Re: libpq async duplicate error results

Tom, you worked on reorganzing the error message handling in libpq in
PostgreSQL 14 (commit ffa2e4670123124b92f037d335a1e844c3782d3f). Any
thoughts on this?

On 25.01.22 09:32, Peter Eisentraut wrote:
>
> This issue was discovered by Fabien in the SHOW_ALL_RESULTS thread.  I'm
> posting it here separately, because I think it ought to be addressed in
> libpq rather than with a workaround in psql, as proposed over there.
>
> When using PQsendQuery() + PQgetResult() and the server crashes during
> the execution of the command, PQgetResult() then returns two result sets
> with partially duplicated error messages, like this from the attached
> test program:
>
> command = SELECT 'before';
> result 1 status = PGRES_TUPLES_OK
> error message = ""
>
> command = SELECT pg_terminate_backend(pg_backend_pid());
> result 1 status = PGRES_FATAL_ERROR
> error message = "FATAL:  terminating connection due to administrator
> command
> "
> result 2 status = PGRES_FATAL_ERROR
> error message = "FATAL:  terminating connection due to administrator
> command
> server closed the connection unexpectedly
>     This probably means the server terminated abnormally
>     before or while processing the request.
> "
>
> command = SELECT 'after';
> PQsendQuery() error: no connection to the server
>
>
> This is hidden in normal use because PQexec() throws away all but the
> last result set, but the extra one is still generated internally.
>
> Apparently, this has changed between PG13 and PG14.  In PG13 and
> earlier, the output is
>
> command = SELECT 'before';
> result 1 status = PGRES_TUPLES_OK
> error message = ""
>
> command = SELECT pg_terminate_backend(pg_backend_pid());
> result 1 status = PGRES_FATAL_ERROR
> error message = "FATAL:  terminating connection due to administrator
> command
> "
> result 2 status = PGRES_FATAL_ERROR
> error message = "server closed the connection unexpectedly
>     This probably means the server terminated abnormally
>     before or while processing the request.
> "
>
> command = SELECT 'after';
> PQsendQuery() error: no connection to the server
>
> In PG13, PQexec() concatenates all the error messages from multiple
> results, so a user of PQexec() sees the same output before and after.
> But for users of the lower-level APIs, things have become a bit more
> confusing.
>
> Also, why are there multiple results being generated in the first place?
>
>
> [0]:
> https://www.postgresql.org/message-id/alpine.DEB.2.22.394.2112230703530.2668598@pseudo