Re: dblink query interruptibility

On Thu, Jan 25, 2024 at 5:45 AM Noah Misch <noah(at)leadboat(dot)com> wrote:
>
> On Thu, Jan 25, 2024 at 04:23:39AM +0900, Fujii Masao wrote:
> > I found that this patch was committed at d3c5f37dd5 and changed the
> > error message in postgres_fdw slightly. Here's an example:
> >
> > #1. Begin a new transaction.
> > #2. Execute a query accessing to a foreign table, like SELECT * FROM
> > <foreign table>
> > #3. Terminate the *remote* session corresponding to the foreign table.
> > #4. Commit the transaction, and then currently the following error
> > message is output.
> >
> > ERROR: 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.
> > invalid socket
> >
> > Previously, before commit d3c5f37dd5, the error message at #4 did not
> > include "invalid socket." Now, after the commit, it does. Is this
> > change intentional?
>
> No. It's a consequence of an intentional change in libpq call sequence, but I
> was unaware I was changing an error message.
>
> > + /* Consume whatever data is available from the socket */
> > + if (PQconsumeInput(conn) == 0)
> > + {
> > + /* trouble; expect PQgetResult() to return NULL */
> > + break;
> > + }
> > + }
> > +
> > + /* Now we can collect and return the next PGresult */
> > + return PQgetResult(conn);
> >
> > This code appears to cause the change. When the remote session ends,
> > PQconsumeInput() returns 0 and marks conn->socket as invalid.
> > Subsequent PQgetResult() calls pqWait(), detecting the invalid socket
> > and appending "invalid socket" to the error message.
> >
> > I think the "invalid socket" message is unsuitable in this scenario,
> > and PQgetResult() should not be called after PQconsumeInput() returns
> > 0. Thought?
>
> The documentation is absolute about the necessity of PQgetResult():

The documentation looks unclear to me regarding what should be done
when PQconsumeInput() returns 0. So I'm not sure if PQgetResult()
must be called even in that case.

As far as I read some functions like libpqrcv_PQgetResult() that use
PQconsumeInput(), it appears that they basically report the error message
using PQerrorMessage(), without calling PQgetResult(),
when PQconsumeInput() returns 0.

Regards,

--
Fujii Masao