallelWorkersToExit
(pcxt=0x5639bbe8a118, pcxt=0x5639bbe8a118) at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/access/transam/parallel.c:655
#7 0x00005639b9af4417 in DestroyParallelContext (pcxt=0x5639bbe8a118) at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/access/transam/parallel.c:737
#8 0x00005639b9af4a28 in AtEOXact_Parallel (isCommit=isCommit(at)entry=0
'\000') at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/access/transam/parallel.c:1006
#9 0x00005639b9affde7 in AbortTransaction () at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/access/transam/xact.c:2538
#10 0x00005639b9b00545 in AbortCurrentTransaction () at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/access/transam/xact.c:3097
#11 0x00005639b9d4bd6d in PostgresMain (argc=1, argv=argv(at)entry=0x5639bbe9ae40,
dbname=0x5639bbe9ad58 "marjory", username=0x5639bbe39a08 "reporter") at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/tcop/postgres.c:3879
#12 0x00005639b9a850d9 in BackendRun (port=0x5639bbe978f0) at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/postmaster/postmaster.c:4405
#13 BackendStartup (port=0x5639bbe978f0) at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/postmaster/postmaster.c:4077
#14 ServerLoop () at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/postmaster/postmaster.c:1755
#15 0x00005639b9cdb78b in PostmasterMain (argc=5, argv=<optimized out>) at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/postmaster/postmaster.c:1363
#16 0x00005639b9a864d5 in main (argc=5, argv=0x5639bbe37850) at
/build/postgresql-10-drhiey/postgresql-10-10.3/build/../src/backend/main/main.c:228
```
Joe, cc'd here is a colleague who will be able to help out in future.

Thanks for all the help on this,
David