oss_utils.cc uses deprecated libxml functions

[josepharhar]

xmlSAXParseMemory, which is used in oss_utils.cc, has been marked as deprecated in libxml, which I discovered while rolling new commits into chromium's copy of libxml:
https://chromium-review.googlesource.com/c/chromium/src/+/3863846
https://ci.chromium.org/ui/p/chromium/builders/try/linux-rel/1116534/overview

I'm not sure exactly how to migrate this off of xmlSAXParseMemory, but the comment says to use xmlNewSAXParserCtxt and xmlCtxtReadMemory instead.

cc @nwellnhof @b-maldoca @dtao-oss

[nwellnhof]

This should work:

xmlParserCtxtPtr ctxt = xmlNewParserCtxt();
*ctxt->sax = sax_handler;
xmlDocPtr doc = xmlCtxtReadMemory(ctxt, buffer, size,
        NULL /* URL */, NULL /* encoding */, 0 /* options */);
xmlFreeParserCtxt(ctxt);

(The main benefit is that parser options are passed directly and not initialized from mutable globals.)

[nwellnhof]

xmlNewSAXParserCtxt is only available in the libxml2 master branch, though. If you have to support older libxml2 releases, things become more complicated. Besides, the deprecation status of the old SAX API functions isn't final yet, and these functions will be kept around for at least several years.

[josepharhar]

Thanks so much! I think I'll just un-deprecate xmlSAXParseMemory in chromium for now until this gets fixed here and then rolled into chromium.

[nwellnhof]

Thank you for providing feedback on the libxml2 master branch! Ultimately, I'll probably make the deprecation warning link to a web page with instructions how to migrate some of the more widely used functions.

I also updated the code snippet above to make it work with old libxml2 releases.

[bobbybrown553]

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