gh-148427: Fix bare except in expatreader.external_entity_ref()

Change bare `except:` to `except Exception:` so that KeyboardInterrupt
and SystemExit propagate instead of being silently swallowed during
external entity parsing. Move entity stack cleanup into a `finally`
clause so the parser state is restored on both success and error paths.

Author: WYSIATI

Lib/test/test_sax.py

@@ -1055,6 +1055,65 @@ def test_expat_entityresolver_default(self):
         self.assertEqual(result.getvalue(), start +
                          b"<doc></doc>")
 
+    def test_external_entity_ref_keyboard_interrupt(self):
+        # gh-148427: KeyboardInterrupt must propagate, not be swallowed
+        class KBHandler(handler.ContentHandler):
+            def startElement(self, name, attrs):
+                if name == 'entity':
+                    raise KeyboardInterrupt('test')
+
+        parser = create_parser()
+        parser.setFeature(feature_external_ges, True)
+        parser.setEntityResolver(self.TestEntityResolver())
+        parser.setContentHandler(KBHandler())
+
+        with self.assertRaises(KeyboardInterrupt):
+            parser.feed('<!DOCTYPE doc [\n')
+            parser.feed('  <!ENTITY test SYSTEM "whatever">\n')
+            parser.feed(']>\n')
+            parser.feed('<doc>&test;</doc>')
+            parser.close()
+
+    def test_external_entity_ref_system_exit(self):
+        # gh-148427: SystemExit must propagate, not be swallowed
+        class ExitHandler(handler.ContentHandler):
+            def startElement(self, name, attrs):
+                if name == 'entity':
+                    raise SystemExit(42)
+
+        parser = create_parser()
+        parser.setFeature(feature_external_ges, True)
+        parser.setEntityResolver(self.TestEntityResolver())
+        parser.setContentHandler(ExitHandler())
+
+        with self.assertRaises(SystemExit):
+            parser.feed('<!DOCTYPE doc [\n')
+            parser.feed('  <!ENTITY test SYSTEM "whatever">\n')
+            parser.feed(']>\n')
+            parser.feed('<doc>&test;</doc>')
+            parser.close()
+
+    def test_external_entity_ref_stack_cleanup(self):
+        # gh-148427: _entity_stack must be cleaned up after errors
+        class ErrorHandler(handler.ContentHandler):
+            def startElement(self, name, attrs):
+                if name == 'entity':
+                    raise ValueError('test error')
+
+        parser = create_parser()
+        parser.setFeature(feature_external_ges, True)
+        parser.setEntityResolver(self.TestEntityResolver())
+        parser.setContentHandler(ErrorHandler())
+
+        with self.assertRaises(SAXParseException):
+            parser.feed('<!DOCTYPE doc [\n')
+            parser.feed('  <!ENTITY test SYSTEM "whatever">\n')
+            parser.feed(']>\n')
+            parser.feed('<doc>&test;</doc>')
+            parser.close()
+
+        self.assertEqual(len(parser._entity_stack), 0)
+
     # ===== Attributes support
 
     class AttrGatherer(ContentHandler):

Lib/xml/sax/expatreader.py

@@ -424,11 +424,11 @@ def external_entity_ref(self, context, base, sysid, pubid):
 
         try:
             xmlreader.IncrementalParser.parse(self, source)
-        except:
-            return 0  # FIXME: save error info here?
-
-        (self._parser, self._source) = self._entity_stack[-1]
-        del self._entity_stack[-1]
+        except Exception:
+            return 0
+        finally:
+            (self._parser, self._source) = self._entity_stack[-1]
+            del self._entity_stack[-1]
         return 1
 
     def skipped_entity_handler(self, name, is_pe):

Misc/NEWS.d/next/Library/2026-04-12-09-52-14.gh-issue-148427.xR7v3m.rst

@@ -0,0 +1,4 @@
+Fix bare ``except`` clause in :mod:`xml.sax` expatreader's external entity
+handler that silently swallowed :exc:`KeyboardInterrupt` and
+:exc:`SystemExit`. Also fix entity parser stack leak on errors by moving
+cleanup into a ``finally`` clause.