Re: [EXTERNAL] Re: Add non-blocking version of PQcancel

On 2024-Mar-13, Jelte Fennema-Nio wrote:

> I agree it's probably a timing issue. The cancel being received after
> the query is done seems very unlikely, since the query takes 180
> seconds (assuming PG_TEST_TIMEOUT_DEFAULT is not lowered for these
> animals). I think it's more likely that the cancel request arrives too
> early, and thus being ignored because no query is running yet. The
> test already had logic to wait until the query backend was in the
> "active" state, before sending a cancel to solve that issue. But my
> guess is that that somehow isn't enough.
>
> Sadly I'm having a hard time reliably reproducing this race condition
> locally. So it's hard to be sure what is happening here. Attached is a
> patch with a wild guess as to what the issue might be (i.e. seeing an
> outdated "active" state and thus passing the check even though the
> query is not running yet)

I tried leaving the original running in my laptop to see if I could
reproduce it, but got no hits ... and we didn't get any other failures
apart from the three ones already reported ... so it's not terribly high
probability. Anyway I pushed your patch now since the theory seems
plausible; let's see if we still get the issue to reproduce. If it
does, we could make the script more verbose to hunt for further clues.

--
Álvaro Herrera PostgreSQL Developer — https://www.EnterpriseDB.com/
"Here's a general engineering tip: if the non-fun part is too complex for you
to figure out, that might indicate the fun part is too ambitious." (John Naylor)
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