gh-146169: correctly handle re-entrant parsing calls in Expat handlers

[picnixz]

• Issue: Segfault/UB from expat when re-entering the XML Parser #146169

[picnixz]

@hartwork I'd like your feedback here to know whether there are other places that I need to consider. I will work on the encoding issue described in the GH issue afterwards unless they are interconnected. TiA!

[hartwork]

@hartwork I'd like your feedback here to know whether there are other places that I need to consider. I will work on the encoding issue described in the GH issue afterwards unless they are interconnected. TiA!

@picnixz they seem interconnected, yes. Here's what I found:

[picnixz]

For the encoding issue, I will actually do it in a separate PR. It's not really a re-entrancy call issue, but rather an issue where you call XML_Parse() first and then you recall it later with a different string and hence have a different possible encoding.

[hartwork]

@picnixz probably good to merge by now(, I lack permissions to approve).

[picnixz]

Thanks! I've left my dev session so I'll merge it tomorrow. Thanks for the review! I'll also address the encoding issue separately