Re: dropdb --force

On Thu, Nov 7, 2019 at 10:46 AM Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>
> čt 7. 11. 2019 v 3:42 odesílatel Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> napsal:
>>
>> On Wed, Nov 6, 2019 at 11:46 PM Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>> >
>> > st 6. 11. 2019 v 14:59 odesílatel Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> napsal:
>> >>
>> >> Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:
>> >> > I think there is still a window where the same problem can happen, say
>> >> > the signal has been sent by SendProcSignal to the required process and
>> >> > it releases the ProcArrayLock. Now, the target process exits and a
>> >> > new process gets the same pid before the signal is received.
>> >>
>> >> In principle, no use of Unix signals is ever safe against this sort
>> >> of race condition --- process A can never know that process B didn't
>> >> exit immediately before A does kill(B, n). In practice, it's okay
>> >> because the kernel is expected not to reassign a dead PID for some
>> >> reasonable grace period [1]. I'd be inclined to lean more heavily
>> >> on that expectation than anything internal to Postgres. That is,
>> >> remembering the PID we want to kill for some small number of
>> >> microseconds is probably a safer API than anything that depends on
>> >> the contents of the ProcArray, because there indeed *isn't* any
>> >> guarantee that a ProcArray entry won't be recycled immediately.
>> >>
>>
>> Right, this makes sense. I think I was overly paranoid about this
>> behavior even though that was used at a few other places as this patch
>> might need to rely on many pids not being reused after the lock is
>> released.
>>
>> >
>> >
>> > so we can return back to just simple killing.
>> >
>>
>> I think so. I think we might want to add a comment about this race
>> condition and add or reference to comments in pg_signal_backend which
>> mentions the same race condition.
>
>
> Please, can you do it. It's bad task for my with my bad English.
>

Okay, no problem. I will pick the previous version and do this. I
will post the patch in a day or so for your review.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com