[3.15] gh-154738: Propagate reparse-deferral setting to pyexpat subparsers (GH-154739) (GH-154990)

(cherry picked from commit 7d76013)

Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com>

Author: miss-islington

Lib/test/test_pyexpat.py

@@ -996,6 +996,13 @@ def test_parent_parser_outlives_its_subparsers__chain(self):
         del parser
         del subparser
 
+    def test_subparser_inherits_reparse_deferral(self):
+        for enabled in (True, False):
+            parser = expat.ParserCreate()
+            parser.SetReparseDeferralEnabled(enabled)
+            subparser = parser.ExternalEntityParserCreate(None)
+            self.assertEqual(subparser.GetReparseDeferralEnabled(), enabled)
+
 
 class ExternalEntityParserCreateErrorTest(unittest.TestCase):
     """ExternalEntityParserCreate error paths should not crash or leak

Misc/NEWS.d/next/Library/2026-07-26-00-00-00.gh-issue-154738.Rd5Pxe.rst

@@ -0,0 +1,3 @@
+Fix :meth:`!ExternalEntityParserCreate` not propagating the reparse-deferral
+setting to the subparser, which left :meth:`!GetReparseDeferralEnabled`
+returning an uninitialized value.  Patch by tonghuaroot.

Modules/pyexpat.c

@@ -1117,6 +1117,7 @@ pyexpat_xmlparser_ExternalEntityParserCreate_impl(xmlparseobject *self,
     new_parser->specified_attributes = self->specified_attributes;
     new_parser->in_callback = 0;
     new_parser->ns_prefixes = self->ns_prefixes;
+    new_parser->reparse_deferral_enabled = self->reparse_deferral_enabled;
     new_parser->itself = XML_ExternalEntityParserCreate(self->itself, context,
                                                         encoding);
     // The new subparser will make use of the parent XML_Parser inside of Expat.