Re: Strange failure in LWLock on skink in REL9_5_STABLE

On 2018-09-20 22:59:29 -0400, Tom Lane wrote:
> Thomas Munro <thomas.munro@enterprisedb.com> writes:
> > Andres pinged me off-list to point out this failure after my commit fb389498be:
> 
> > ! FATAL:  semop(id=332464133) failed: Invalid argument
> 
> I was just looking at that, and my guess is that it was caused by
> something doing an ipcrm or equivalent, and is unrelated to your patch.
> Especially since skink has succeeded with that patch in several other
> branches.

I'm (hopefully) the only person with access to that machine, and I
certainly didn't do so. Nor are there script I know of that'd do
so. There's not been a lot of instability on skink, so it's certainly
quite weird.

I'm quite suspicious of the logic around:

		/*
		 * If we received a query cancel or termination signal, we will have
		 * EINTR set here.  If the caller said that errors are OK here, check
		 * for interrupts immediately.
		 */
		if (errno == EINTR && elevel >= ERROR)
			CHECK_FOR_INTERRUPTS();

because it seems far from guaranteed to do anything meaningful as I
don't see a guarantee that interrupts are active at that point (e.g. it
seems quite reasonable to hold an lwlock while resizing).

Afaict that might cause problems at a later stage, because at that point
we've not adjusted the actual mapping, but *have* ftruncate()ed it. If
there's actual data in the mapping, that certainly could cause trouble.

In fact, while this commit has expanded the size of the problem, I fail
to see how the error handling for resizing is correct. It's fine to fail
in the ftruncate() itself - at that point no changes have been made -,
but I don't think it's currently ok for posix_fallocate() to ever error
out.

It's not clear to me how that'd be problematic in 9.5 of all releases
however.

> If it's repeatable, then it would be time to get excited.

Yea, I guess we'll have to wait :/.

Greetings,

Andres Freund