gh-156796: Finalize the parser of an external entity in xml.sax

[skdas20]

ExpatParser.close() returned early whenever _entity_stack was non-empty:

if (self._entity_stack or self._parser is None or
    isinstance(self._parser, _ClosedParser)):
    # If we are completing an external entity, do nothing here
    return

external_entity_ref() swaps in a parser from ExternalEntityParserCreate() and then calls IncrementalParser.parse(), which ends with self.close(). Because the stack is non-empty at that point, feed(b"", isFinal=True) was never reached, so expat never checked the entity's input for completeness and content that is not well-formed was accepted silently.

The guard is still needed — ending the document while an entity is being parsed would be wrong — so this finalizes the entity's parser and returns without calling endDocument(). The None/_ClosedParser checks are hoisted above it so they still apply on that path.

Verification

The reproducer from the issue silently succeeds on main. With this change it fails as you described:

xml.sax._exceptions.SAXParseException: <unknown>:1:8: error in processing external entity reference

test_expat_entityresolver_not_well_formed in Lib/test/test_sax.py covers it, and fails on unpatched main. The full test_sax passes (188 tests), which matches your note that finalizing the entity parser leaves the existing suite green.

I confirmed the behaviour is longstanding — the reproducer is silently accepted on 3.12 as well.

Fixes #156796

• Issue: xml.sax: the content of an external entity is not checked to be well-formed #156796

[maurycy]

Perhaps the second sentence could be ommited, since it's an implementation detail?

Suggested change

	Fix :mod:`xml.sax` not reporting an external entity whose content is not
	well-formed. The parser created for the entity was never finalized, so errors
	which are only detectable at the end of its input, such as an unclosed
	element, were silently ignored.
	Fix :mod:`xml.sax` silently accepting an external entity whose content is not
	well-formed.