Re: [HACKERS] possible encoding issues with libxml2 functions

xpath-bugfix.patch affected only xml values containing an xml declaration with
"encoding" attribute.  In UTF8 databases, this latest proposal
(xpath-parsing-error-fix.patch) is equivalent to xpath-bugfix.patch.  In
non-UTF8 databases, xpath-parsing-error-fix.patch affects all xml values
containing non-ASCII data.  In a LATIN1 database, the following works today
but breaks under your latest proposal:

  SELECT xpath('text()', ('<x>' || convert_from('\xc2b0', 'LATIN1') || '</x>')::xml);

It's acceptable to break that, since the documentation explicitly disclaims
support for it.  xpath-bugfix.patch breaks different use cases, which are
likewise acceptable to break.  See my 2017-08-08 review for details.

We have xpath-bugfix.patch and xpath-parsing-error-fix.patch.  Both are
equivalent under supported use cases (xpath in UTF8 databases).  Among
non-supported use cases, they each make different things better and different
things worse.  We should prefer to back-patch the version harming fewer
applications.  I expect non-ASCII data is more common than xml declarations
with "encoding" attribute, so xpath-bugfix.patch will harm fewer applications.

Having said that, I now see a third option.  Condition this thread's patch's
effects on GetDatabaseEncoding()==PG_UTF8.  That way, we fix supported cases,
and we remain bug-compatible in unsupported cases.  I think that's better than
the other options discussed so far.  If you agree, please send a patch based
on xpath-bugfix.patch with the GetDatabaseEncoding()==PG_UTF8 change and the
two edits I described earlier.

Thanks,
nm