On 2017-09-19 13:15:28 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > On 2017-09-19 13:00:33 -0400, Robert Haas wrote:
> >> You mean, in the postmaster?
>
> > Yes. We try to avoid touch shmem there, but it's not like we're
> > succeeding fully. See e.g. the pgstat_get_crashed_backend_activity()
> > calls (which do rely on shmem being ok to some extent), pmsignal,
> > BackgroundWorkerStateChange(), ...
>
> Well, the point is to avoid touching data structures that could be
> corrupted enough to confuse the postmaster. I don't have any problem with
> adding some more functionality to pmsignal, say.
Given that we're ok with reading pgstat shared memory entries, I think
adding a carefully coded variant of SendProcSignal() should be doable in
a safe manner.
Something roughly like
int
PostmasterSendProcSignal(pid_t pid, ProcSignalReason reason)
{ volatile ProcSignalSlot *slot;
/* * As this is running in postmaster, be careful not to dereference * any pointers from shared memory that
couldbe corrupted, and to * not to throw errors. */
for (i = 0; i < NumProcSignalSlots; i++) { slot = &ProcSignalSlots[i];
if (slot->pss_pid == pid) { /* * The note about race conditions in SendProcSignal
applies * here, too */
/* Atomically set the proper flag */ slot->pss_signalFlags[reason] = true; /* Send
signal*/ return kill(pid, SIGUSR1); } }
errno = ESRCH; return -1;
}
As all the memory offsets are computed based on postmaster process-local
variables, this should be safe.
I'd rather like to avoid a copy of the procsignal infrastructure if we
don't need it...
Greetings,
Andres Freund
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