Support MSC2033: Device ID on whoami

MSC: matrix-org/matrix-spec-proposals#2033

The MSC has passed FCP, which means stable endpoints can be used.

synapse/rest/client/v2_alpha/account.py

@@ -884,7 +884,10 @@ def __init__(self, hs):
     async def on_GET(self, request):
         requester = await self.auth.get_user_by_req(request)
 
-        return 200, {"user_id": requester.user.to_string()}
+        return 200, {
+            "user_id": requester.user.to_string(),
+            "device_id": requester.device_id,
+        }
 
 
 def register_servlets(hs, http_server):

tests/rest/client/v2_alpha/test_account.py

@@ -458,6 +458,29 @@ def deactivate(self, user_id, tok):
         self.assertEqual(channel.code, 200)
 
 
+class WhoamiTestCase(unittest.HomeserverTestCase):
+
+    servlets = [
+        synapse.rest.admin.register_servlets_for_client_rest_resource,
+        login.register_servlets,
+        account.register_servlets,
+        register.register_servlets,
+    ]
+
+    def test_GET_whoami(self):
+        device_id = "wouldgohere"
+        user_id = self.register_user("kermit", "test")
+        tok = self.login("kermit", "test", device_id=device_id)
+
+        whoami = self.whoami(tok)
+        self.assertObjectHasAttributes({"user_id": user_id, "device_id": device_id}, whoami)
+
+    def whoami(self, tok):
+        channel = self.make_request("GET", "account/whoami", {}, access_token=tok)
+        self.assertEqual(channel.code, 200)
+        return channel.json_body
+
+
 class ThreepidEmailRestTestCase(unittest.HomeserverTestCase):
 
     servlets = [