Re: A new function to wait for the backend exit after termination

On Thu, Mar 18, 2021 at 12:46 PM Fujii Masao
<masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
> On 2021/03/17 11:58, Kyotaro Horiguchi wrote:
> > The first suggested signature for pg_terminate_backend() with timeout
> > was pg_terminate_backend(pid, timeout). The current signature (pid,
> > wait?, timeout) looks redundant. Maybe the reason for rejecting 0
> > astimeout is pg_terminate_backend(pid, true, 0) looks odd but it we
> > can wait forever in that case (as other features does).
>
> I'm afraid that "waiting forever" can cause something like deadlock situation,
> as follows. We have no mechanism to detect this for now.
>
> 1. backend 1 took the lock on the relation A.
> 2. backend 2 took the lock on the relation B.
> 3. backend 1 tries to take the lock on the relation B and is waiting for
> the lock to be released.
> 4. backend 2 accidentally executes pg_wait_for_backend_termination() with
> the pid of backend 1, and then is waiting for backend 1 to be terminated.

Yeah this can happen.

So, as stated upthread, how about a timeout 0 (which is default)
telling "don't wait", erroring out on negative value and when
specified a positive milliseconds value, then wait for that amount of
time. With this semantics, we can remove the wait flag for
pg_terminate_backend(pid, 0). Thoughts?

And for pg_wait_for_backend_termination timeout 0 or negative, we
error out. Thoughts?

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com