Leave out optional keys from /sync (#9919)

This leaves out all optional keys from /sync. This should be fine for all clients tested against conduit already, but it may break some clients, as such we should check, that at least most of them don't break horribly and maybe back out some of the individual changes. (We can probably always leave out groups for example, while the others may cause more issues.)

Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>

4 files changed, 51 insertions, 50 deletions

diff --git a/changelog.d/9919.feature b/changelog.d/9919.feature
new file mode 100644
index 0000000000..07747505d2
--- /dev/null
+++ b/changelog.d/9919.feature
@@ -0,0 +1 @@
+Omit empty fields from the `/sync` response.  Contributed by @deepbluev7.
diff --git a/synapse/rest/client/v2_alpha/sync.py b/synapse/rest/client/v2_alpha/sync.py
index 95ee3f1b84..5f85653330 100644
--- a/synapse/rest/client/v2_alpha/sync.py
+++ b/synapse/rest/client/v2_alpha/sync.py
@@ -14,6 +14,7 @@
 
 import itertools
 import logging
+from collections import defaultdict
 from typing import TYPE_CHECKING, Tuple
 
 from synapse.api.constants import PresenceState
@@ -229,24 +230,49 @@ class SyncRestServlet(RestServlet):
         )
 
         logger.debug("building sync response dict")
-        return {
-            "account_data": {"events": sync_result.account_data},
-            "to_device": {"events": sync_result.to_device},
-            "device_lists": {
-                "changed": list(sync_result.device_lists.changed),
-                "left": list(sync_result.device_lists.left),
-            },
-            "presence": SyncRestServlet.encode_presence(sync_result.presence, time_now),
-            "rooms": {"join": joined, "invite": invited, "leave": archived},
-            "groups": {
-                "join": sync_result.groups.join,
-                "invite": sync_result.groups.invite,
-                "leave": sync_result.groups.leave,
-            },
-            "device_one_time_keys_count": sync_result.device_one_time_keys_count,
-            "org.matrix.msc2732.device_unused_fallback_key_types": sync_result.device_unused_fallback_key_types,
-            "next_batch": await sync_result.next_batch.to_string(self.store),
-        }
+
+        response: dict = defaultdict(dict)
+        response["next_batch"] = await sync_result.next_batch.to_string(self.store)
+
+        if sync_result.account_data:
+            response["account_data"] = {"events": sync_result.account_data}
+        if sync_result.presence:
+            response["presence"] = SyncRestServlet.encode_presence(
+                sync_result.presence, time_now
+            )
+
+        if sync_result.to_device:
+            response["to_device"] = {"events": sync_result.to_device}
+
+        if sync_result.device_lists.changed:
+            response["device_lists"]["changed"] = list(sync_result.device_lists.changed)
+        if sync_result.device_lists.left:
+            response["device_lists"]["left"] = list(sync_result.device_lists.left)
+
+        if sync_result.device_one_time_keys_count:
+            response[
+                "device_one_time_keys_count"
+            ] = sync_result.device_one_time_keys_count
+        if sync_result.device_unused_fallback_key_types:
+            response[
+                "org.matrix.msc2732.device_unused_fallback_key_types"
+            ] = sync_result.device_unused_fallback_key_types
+
+        if joined:
+            response["rooms"]["join"] = joined
+        if invited:
+            response["rooms"]["invite"] = invited
+        if archived:
+            response["rooms"]["leave"] = archived
+
+        if sync_result.groups.join:
+            response["groups"]["join"] = sync_result.groups.join
+        if sync_result.groups.invite:
+            response["groups"]["invite"] = sync_result.groups.invite
+        if sync_result.groups.leave:
+            response["groups"]["leave"] = sync_result.groups.leave
+
+        return response
 
     @staticmethod
     def encode_presence(events, time_now):
diff --git a/tests/rest/client/v2_alpha/test_sync.py b/tests/rest/client/v2_alpha/test_sync.py
index dbcbdf159a..74be5176d0 100644
--- a/tests/rest/client/v2_alpha/test_sync.py
+++ b/tests/rest/client/v2_alpha/test_sync.py
@@ -37,35 +37,7 @@ class FilterTestCase(unittest.HomeserverTestCase):
         channel = self.make_request("GET", "/sync")
 
         self.assertEqual(channel.code, 200)
-        self.assertTrue(
-            {
-                "next_batch",
-                "rooms",
-                "presence",
-                "account_data",
-                "to_device",
-                "device_lists",
-            }.issubset(set(channel.json_body.keys()))
-        )
-
-    def test_sync_presence_disabled(self):
-        """
-        When presence is disabled, the key does not appear in /sync.
-        """
-        self.hs.config.use_presence = False
-
-        channel = self.make_request("GET", "/sync")
-
-        self.assertEqual(channel.code, 200)
-        self.assertTrue(
-            {
-                "next_batch",
-                "rooms",
-                "account_data",
-                "to_device",
-                "device_lists",
-            }.issubset(set(channel.json_body.keys()))
-        )
+        self.assertIn("next_batch", channel.json_body)
 
 
 class SyncFilterTestCase(unittest.HomeserverTestCase):
diff --git a/tests/server_notices/test_resource_limits_server_notices.py b/tests/server_notices/test_resource_limits_server_notices.py
index d46521ccdc..3245aa91ca 100644
--- a/tests/server_notices/test_resource_limits_server_notices.py
+++ b/tests/server_notices/test_resource_limits_server_notices.py
@@ -306,8 +306,9 @@ class TestResourceLimitsServerNoticesWithRealRooms(unittest.HomeserverTestCase):
 
         channel = self.make_request("GET", "/sync?timeout=0", access_token=tok)
 
-        invites = channel.json_body["rooms"]["invite"]
-        self.assertEqual(len(invites), 0, invites)
+        self.assertNotIn(
+            "rooms", channel.json_body, "Got invites without server notice"
+        )
 
     def test_invite_with_notice(self):
         """Tests that, if the MAU limit is hit, the server notices user invites each user
@@ -364,7 +365,8 @@ class TestResourceLimitsServerNoticesWithRealRooms(unittest.HomeserverTestCase):
             # We could also pick another user and sync with it, which would return an
             # invite to a system notices room, but it doesn't matter which user we're
             # using so we use the last one because it saves us an extra sync.
-            invites = channel.json_body["rooms"]["invite"]
+            if "rooms" in channel.json_body:
+                invites = channel.json_body["rooms"]["invite"]
 
         # Make sure we have an invite to process.
         self.assertEqual(len(invites), 1, invites)