pgsql: psql: Fix issues with deferred errors in pipelines

psql: Fix issues with deferred errors in pipelines

When an error is raised while processing a Sync message in a pipeline,
like a deferred constraint violation, the error was not associated with
the piped command and was not counted in available_results. This caused
assertion failures in discardAbortedPipelineResults(), keeping an
incorrect state at pipeline exit, because the code assumed that the
number of available and requested results would always be positive,
expecting all the counters to be 0 at the end of a pipeline.

This commit switches discardAbortedPipelineResults() and
ExecQueryAndProcessResults() to take a softer approach when consuming
and draining the results after an error. If there are still piped syncs
in the pipeline when it ends, we now attempt to consume them before
leaving the pipeline mode.

Alexander has been able to reach two assertion failures through his
testing. While investigating more this issue, I have bumped into two
more. Most of these cases are covered by the regression tests added in
this commit, plus some cases with mixes of pipelines, deferred errors
and results fetched. Some of the tests discussed (like the backend
termination one) could not be included in this commit but have been
tested manually. Another test scenario discussed involved the injection
of an error state in the backend, that was able to trick libpq
internally and put its queue out of sync. This scenario is not going to
happen in practice, but if we were to do something about it we would
need to make libpq understand that it needs to fail in some cases but
not block.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/19494-97a86d84fee71c47@postgresql.org
Backpatch-through: 18

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d21604e17e491eef39626a2eff518b9731fefb91

Modified Files
--------------
src/bin/psql/common.c | 76 +++++++++++++----
src/test/regress/expected/psql_pipeline.out | 124 ++++++++++++++++++++++++++++
src/test/regress/sql/psql_pipeline.sql | 63 ++++++++++++++
3 files changed, 245 insertions(+), 18 deletions(-)