Re: PQConsumeinput stuck on recv

Hi,

On 2018-02-23 13:33:18 -0300, Andre Oliveira Freitas wrote:
> Since it's been happening for a few weeks now, every time it freezes we
> take a gcore dump and check it in gdb... and after a lot of hair pulling
> and learning about the innards of the VoIP software we see that most often
> the software is stuck in this call trace:
>
> #0 in __libc_recv (fd=409, buf=0x7f2c4802e6c0, n=16384, flags=1898970523)
> at ../sysdeps/unix/sysv/linux/x86_64/recv.c:33
> #1 in ?? () from /usr/lib/x86_64-linux-gnu/libpq.so.5
> #2 in ?? () from /usr/lib/x86_64-linux-gnu/libpq.so.5
> #3 in PQconsumeInput () from /usr/lib/x86_64-linux-gnu/libpq.so.5

So it's just receiving data from the network. Have you verified whether
the connection is actually stable? Any chance it's just waiting for the
network to time out. Might be worth configuring tcp timeouts, to make
sure its unrelated to that.

What is the server showing as activity while the client is waiting?
Could you show the corresponding pg_stat_activity row?

> The software shares a database connection between threads, and controls its
> access through a mutex, so once one thread that acquires the mutex gets
> stuck in the location above, all other threads starts pilling up behind the
> mutex, and that's apparently the reason the software stops responding for
> most of its functions (while other functions that do not depend on the
> database works normally).

Hm, have you compiled libpq with threading support? Or use a
distribution that compiles it with that? While I don't see an obvious
connection to that stacktrace it seems worthwhile to verify.

A mutex protecting individual connection, while also necessary if
connections are shared, doesn't achieve the same.

> I wonder if anyone has any tip on what to look for next...

Any chance you're occasionally forking and then interacting with the
connection in the forked process?

Greetings,

Andres Freund