bug#26533: 26.0.50; xml-parse-region's symbol-qname argument is ignored

[Christopher Wellons]

Thanks, David! Your fix works fine as far as I can tell.

I'm using this trick in Elfeed (a syndication feed reader) as a fast
method to strip all namespaces from the XML as it's being parsed. As you
said, there's a lot of invalid XML in the wild. I've found it works a
lot better to ignore namespaces and strictness, instead extracting the
required information heuristically as long as it's reasonably close.
Otherwise there would be a whole lot more feeds that wouldn't work well,
or at all, in Elfeed.

I had noticed with symbol-qnames that xml-parse-region drops unknown
namespaces. Since this information comes from an alist, that seemed like
reasonable behavior and I assumed it was intentional -- though signaling
an error would also be reasonable. To tightly control which namespaces
are stripped, I bind xml-default-ns to my own alist for that call. This
feels like the natural and lispy way to use this function.

The file that binds xml-default-ns requires the xml package explicitly,
so there's no risk of it autoloading while it's bound. Though that's an
interesting consequence I hadn't considered before. I _have_ seen
similar issues with accept-process-output when arbitrary process events
are handled while the stack is in an unusual state.