Tailcalls

src/dispatch/coroutine.py

@@ -231,6 +231,21 @@ def callback(cls, state: Any, calls: None | list[Call] = None) -> Output:
 
         return Output(coroutine_pb2.ExecuteResponse(poll=poll))
 
+    @classmethod
+    def tailcall(cls, call: Call) -> Output:
+        """Exit the coroutine instructing the orchestrator to call the provided
+        coroutine."""
+        input_bytes = _pb_any_pickle(call.input)
+        x = coroutine_pb2.Call(
+            coroutine_uri=call.coroutine_uri,
+            coroutine_version=call.coroutine_version,
+            correlation_id=call.correlation_id,
+            input=input_bytes,
+        )
+        return Output(
+            coroutine_pb2.ExecuteResponse(exit=coroutine_pb2.Exit(tail_call=x))
+        )
+
 
 # Note: contrary to other classes here Call is not just a wrapper around its
 # associated protobuf class, because it is reasonable for a human to write the

tests/test_fastapi.py

@@ -291,3 +291,18 @@ def mycoro(input: Input) -> Output:
         self.assertEqual("foo", resp.exit.result.error.type)
         self.assertEqual("bar", resp.exit.result.error.message)
         self.assertEqual(Status.THROTTLED, resp.status)
+
+    def test_tailcall(self):
+        @self.app.dispatch_coroutine()
+        def other_coroutine(input: Input) -> Output:
+            return Output.value(f"Hello {input.input}")
+
+        @self.app.dispatch_coroutine()
+        def mycoro(input: Input) -> Output:
+            return Output.tailcall(other_coroutine.call_with(42))
+
+        resp = self.execute(mycoro)
+        self.assertEqual(other_coroutine.uri, resp.exit.tail_call.coroutine_uri)
+        self.assertEqual(
+            42, dispatch.coroutine._any_unpickle(resp.exit.tail_call.input)
+        )