From: | Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com> |
---|---|
To: | "Higuchi, Daisuke" <higuchi(dot)daisuke(at)jp(dot)fujitsu(dot)com> |
Cc: | "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: [Bug fix] PQsendQuery occurs error when target_session_attrs is set to read-write |
Date: | 2017-02-01 10:28:18 |
Message-ID: | CAFjFpReS0OHrFVh2xRJ2rHd8S8+vyzJixjipwxKPt1sDhLpYBQ@mail.gmail.com |
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On Tue, Jan 31, 2017 at 9:53 AM, Higuchi, Daisuke
<higuchi(dot)daisuke(at)jp(dot)fujitsu(dot)com> wrote:
> Hello,
>
> This this is my first posting to the mailing list.
>
> I am interested in multiple hosts of libpq [1], then I found the bug in this feature.
> When I set "target_session_attrs" to "any" and call PQsendQuery, my application is succeeded.
> However, when I set "target_session_attrs" to "read-write" and call PQsendQuery, "another command is already in progress" is occurred.
> I attached the test application to reproduce this problem.
>
> I think this is because PQgetResult is not called until PQgetResult has returned a null pointer.
> So, I attached the patch for fix this.
>
> [1] https://www.postgresql.org/message-id/flat/20150818041850(dot)GA5092(at)wagner(dot)pp(dot)ru#20150818041850(dot)GA5092@wagner.pp.ru
>
Interestingly, when I don't set PGPORT, PGHOST I get the same error
with the C program. The patch fixes the problem.
Per the documentation [1], "PQgetResult must be called repeatedly
until it returns a null pointer, indicating that the command is
done.". The code in PQgetResult() under CONNECTION_CHECK_WRITABLE
case, violates that. The patch fixes it. The patch however jumps to
keep_going without changing conn->status, which means that it will end
up again in the same case. While that's fine, may be we should use a
local loop on PQgetResult() to keep the code readable.
I would have added a test with the patch, but it seems we don't have
much tests in interfaces/libpq. The bug is pretty trivial and would
have been caught easily had we had any tests.
[1] https://www.postgresql.org/docs/devel/static/libpq-async.html
--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company
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