Implement Detached Actor (#6036)

* Arg propagation works

* Implement persistent actor

* Add doc

* Initialize is_persistent_

* Rename persistent->detached

* Address comment

* Make test passes

* Address comment

* Python2 compatiblity

* Fix naming, py2

* Lint

python/ray/tests/test_actor.py

@@ -21,12 +21,9 @@ import ray.ray_constants as ray_constants
 import ray.tests.utils
 import ray.tests.cluster_utils
 from ray.tests.conftest import generate_internal_config_map
-from ray.tests.utils import (
-    relevant_errors,
-    wait_for_condition,
-    wait_for_errors,
-    wait_for_pid_to_exit,
-)
+from ray.tests.utils import (relevant_errors, wait_for_condition,
+                             wait_for_errors, wait_for_pid_to_exit,
+                             run_string_as_driver)
 
 
 @pytest.fixture
@@ -2816,3 +2813,37 @@ def test_ray_wait_dead_actor(ray_start_cluster):
     failure_detected = False
     while not failure_detected:
         failure_detected = ray.get(parent_actor.wait.remote())
+
+
+def test_detached_actor(ray_start_regular):
+    @ray.remote
+    class DetachedActor(object):
+        def ping(self):
+            return "pong"
+
+    with pytest.raises(Exception, match="Detached actors must be named"):
+        DetachedActor._remote(detached=True)
+
+    with pytest.raises(ValueError, match="Please use a different name"):
+        _ = DetachedActor._remote(name="d_actor")
+        DetachedActor._remote(name="d_actor")
+
+    redis_address = ray_start_regular["redis_address"]
+
+    actor_name = "DetachedActor"
+    driver_script = """
+import ray
+ray.init(address="{}")
+
+@ray.remote
+class DetachedActor(object):
+    def ping(self):
+        return "pong"
+
+actor = DetachedActor._remote(name="{}", detached=True)
+ray.get(actor.ping.remote())
+""".format(redis_address, actor_name)
+
+    run_string_as_driver(driver_script)
+    detached_actor = ray.experimental.get_actor(actor_name)
+    assert ray.get(detached_actor.ping.remote()) == "pong"