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

On Thu, Mar 18, 2021 at 1:11 PM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> 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?

Attaching v11 patch that removed the wait boolean flag in the
pg_terminate_backend and timeout 0 indicates "no wait", negative value
"errors out", positive value "waits for those many milliseconds". Also
addressed other review comments that I received upthread. Please
review v11 further.

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