Fix noop checks when updating device keys

changelog.d/4164.bugfix

@@ -0,0 +1 @@
+Fix noop checks when updating device keys, reducing spurious device list update notifications.

synapse/storage/end_to_end_keys.py

@@ -40,7 +40,10 @@ def _set_e2e_device_keys_txn(txn):
                 allow_none=True,
             )
 
-            new_key_json = encode_canonical_json(device_keys)
+            # In py3 we need old_key_json to match new_key_json type. The DB
+            # returns unicode while encode_canonical_json returns bytes.
+            new_key_json = encode_canonical_json(device_keys).decode("utf-8")
+
             if old_key_json == new_key_json:
                 return False
 

tests/storage/test_end_to_end_keys.py

@@ -44,6 +44,21 @@ def test_key_without_device_name(self):
         dev = res["user"]["device"]
         self.assertDictContainsSubset({"keys": json, "device_display_name": None}, dev)
 
+    @defer.inlineCallbacks
+    def test_reupload_key(self):
+        now = 1470174257070
+        json = {"key": "value"}
+
+        yield self.store.store_device("user", "device", None)
+
+        changed = yield self.store.set_e2e_device_keys("user", "device", now, json)
+        self.assertTrue(changed)
+
+        # If we try to upload the same key then we should be told nothing
+        # changed
+        changed = yield self.store.set_e2e_device_keys("user", "device", now, json)
+        self.assertFalse(changed)
+
     @defer.inlineCallbacks
     def test_get_key_with_device_name(self):
         now = 1470174257070