pgsql: Fix waits of REINDEX CONCURRENTLY for indexes with predicates or

Fix waits of REINDEX CONCURRENTLY for indexes with predicates or expressions

As introduced by f9900df5f94, a REINDEX CONCURRENTLY job done for an
index with predicates or expressions would set PROC_IN_SAFE_IC in its
MyProc->statusFlags, causing it to be ignored by other concurrent
operations.

Such concurrent index rebuilds should never be ignored, as a predicate
or an expression could call a user-defined function that accesses a
different table than the table where the index is rebuilt.

A test that uses injection points is added, backpatched down to 17.
Michail has proposed a different test, but I have added something
simpler with more coverage.

Oversight in f9900df5f949.

Author: Michail Nikolaev
Discussion: https://postgr.es/m/CANtu0oj9A3kZVduFTG0vrmGnKB+DCHgEpzOp0qAyOgmks84j0w@mail.gmail.com
Backpatch-through: 14

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/edb0f6e41b09d261326c5340acad3a2cea59d718

Modified Files
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src/backend/commands/indexcmds.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)