pgsql: Fix duplicate arbiter detection during REINDEX CONCURRENTLY on p

Fix duplicate arbiter detection during REINDEX CONCURRENTLY on partitions

Commit 90eae926a fixed ON CONFLICT handling during REINDEX CONCURRENTLY
on partitioned tables by treating unparented indexes as potential
arbiters.  However, there's a remaining race condition: when pg_inherits
records are swapped between consecutive calls to get_partition_ancestors(),
two different child indexes can appear to have the same parent, causing
duplicate entries in the arbiter list and triggering "invalid arbiter
index list" errors.

Note that this is not a new problem introduced by 90eae926a.  The same
error could occur before that commit in a slightly different scenario:
an index is selected during planning, then index_concurrently_swap()
commits, and a subsequent call to get_partition_ancestors() uses a new
catalog snapshot that sees zero ancestors for that index.

Fix by tracking which parent indexes have already been processed.  If a
subsequent call to get_partition_ancestors() returns a parent we've
already seen, treat that index as unparented instead, allowing it to be
matched via IsIndexCompatibleAsArbiter() like other concurrent reindex
scenarios.

Author: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/e5a8c1df-04e5-4343-85ef-5df2a7e3d90c@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e6d6e32f4240bb967460aabdd2db51181cff242f

Modified Files
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src/backend/executor/execPartition.c               | 35 +++++++++++++----
.../test_misc/t/010_index_concurrently_upsert.pl   | 45 ++++++++++++++++++++++
2 files changed, 72 insertions(+), 8 deletions(-)