pgsql: Fix TRUNCATE .. CASCADE on partitions

Fix TRUNCATE .. CASCADE on partitions

When running TRUNCATE CASCADE on a child of a partitioned table
referenced by another partitioned table, the truncate was not applied to
partitions of the referencing table; this could leave rows violating the
constraint in the referencing partitioned table.  Repair by walking the
pg_constraint chain all the way up to the topmost referencing table.

Note: any partitioned tables containing FKs that reference other
partitioned tables should be checked for possible violating rows, if
TRUNCATE has occurred in partitions of the referenced table.

Reported-by: Christophe Courtois
Author: Jehan-Guillaume de Rorthais
Discussion: https://postgr.es/m/20200204183906.115f693e@firost

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ce054a8cd4f4faf3479050feb5d8fa08545f4c5c

Modified Files
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doc/src/sgml/ref/truncate.sgml         |  3 ++
src/backend/catalog/heap.c             | 84 +++++++++++++++++++++++++++++++++-
src/test/regress/expected/truncate.out | 50 ++++++++++++++++++++
src/test/regress/sql/truncate.sql      | 38 +++++++++++++++
4 files changed, 173 insertions(+), 2 deletions(-)