[PATCH] contrib/xml2: xslt_process() should report XSLT-related error details

Dear PG hackers,

here is a patch that improves the xslt_process() error handling. Error
handling was one of the points criticized earlier [1] and as far as I
understand it is one of the reasons that xslt_process() was never adopted
into core and contrib/xml2 is considered deprecated since PG 8. However the
point is also still on the TODO list ("Report errors returned by the XSLT
library") and xslt_process() has been asked for several times on the mailing
list. Memory handling bugs that have been reported earlier [2] have obviously
long been fixed.

The point raised in [1] was that xslt_process() would not report missing
stylesheet parameters at all. This hasn't been the case at the current HEAD.
However, xslt_process() still wouldn't report any details about errors
related to XSLT processing. This is what is fixed by this patch. It now
installs an XSLT error handler. See the patch comment for details. Since the
goal is to eventually get xslt_process() adopted into core, I also got rid of
PG_XML_STRICTNESS_LEGACY.

Perhaps you can tell me what else is preventing adoption into core. I believe
that xslt_process() should also accept the `xml` type as an alternative to
strings. Strings should be kept for backwards compatibility, though. Also,
the stylesheet parameter passing is very crude and limited. I suggest, that
the current method of passing them as strings (see parse_params()) is
deprecated and we don't try to tackle its limitations, but will instead
accept hstores. If you agree, I will work on these two tasks next.

Yours sincerely,
Robin Haberkorn

[1]: https://www.postgresql.org/message-id/4C5ECEF9.3030806%40mlfowler.com
[2]: https://www.postgresql.org/message-id/flat/4A6A276A.6090405%40dunslane.net

--
Robin Haberkorn
Senior Software Engineer

B1 Systems GmbH
Osterfeldstraße 7 / 85088 Vohburg / https://www.b1-systems.de
GF: Ralph Dehner / Unternehmenssitz: Vohburg / AG: Ingolstadt, HRB 3537