On demand create message meta class for upb python

PiperOrigin-RevId: 565733360

python/google/protobuf/internal/factory_test1.proto

@@ -52,6 +52,7 @@ message Factory1Message {
   optional NestedFactory1Message nested_factory_1_message = 3;
   optional int32 scalar_value = 4;
   repeated string list_value = 5;
+  map<string, string> map_field = 6;
 
   extensions 1000 to max;
 }

python/google/protobuf/internal/message_factory_test.py

@@ -33,6 +33,7 @@
 __author__ = 'matthewtoia@google.com (Matt Toia)'
 
 import unittest
+import gc
 
 from google.protobuf import descriptor_pb2
 from google.protobuf.internal import api_implementation
@@ -285,5 +286,29 @@ class MessageFactoryTest(unittest.TestCase):
     self.assertEqual(345, m.Extensions[ext2].setting)
 
 
+  def testOndemandCreateMetaClass(self):
+    def loadFile():
+      f = descriptor_pb2.FileDescriptorProto.FromString(
+        factory_test1_pb2.DESCRIPTOR.serialized_pb)
+      return message_factory.GetMessages([f])
+
+    messages = loadFile()
+    data = factory_test1_pb2.Factory1Message()
+    data.map_field['hello'] = 'welcome'
+    # Force GC to collect. UPB python will clean up the map entry class.
+    # cpp extension and pure python will still keep the map entry class.
+    gc.collect()
+    message = messages['google.protobuf.python.internal.Factory1Message']()
+    message.ParseFromString(data.SerializeToString())
+    value = message.map_field
+    values = [
+        # The entry class will be created on demand in upb python.
+        value.GetEntryClass()(key=k, value=value[k]) for k in sorted(value)
+    ]
+    gc.collect()
+    self.assertEqual(1, len(values))
+    self.assertEqual('hello', values[0].key)
+    self.assertEqual('welcome', values[0].value)
+
 if __name__ == '__main__':
   unittest.main()