Re: [HACKERS] proposal - Default namespaces for XPath expressions (PostgreSQL 11)

Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> [ xml-xpath-default-ns-7.patch ]

At Andrew's prompting, I took a look over this patch. I don't know much
of anything about XML, so I have no idea as to standards compliance here,
but I do have some comments:

* I'm fairly uncomfortable with the idea that we're going to maintain
our own XPath parser. That seems like a recipe for lots of future
work ... and the code is far too underdocumented for anyone to actually
maintain it. (Case in point: _transformXPath's arguments are not
documented at all, and in fact there's hardly a word about what it's
even supposed to *do*.)

* I think the business with "pgdefnamespace.pgsqlxml.internal" is just
plain awful. It's a wart, and I don't think it's even saving you any
code once you account for all the places where you have to throw errors
for somebody trying to use that as a regular name. This should be done
with out-of-band signaling if possible. The existing convention above
this code is that a NULL pointer means a default namespace; can't that
be continued throughout? If that's not practical, can you pick a string
that simply can't syntactically be a namespace name? (In the SQL world,
for example, an empty string is an illegal identifier so that that could
be used for the purpose. But I don't know if that applies to XML.)
Or perhaps you can build a random name that is chosen just to make it
different from any of the listed namespaces? If none of those work,
I think passing around an explicit "isdefault" boolean alongside the name
would be better than having a reserved word.

* _transformXPath recurses, so doesn't it need check_stack_depth()?

* I'm not especially in love with using function names that start
with an underscore. I do not think that adds anything, and it's
unlike the style in most places in PG.

* This is a completely unhelpful error message:
+ if (!parser->buffer_is_empty)
+ elog(ERROR, "internal error");
If you can't be bothered to write a message that says something
useful, either drop the test or turn it into an Assert. I see some
other internal errors that aren't up to project norms either.

* Either get rid of the "elog(DEBUG1)"s, or greatly lower their
message priority. They might've been appropriate for developing
this patch, but they are not okay to commit that way.

* Try to reduce the amount of random whitespace changes in the patch.

* .h files should never #include "postgres.h", per project policy.

* I'm not sure I'd bother with putting the new code into a separate
file rather than cramming it into xml.c. The main reason why not
is that you're going to get "empty translation unit" warnings from
some compilers in builds without USE_LIBXML.

* Documentation, comments, and error messages could all use some
copy-editing by a native English speaker (you knew that of course).

regards, tom lane