pyexpat: ExternalEntityParserCreate does not propagate the reparse-deferral setting

[tonghuaroot]

Bug description

xmlparser.ExternalEntityParserCreate() copies the parent parser's fields onto the new subparser, but misses reparse_deferral_enabled. Since PyObject_GC_New does not zero the struct, that field is read uninitialized, so GetReparseDeferralEnabled() on the subparser returns a garbage value.

from xml.parsers import expat

p = expat.ParserCreate()
p.SetReparseDeferralEnabled(False)
sub = p.ExternalEntityParserCreate(None)
print(sub.GetReparseDeferralEnabled())  # True, should be False

Expat's own C-level state is inherited correctly (parserCreate saves and restores it), so the actual parsing behaviour is fine; only the Python-visible cached value is wrong.

CPython versions tested on

3.13, 3.14, main

Operating systems tested on

macOS

Linked PRs

• gh-154738: Propagate reparse-deferral setting to pyexpat subparsers #154739