pgsql: Fix possible index corruption in pg_trgm GiST indexes

Fix possible index corruption in pg_trgm GiST indexes

Commit 911e7020770, which added configurable signature lengths,
introduced an error in how gist_trgm_ops index entries are combined.
Some entries could lose the flag identifying their format, causing
later inserts to treat them as empty and lose information about
existing entries. As a result, queries using these indexes could
miss matching rows.

Fix this by preserving the format flag when combining index entries,
so that subsequent inserts correctly retain the information they
represent.

This fix does not repair existing indexes. Users should REINDEX all
existing gist_trgm_ops indexes after upgrading to a release containing
this fix.

Backpatch to all supported versions.

Author: Nate Clark <natec425(at)gmail(dot)com>
Reviewed-by: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/19720-b3e83f5e99c485c5@postgresql.org
Backpatch-through: 14

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/145a7175ee0abf9c0894662f34a42988e5ea6276

Modified Files
--------------
contrib/pg_trgm/trgm_gist.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)