gh-82951: Fix serializing by name in pickle protocols < 4 (GH-122149)

Serializing objects with complex __qualname__ (such as unbound methods and
nested classes) by name no longer involves serializing parent objects by value
in pickle protocols < 4.

2 files changed, 43 insertions, 9 deletions

diff --git a/Lib/pickle.py b/Lib/pickle.py
index 115bd89..2d76498 100644
--- a/Lib/pickle.py
+++ b/Lib/pickle.py
@@ -1110,11 +1110,35 @@ class _Pickler:
             self.save(module_name)
             self.save(name)
             write(STACK_GLOBAL)
-        elif parent is not module:
-            self.save_reduce(getattr, (parent, lastname))
-        elif self.proto >= 3:
-            write(GLOBAL + bytes(module_name, "utf-8") + b'\n' +
-                  bytes(name, "utf-8") + b'\n')
+        elif '.' in name:
+            # In protocol < 4, objects with multi-part __qualname__
+            # are represented as
+            # getattr(getattr(..., attrname1), attrname2).
+            dotted_path = name.split('.')
+            name = dotted_path.pop(0)
+            save = self.save
+            for attrname in dotted_path:
+                save(getattr)
+                if self.proto < 2:
+                    write(MARK)
+            self._save_toplevel_by_name(module_name, name)
+            for attrname in dotted_path:
+                save(attrname)
+                if self.proto < 2:
+                    write(TUPLE)
+                else:
+                    write(TUPLE2)
+                write(REDUCE)
+        else:
+            self._save_toplevel_by_name(module_name, name)
+
+        self.memoize(obj)
+
+    def _save_toplevel_by_name(self, module_name, name):
+        if self.proto >= 3:
+            # Non-ASCII identifiers are supported only with protocols >= 3.
+            self.write(GLOBAL + bytes(module_name, "utf-8") + b'\n' +
+                       bytes(name, "utf-8") + b'\n')
         else:
             if self.fix_imports:
                 r_name_mapping = _compat_pickle.REVERSE_NAME_MAPPING
@@ -1124,15 +1148,13 @@ class _Pickler:
                 elif module_name in r_import_mapping:
                     module_name = r_import_mapping[module_name]
             try:
-                write(GLOBAL + bytes(module_name, "ascii") + b'\n' +
-                      bytes(name, "ascii") + b'\n')
+                self.write(GLOBAL + bytes(module_name, "ascii") + b'\n' +
+                           bytes(name, "ascii") + b'\n')
             except UnicodeEncodeError:
                 raise PicklingError(
                     "can't pickle global identifier '%s.%s' using "
                     "pickle protocol %i" % (module, name, self.proto)) from None
 
-        self.memoize(obj)
-
     def save_type(self, obj):
         if obj is type(None):
             return self.save_reduce(type, (None,), obj=obj)
diff --git a/Lib/test/pickletester.py b/Lib/test/pickletester.py
index 9922591..1366322 100644
--- a/Lib/test/pickletester.py
+++ b/Lib/test/pickletester.py
@@ -2818,6 +2818,18 @@ class AbstractPickleTests:
                 self.assertIs(unpickled, Recursive)
         del Recursive.mod # break reference loop
 
+    def test_recursive_nested_names2(self):
+        global Recursive
+        class Recursive:
+            pass
+        Recursive.ref = Recursive
+        Recursive.__qualname__ = 'Recursive.ref'
+        for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+            with self.subTest(proto=proto):
+                unpickled = self.loads(self.dumps(Recursive, proto))
+                self.assertIs(unpickled, Recursive)
+        del Recursive.ref # break reference loop
+
     def test_py_methods(self):
         global PyMethodsTest
         class PyMethodsTest: