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

Hello,

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.

Thanks for considering,
Denis

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  —  https://www.EnterpriseDB.com/