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

Hello Tom!

On Tue Apr 22, 2025 at 18:29:02 GMT +03, Tom Lane wrote:
> Hi Robin, thank you for the patch! This has arrived too late to
> be considered for PG v18, but please add the thread to the open
> commitfest

I will do that. But what about the other changes I proposed (xml data type
and hstore support)? Should they all be discussed in this thread or should I
create separate threads and consequently separate patch entries for the
Commitfest? And should they all be based on the master branch or can they be
based upon each other? Do you think they will help to bring xslt_process()
closer to adoption into core?

> Just from a quick eyeballing of the patch, it seems generally
> sane, but I wonder why you have xml_generic_error_handler
> creating and then destroying an errorBuf instead of just
> appending the results directly to xmlerrcxt->err_buf. The
> extra buffer doesn't seem to be doing much except increasing
> our odds of an OOM failure right there.

You are right. It was a leftover from thinning out xml_errorHandler().
I attached a new version of the patch where this is fixed.
I also ran all touched files through pgindent.

> I'm also a little suspicious of
>
> - xmlerrcxt = pgxml_parser_init(PG_XML_STRICTNESS_LEGACY);
> + xmlerrcxt = pgxml_parser_init(PG_XML_STRICTNESS_ALL);
>
> as that looks like it is probably changing the behavior of
> the module in ways bigger than just providing more error
> details. This code has been in legacy status for so long
> that I feel a little hesitant about doing that.

You wouldn't actually need that change to get more error details as the
details are provided by xml_generic_error_handler() which ignores the
strictness level. The reason is that libxml generic error handlers cannot
discern between warnings and errors. The only thing that the strictness level
does is affecting how xml_errorHandler() works: With STRICTNESS_ALL, it can
actually log warnings without appending anything to the err_buf and set the
err_occurred flag which is apparently useful because some libxml functions
can result in error-level messages without the function calls signalling
failure. At least, otherwise the code doesn't make sense to me. In other
words, using the STRICTNESS_ALL should allow you to see warnings, you
wouldn't otherwise see and catch errors, you wouldn't otherwise catch -- at
least if they occur in libxml itself. As far as I understand from the code
comment xml_errorHandler(), STRICTNESS_LEGACY was introduced, just so you
don't have to touch the existing contrib/xml2 code that did not check
err_occurred:

/*
* Legacy error handling mode. err_occurred is never set, we just add the
* message to err_buf. This mode exists because the xml2 contrib module
* uses our error-handling infrastructure, but we don't want to change its
* behaviour since it's deprecated anyway. This is also why we don't
* distinguish between notices, warnings and errors here --- the old-style
* generic error handler wouldn't have done that either.
*/

Since we now check that flag in xslt_proc.c, we should be fine.
But I might be overlooking something.

> If we are willing to do it, should we go further and clean out the use
> of PG_XML_STRICTNESS_LEGACY in xml2/xpath.c as well?

I think this is a separate construction site. We don't necessarily have
to touch xpath.c and I didn't plan to do so in the near future.
We should first determine whether the XPath support is actually worth
keeping (should be adopted into core) or rather stay deprecated.

Best regards,
Robin Haberkorn

--
Robin Haberkorn
Senior Software Engineer

B1 Systems GmbH
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