Re: Cannot shutdown subscriber after DROP SUBSCRIPTION

On Sat, Feb 4, 2017 at 12:49 AM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> On Fri, Feb 3, 2017 at 10:56 AM, Kyotaro HORIGUCHI
> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> At Fri, 3 Feb 2017 01:02:47 +0900, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote in <CAHGQGwHqQVHmQ7wM=eLNnp1_oy-GVSSAcaJXWjE4nc2twSqXRg(at)mail(dot)gmail(dot)com>
>>> On Thu, Feb 2, 2017 at 2:36 PM, Michael Paquier
>>> <michael(dot)paquier(at)gmail(dot)com> wrote:
>>> > On Thu, Feb 2, 2017 at 2:13 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> >> Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> writes:
>>> >>> Then, the reason for the TRY-CATCH cluase is that I found that
>>> >>> some functions called from there can throw exceptions.
>>> >>
>>> >> Yes, but all LWLocks should be released by normal error recovery.
>>> >> It should not be necessary for this code to clean that up by hand.
>>> >> If it were necessary, there would be TRY-CATCH around every single
>>> >> LWLockAcquire in the backend, and we'd have an unreadable and
>>> >> unmaintainable system. Please don't add a TRY-CATCH unless it's
>>> >> *necessary* -- and you haven't explained why this one is.
>>>
>>> Yes.
>>
>> Thank you for the suggestion. I minunderstood that.
>>
>>> > Putting hands into the code and at the problem, I can see that
>>> > dropping a subscription on a node makes it unresponsive in case of a
>>> > stop. And that's just because calls to LWLockRelease are missing as in
>>> > the patch attached. A try/catch problem should not be necessary.
>>>
>>> Thanks for the patch!
>>>
>>> With the patch, LogicalRepLauncherLock is released at the end of
>>> DropSubscription(). But ISTM that the lock should be released just after
>>> logicalrep_worker_stop() and there is no need to protect the removal of
>>> replication slot with the lock.
>>
>> That's true. logicalrep_worker_stop returns after confirmig that
>> worker->proc is cleard, so no false relaunch cannot be caused.
>> After all, logicalrep_worker_stop is surrounded by
>> LWLockAcquire/Relase pair. So it can be moved into the funciton
>> and make the lock secrion to be more narrower.

If we do this, Assert(LWLockHeldByMe(LogicalRepLauncherLock)) should be
removed and the comment for logicalrep_worker_stop() should be updated.

Your approach may cause the deadlock. The launcher takes LogicalRepWorkerLock
while holding LogicalRepLauncherLock. OTOH, with your approach,
logicalrep_worker_stop() takes LogicalRepLauncherLock while holding
LogicalRepWorkerLock.

Therefore I pushed the simple patch which adds LWLockRelease() just after
logicalrep_worker_stop().

Another problem that I found while reading the code is that the launcher can
start up the worker with the subscription that DROP SUBSCRIPTION just removed.
That is, DROP SUBSCRIPTION removes the target entry from pg_subscription,
but the launcher can see it and start new worker until the transaction for
DROP has been committed.

To fix this issue, I think that DROP SUBSCRIPTION should take
AccessExclusiveLock on pg_subscription, instead of RowExclusiveLock,
so that the launcher cannot see the entry to be being removed.

Regards,

--
Fujii Masao