Re: Race conditions in 019_replslot_limit.pl

Hi,

On 2022-02-17 21:55:21 -0800, Andres Freund wrote:
> Isn't it pretty bonkers that we allow error processing to get stuck behind
> network traffic, *before* we have have released resources (locks etc)?

This is particularly likely to be a problem for walsenders, because they often
have a large output buffer filled, because walsender uses
pq_putmessage_noblock() to send WAL data. Which obviously can be large.

In the stacktrace upthread you can see:
#3 0x00007faf4b70f48b in secure_write (port=0x7faf4c22da50, ptr=0x7faf4c2f1210, len=21470) at /home/andres/src/postgresql/src/backend/libpq/be-secure.c:29

which certainly is more than in most other cases of error messages being
sent. And it obviously might not be the first to have gone out.

> I wonder if we should try to send, but do it in a nonblocking way.

I think we should probably do so at least during FATAL error processing. But
also consider doing so for ERROR, because not releasing resources after
getting cancelled / terminated is pretty nasty imo.

Greetings,

Andres Freund