Re: [PATCH] Reset single-row processing mode at end of pipeline commands queue

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: Denis Laxalde <denis(dot)laxalde(at)dalibo(dot)com>
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: [PATCH] Reset single-row processing mode at end of pipeline commands queue
Date: 2022-10-14 17:34:58
Message-ID: 20221014173458.iacteadybkaja5sy@alvherre.pgsql
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Hello Denis,

On 2022-Oct-07, Denis Laxalde wrote:

> I'm trying to make single-row mode and pipeline mode work together in
> Psycopg using libpq. I think there is something wrong with respect to the
> single-row mode flag, not being correctly reset, in some situations.
> The minimal case I'm considering is (in a pipeline):
> * send query 1,
> * get its results in single-row mode,
> * send query 2,
> * get its results *not* in single-row mode.
> It seems that, as the command queue in the pipeline is empty after getting
> the results of query 1, the single-row mode flag is not reset and is still
> active for query 2, thus leading to an unexpected PGRES_SINGLE_TUPLE status.
> The attached patch demonstrates this in the test suite. It also suggests to
> move the statement resetting single-row mode up in pqPipelineProcessQueue(),
> before exiting the function when the command queue is empty in particular.

Your suggestion to move the code up seems correct to me. Therefore, I
have pushed this including the added test code. Thanks for an excellent
report and patch.

Álvaro Herrera Breisgau, Deutschland —

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