Re: [PG19] Promotion hangs behind a backend blocked on a stuck client

On Wed, Oct 21, 2026 at 2:36 PM Hayato Kuroda (Fujitsu)
<kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
>
> ```
> The setting takes effect only for connections
> + established after it is configured,
> ```
>
> Hmm, is it correct?
> There is an assign hook which can update the socket option. My below experiment
> also indicated that TCP_USER_TIMEOUT can be enabled for established connections.
>
> 1. Connected via TCP connection
> $ psql -h 127.0.0.1 -U postgres
>
> 2. Set the tcp_user_timeout
>
> ```
> postgres=# SHOW tcp_user_timeout ;
> tcp_user_timeout
> ------------------
> 0
> (1 row)
>
> postgres=# SET tcp_user_timeout = '3s';
> SET
> ```
>
> 3. Sent a large amount of data to the client.
>
> ```
> postgres=# COPY (
> SELECT repeat('x', 1000000)
> FROM generate_series(1, 100000)
> ) TO STDOUT;
> ```
>
> 4. Sent a STOP signal to psql process, from another terminal
> ```
> $ kill -STOP 40049
> ```
>
> 5. After sometime the backend could be terminated due to the tcp timeout.
> According to the log:
>
> ```
> LOG: could not send data to client: Connection timed out
> STATEMENT: COPY (
> SELECT repeat('x', 1000000)
> FROM generate_series(1, 100000)
> ) TO STDOUT;
> ```
>
> Note: the backend could retain if step2 was skipped.
>

Thanks for the test but how can the user know when to set it, if not
before connection? Anyway, how about something generic as follows:
- To bound such waits in advance, set
- <varname>tcp_user_timeout</varname> on the standby, where supported by
- the operating system. The setting takes effect only for connections
- established after it is configured, and does not affect a connection
- whose backend is already blocked.
+ To bound such waits, configure <varname>tcp_user_timeout</varname> on
+ the standby ahead of time, for example in
+ <filename>postgresql.conf</filename>, where supported by the operating
+ system. Setting it after a backend is already blocked sending data has
+ no effect on that backend.

--
With Regards,
Amit Kapila.