Re: BUG #12918: Segfault in BackendIdGetTransactionIds

root(at)simply(dot)name writes:
> After upgrading from 9.3.6 to 9.4.1 (both installed from packages on
> yum.postgresql.org) we have started getting segfaults of different backends.
> Backtraces of all coredumps look similar:
> (gdb) bt
> #0 0x000000000066bf9b in BackendIdGetTransactionIds (backendID=<value
> optimized out>, xid=0x7f2a1b714798, xmin=0x7f2a1b71479c) at sinvaladt.c:426
> #1 0x00000000006287f4 in pgstat_read_current_status () at pgstat.c:2871
> #2 0x0000000000628879 in pgstat_fetch_stat_numbackends () at pgstat.c:2342

Hmm ... looks to me like BackendIdGetTransactionIds is simply busted.
It supposes that there are no inactive entries in the sinval array
within the range 0 .. lastBackend. But there can be, in which case
dereferencing stateP->proc crashes. The reason it's hard to reproduce
is the relatively narrow window between where pgstat_read_current_status
saw the backend as active and where we're inspecting its sinval entry.

regards, tom lane