pgsql: Update FSM during prune/freeze replay even if freespace is zero

Update FSM during prune/freeze replay even if freespace is zero

add323da40a started updating the visibility map in the same WAL record
as pruning and freezing. This included updating the freespace map during
replay of a record setting the VM, which we've done since ab7dbd681.

add323da40a, however, conditioned doing so on there being > 0 freespace
on the page, which differed from the previous state for records updating
the VM.

The FSM is not WAL-logged and is instead updated heuristically on
standbys. In rare cases, this heuristic could lead to pages with 0
freespace having outdated entries in the FSM. If the standby is later
promoted and vacuum skips these pages because they are marked
all-visible/all-frozen, overly optimistic values would be propagated up
the FSM tree, causing slowness when searching for freespace for new
tuples.

Fix it by always updating the FSM during replay when setting VM bits.

Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reported-by: Alexey Makhmutov <a(dot)makhmutov(at)postgrespro(dot)ru>
Discussion: https://postgr.es/m/ead2f110-c736-48f5-99e1-023dc9acbf0b%40postgrespro.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b4c1b2be300e619aebc3a2c00198009af256bfee

Modified Files
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src/backend/access/heap/heapam_xlog.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)