gh-150560: Fix crash in XML parser on invalid XML with multi-byte encoding

Lib/test/test_pyexpat.py

@@ -426,6 +426,16 @@ def test_unknown_encoding(self):
         with self.assertRaises(LookupError):
             parser.Parse(data, True)
 
+    @support.subTests('sample,exception', [
+        (b'<x> \xa1</x>', UnicodeDecodeError),  # crashed
+        (b'<x> \xa1</x', UnicodeDecodeError),  # crashed
+        (b'<x> \xa1', expat.ExpatError),
+    ])
+    def test_multibyte_encoding_errors(self, sample, exception):
+        parser = expat.ParserCreate()
+        data = b'<?xml version="1.0" encoding="EUC-JP"?>\n' + sample
+        with self.assertRaises(exception):
+            parser.Parse(data, True)
 
 class NamespaceSeparatorTest(unittest.TestCase):
     def test_legal(self):

Lib/test/test_xml_etree.py

@@ -1064,6 +1064,17 @@ def bxml(encoding, body=''):
         self.assertRaises(ValueError, ET.XML, xml('undefined').encode('ascii'))
         self.assertRaises(LookupError, ET.XML, xml('xxx').encode('ascii'))
 
+    @support.subTests('sample,exception', [
+        (b'<x> \xa1</x>', UnicodeDecodeError),  # crashed
+        (b'<x> \xa1</x', UnicodeDecodeError),  # crashed
+        (b'<x> \xa1', None), # ET.ParseError
+    ])
+    def test_multibyte_encoding_errors(self, sample, exception):
+        exception = exception or ET.ParseError
+        data = b'<?xml version="1.0" encoding="EUC-JP"?>\n' + sample
+        with self.assertRaises(exception):
+            ET.XML(data)
+
     def test_methods(self):
         # Test serialization methods.
 

Modules/pyexpat.c

@@ -1473,6 +1473,9 @@ pyexpat_encoding_create(const char *name, PyObject *mapping)
 static int
 pyexpat_encoding_convert(void *data, const char *s)
 {
+    if (PyErr_Occurred()) {
+        return -1;
+    }
     pyexpat_encoding_info *info = (pyexpat_encoding_info *)data;
     int i = (unsigned char)s[0];
     assert(info->map[i] < -1);