Re: Parameter status message not sent?

On 2018-02-14 11:12:30 +0900, Tatsuo Ishii wrote:
> According to the manual, backend sends a parameter status message when
> certain configuration variable has been changed and SIGHUP signal is sent.
> https://www.postgresql.org/docs/10/static/protocol-flow.html#PROTOCOL-ASYNC
> 
>   ParameterStatus messages will be generated whenever the active value
>   changes for any of the parameters the backend believes the frontend
>   should know about. Most commonly this occurs in response to a SET
>   SQL command executed by the frontend, and this case is effectively
>   synchronous ― but it is also possible for parameter status changes
>   to occur because the administrator changed a configuration file and
>   then sent the SIGHUP signal to the server.
> 
> So I connected to PostgreSQL using psql and attached strace to psql.
> Then I changed standard_conforming_strings and executed pg_ctl
> reload. The PostgreSQL log shows:
> 
> 12073 2018-02-14 11:05:22 JST LOG:  received SIGHUP, reloading configuration files
> 12073 2018-02-14 11:05:22 JST LOG:  parameter "standard_conforming_strings" changed to "off"
> 12073 2018-02-14 11:05:22 JST DEBUG:  sending signal 1 to process 12158
> 
> But as far as strace tells, nothing was sent to psql. Is this expected?

It'll only get sent to the client the next time the server processes a
query. We can't just at arbitrary points reload the config file or send
messages out. The SIGHUP handler just sets ConfigReloadPending which
PostgresMain() then processes:

        /*
         * (6) check for any other interesting events that happened while we
         * slept.
         */
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
        }

which'll then, in turn, send out ParameterStatus messages for changed
GUC_REPORT GUCs.

Greetings,

Andres Freund