pgsql: Add strict_multi_assignment and too_many_rows plpgsql checks

Add strict_multi_assignment and too_many_rows plpgsql checks

Until now shadowed_variables was the only plpgsql check supported by
plpgsql.extra_warnings and plpgsql.extra_errors. This patch introduces
two new checks - strict_multi_assignment and too_many_rows. Unlike
shadowed_variables, these new checks are enforced at run-time.

strict_multi_assignment checks that commands allowing multi-assignment
(for example SELECT INTO) have the same number of sources and targets.
too_many_rows checks that queries with an INTO clause return one row
exactly.

These checks are aimed at cases that are technically valid and allowed,
but are often a sign of a bug. Therefore those checks are expected to
be enabled primarily in development and testing environments.

Author: Pavel Stehule
Reviewed-by: Stephen Frost, Tomas Vondra
Discussion: https://www.postgresql.org/message-id/flat/CAFj8pRA2kKRDKpUNwLY0GeG1OqOp+tLS2yQA1V41gzuSz-hCng(at)mail(dot)gmail(dot)com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/167075be3ab1547e186096bb8e6e448cd8eea5af

Modified Files
--------------
doc/src/sgml/plpgsql.sgml | 115 +++++++++++++++++++++++++++-------
src/pl/plpgsql/src/pl_exec.c | 108 ++++++++++++++++++++++++++++---
src/pl/plpgsql/src/pl_handler.c | 4 ++
src/pl/plpgsql/src/plpgsql.h | 10 +--
src/test/regress/expected/plpgsql.out | 107 +++++++++++++++++++++++++++++++
src/test/regress/sql/plpgsql.sql | 89 ++++++++++++++++++++++++++
6 files changed, 400 insertions(+), 33 deletions(-)