Fix issue with codecs returning passed-in payloads (#526)

Fixes #525

temporalio/bridge/worker.py

@@ -175,6 +175,8 @@ async def _apply_to_payloads(
     if len(payloads) == 0:
         return
     new_payloads = await cb(payloads)
+    if new_payloads is payloads:
+        return
     del payloads[:]
     # TODO(cretz): Copy too expensive?
     payloads.extend(new_payloads)
@@ -189,9 +191,7 @@ async def _apply_to_payload(
 ) -> None:
     """Apply API payload callback to payload."""
     new_payload = (await cb([payload]))[0]
-    payload.metadata.clear()
-    payload.metadata.update(new_payload.metadata)
-    payload.data = new_payload.data
+    payload.CopyFrom(new_payload)
 
 
 async def _decode_payloads(

tests/worker/test_workflow.py

@@ -1485,6 +1485,24 @@ async def test_workflow_with_codec(client: Client, env: WorkflowEnvironment):
     await test_workflow_update_handlers_happy(client, env)
 
 
+class PassThroughCodec(PayloadCodec):
+    async def encode(self, payloads: Sequence[Payload]) -> List[Payload]:
+        return list(payloads)
+
+    async def decode(self, payloads: Sequence[Payload]) -> List[Payload]:
+        return list(payloads)
+
+
+async def test_workflow_with_passthrough_codec(client: Client):
+    # Make client with this codec and run the activity test. This used to fail
+    # because there was a bug where the codec couldn't reuse the passed-in
+    # payloads.
+    config = client.config()
+    config["data_converter"] = DataConverter(payload_codec=PassThroughCodec())
+    client = Client(**config)
+    await test_workflow_simple_activity(client)
+
+
 class CustomWorkflowRunner(WorkflowRunner):
     def __init__(self) -> None:
         super().__init__()