pgsql: Avoid overflow in Levenshtein distance calculations.

Avoid overflow in Levenshtein distance calculations.

levenshtein() and levenshtein_less_equal() let the caller specify
the insertion, deletion, and substitution costs, and
fuzzystrmatch's corresponding SQL functions accept any 32-bit
integer for each. Since the distances are calculated with 32-bit
arithmetic, large costs can cause overflows, thereby producing
nonsensical results. Certain inputs to levenshtein_less_equal()
can even cause out-of-bounds writes. To fix, use 64-bit arithmetic
instead, and error whenever the final result won't fit in the
returned 32-bit integer.

We may want to teach these functions to reject negative costs, too,
but that didn't seem appropriate for a security fix, and therefore
it is left as a future exercise.

Reported-by: Ben Morris in collaboration with Claude and Anthropic Research
Author: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Security: CVE-2026-15742
Backpatch-through: 14

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/61481aa0e4aa66c32a8826732f30d17d4c488d06
Author: Nathan Bossart <nathan(at)postgresql(dot)org>

Modified Files
--------------
contrib/fuzzystrmatch/expected/fuzzystrmatch.out | 14 ++++
contrib/fuzzystrmatch/sql/fuzzystrmatch.sql | 3 +
src/backend/utils/adt/levenshtein.c | 89 ++++++++++++------------
src/backend/utils/adt/varlena.c | 14 ++++
4 files changed, 77 insertions(+), 43 deletions(-)