pgsql: Fix crash on trying to expand a shared memory hash table

Fix crash on trying to expand a shared memory hash table

When a hash table grows large enough we try to expand the directory,
but we shouldn't try to do that if the hash table is marked as
HASH_FIXED_SIZE. Before commit 9fe9ecd516b that was harmless,
although it was a little weird to expand the directory when allocating
the new element was doomed to fail later anyway. But the new
allocator function added in that commit for shared memory hash tables
did not expect to be called after hash table creation at all and would
just crash. Fix by ensuring that the allocator is not called after
hash table creation for HASH_FIXED_SIZE tables.

The crash started with commit 9fe9ecd516b in v19, so backpatch to v19.
To trigger the crash, you needed a shared memory hash table of just
the right size so that we would attempt to resize it: power-of-2 and
at least HASH_SEGSIZE (256) elements. With any other size, you would
run out of allocated elements first.

Author: Konstantin Knizhnik <knizhnik(at)garret(dot)ru>
Reviewed-by: Rahila Syed <rahilasyed90(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/d59221f2-b3d2-41ad-8bf0-d581b42e4cba@garret.ru
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/31ad154242e2dc9fa9bbc4ebbe31f559e2282d9c

Modified Files
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src/backend/utils/hash/dynahash.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)