pgsql: Fix clobbering of proc entry's statusFlags during slot invalidat

Fix clobbering of proc entry's statusFlags during slot invalidation

ReplicationSlotRelease() copied MyProc->statusFlags unconditionally
when resetting PROC_IN_LOGICAL_DECODING, using its pgxactoff to decide
which area to update.

Slot invalidations can be done by the checkpointer or the startup
process, and ReplicationSlotRelease() ignored the fact that pgxactoff is
only meaningful for processes in the proc array, that auxiliary
processes never join. pgxactoff would remain at 0 for these, meaning
that an incorrect set of status flags would be updated.

The consequences of this issue could be various, depending on the
statsFlags lost in the proc slot incorrectly overwritten. Losing
PROC_AFFECTS_ALL_HORIZONS has been mentioned as the worse case, causing
VACUUM to remove rows a standby may needs even if standby feedback is
enabled. Assertions failures are another possibility.

Let's skip the update of statusFlags unless PROC_IN_LOGICAL_DECODING is
set. Nothing but StartupDecodingContext() sets it, and no auxiliary
processes call it.

Author: Vlad Lesin <vladlesin(at)gmail(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/889a06dd-45fc-423b-9dd2-87d5b5dc60c6@gmail.com
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/56b306f8b2edb2a7ec466a42fdc5213f908fb6a5

Modified Files
--------------
src/backend/replication/slot.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)