[PATCH] BUG: Stale LC_MESSAGES translations after SET on windows.

Greetings hackers,

The libintl library provides a libintl_setlocale function that we need
to use to correctly invalidate the libintl translation cache. We are
not doing this as the following shows.

On Windows, changing lc_messages at runtime via SET returns stale
translations from the previous locale. For example:

SET lc_messages = 'de_DE';
SELECT 1/0;
-- FEHLER: Division durch Null (correct, German)

SET lc_messages = 'en_US';
SELECT 1/0;
-- FEHLER: Division durch Null (wrong, still German)

libintl caches translated messages in a binary tree keyed by (msgid,
domain, category). The key struct includes a counter and a pointer to
the translation text and translation length in the mmap'd .mo file for
that locale.

Background:
Assume you have ("PL/pgSQL function %s line %d at %s",
"plpgsql-19",1729) for (msgid, domain, category) respectively. Gettext
uses strcmp for msgid and domain, and integer subtraction for category
as it's comparison function. The search through the tree will short
circuit at a node as soon as one of the three-- compared left to right
as listed in the tuple-- is not a match. If the comparison is negative
then it goes left in the tree, positive it goes right. If all three
components of the key match then it checks that the counter matches
_nl_msg_cat_cntr. If the counter matches then it is a cache hit. If
the counter does not match then this translation was stored in the cache
before the current locale was set with libintl_setlocale-- in other
words it is stale. In this case the cache is invalidated and it is a
cache miss because the locale has changed.

Notice locale is not directly involved in the key structure for windows.
So, if you don't increment the counter by calling libintl_setlocale and
the two locales have the same msgid's for a given domain you will get
the problem shown at the beginning of this email.

The fix adds an explicit call to libintl_setlocale(LC_MESSAGES, ...)
in pg_perm_setlocale() after IsoLocaleName() has produced the POSIX
name. Instead of an explicit call, we could define
_INTL_REDIRECT_MACROS before including libintl.h in c.h. This would
make all setlocale() calls go through libintl_setlocale on MSVC. I
chose to be explicit to limit the blast radius. The attached patch
limits the change to a single spot and resolves the identified bug.

Patch attached.

--
Bryan Green
EDB: https://www.enterprisedb.com