Fixed a bug in msg.MergeFrom(msg2) in Python.

In `FixupMessageAfterMerge()`, the loop over fields was exiting the entire function if a mutable field was found, but this skips other fields that may need fixing up.

PiperOrigin-RevId: 882766354

python/google/protobuf/internal/message_test.py

@@ -3071,6 +3071,34 @@ class PackedFieldTest(unittest.TestCase):
     self.assertEqual(golden_data, message.SerializeToString())
 
 
+@testing_refleaks.TestCase
+class MergeFromTest(unittest.TestCase):
+  def testBugFixupAfterMerge(self):
+    target = more_messages_pb2.LotsOfMessageFields()
+    wrappers = []
+
+    # Access all fields mutably, except for a10 which is accessed read-only.
+    for i in range(1, 21):
+      sub = getattr(target, f'a{i}')
+      wrappers.append(sub)
+      if i != 10:
+        # Access mutable.
+        sub.b = 1
+
+    self.assertEqual(target.a10.b, 0)
+    self.assertFalse(target.HasField('a10'))
+
+    source = more_messages_pb2.LotsOfMessageFields()
+    source.a10.b = 1
+
+    # The read-only message target.a10 needs to be updated to point to the new
+    # message that is created by MergeFrom.
+    target.MergeFrom(source)
+
+    self.assertEqual(target.a10.b, 1)
+    self.assertTrue(target.HasField('a10'))
+
+
 @testing_refleaks.TestCase
 class OversizeProtosTest(unittest.TestCase):
 

python/google/protobuf/internal/more_messages.proto

@@ -342,6 +342,31 @@ message LotsNestedMessage {
   message B255 {}
 }
 
+message LotsOfMessageFields {
+  optional LotsOfMessageFields a1 = 1;
+  optional LotsOfMessageFields a2 = 2;
+  optional LotsOfMessageFields a3 = 3;
+  optional LotsOfMessageFields a4 = 4;
+  optional LotsOfMessageFields a5 = 5;
+  optional LotsOfMessageFields a6 = 6;
+  optional LotsOfMessageFields a7 = 7;
+  optional LotsOfMessageFields a8 = 8;
+  optional LotsOfMessageFields a9 = 9;
+  optional LotsOfMessageFields a10 = 10;
+  optional LotsOfMessageFields a11 = 11;
+  optional LotsOfMessageFields a12 = 12;
+  optional LotsOfMessageFields a13 = 13;
+  optional LotsOfMessageFields a14 = 14;
+  optional LotsOfMessageFields a15 = 15;
+  optional LotsOfMessageFields a16 = 16;
+  optional LotsOfMessageFields a17 = 17;
+  optional LotsOfMessageFields a18 = 18;
+  optional LotsOfMessageFields a19 = 19;
+  optional LotsOfMessageFields a20 = 20;
+
+  optional int32 b = 100;
+}
+
 message RequiredField {
   required int32 id = 1;
 }

python/google/protobuf/pyext/message.cc

@@ -810,7 +810,7 @@ int FixupMessageAfterMerge(CMessage* self) {
         !descriptor->is_repeated()) {
       CMessage* cmsg = reinterpret_cast<CMessage*>(item.second);
       if (cmsg->read_only == false) {
-        return 0;
+        continue;
       }
       Message* message = self->message;
       const Reflection* reflection = message->GetReflection();