Re: [HACKERS] [Bug fix] PQsendQuery occurs error whentarget_session_attrs is set to read-write

On Tue, Jan 31, 2017 at 9:53 AM, Higuchi, Daisuke
<higuchi.daisuke@jp.fujitsu.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.GA5092@wagner.pp.ru#20150818041850.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