From 4cee8c7b9995b4075ac3a41bb3e53c5a7b4377a1 Mon Sep 17 00:00:00 2001
From: Eric Eastwood <erice@element.io>
Date: Mon, 18 Aug 2025 09:28:59 -0500
Subject: [PATCH] Instrument `encode_response` with tracing in Sliding Sync
 requests (#18815)

Just filling in the hole I see in the trace after
`current_sync_for_user`.
---
 changelog.d/18815.misc      |  1 +
 synapse/events/utils.py     | 11 +++++++++--
 synapse/rest/client/sync.py |  4 ++++
 3 files changed, 14 insertions(+), 2 deletions(-)
 create mode 100644 changelog.d/18815.misc

diff --git a/changelog.d/18815.misc b/changelog.d/18815.misc
new file mode 100644
index 000000000..ad369e592
--- /dev/null
+++ b/changelog.d/18815.misc
@@ -0,0 +1 @@
+Instrument the `encode_response` part of Sliding Sync requests for more complete traces in Jaeger.
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index cae27136c..942072cf8 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -26,8 +26,8 @@ from typing import (
     Any,
     Awaitable,
     Callable,
+    Collection,
     Dict,
-    Iterable,
     List,
     Mapping,
     Match,
@@ -49,6 +49,7 @@ from synapse.api.constants import (
 )
 from synapse.api.errors import Codes, SynapseError
 from synapse.api.room_versions import RoomVersion
+from synapse.logging.opentracing import SynapseTags, set_tag, trace
 from synapse.types import JsonDict, Requester
 
 from . import EventBase, StrippedStateEvent, make_event_from_dict
@@ -710,9 +711,10 @@ class EventClientSerializer:
                 "m.relations", {}
             ).update(serialized_aggregations)
 
+    @trace
     async def serialize_events(
         self,
-        events: Iterable[Union[JsonDict, EventBase]],
+        events: Collection[Union[JsonDict, EventBase]],
         time_now: int,
         *,
         config: SerializeEventConfig = _DEFAULT_SERIALIZE_EVENT_CONFIG,
@@ -731,6 +733,11 @@ class EventClientSerializer:
         Returns:
             The list of serialized events
         """
+        set_tag(
+            SynapseTags.FUNC_ARG_PREFIX + "events.length",
+            str(len(events)),
+        )
+
         return [
             await self.serialize_event(
                 event,
diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index b6789a059..5e0596d1b 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -1017,6 +1017,7 @@ class SlidingSyncRestServlet(RestServlet):
 
         return 200, response_content
 
+    @trace_with_opname("sliding_sync.encode_response")
     async def encode_response(
         self,
         requester: Requester,
@@ -1037,6 +1038,7 @@ class SlidingSyncRestServlet(RestServlet):
 
         return response
 
+    @trace_with_opname("sliding_sync.encode_lists")
     def encode_lists(
         self, lists: Mapping[str, SlidingSyncResult.SlidingWindowList]
     ) -> JsonDict:
@@ -1058,6 +1060,7 @@ class SlidingSyncRestServlet(RestServlet):
 
         return serialized_lists
 
+    @trace_with_opname("sliding_sync.encode_rooms")
     async def encode_rooms(
         self,
         requester: Requester,
@@ -1178,6 +1181,7 @@ class SlidingSyncRestServlet(RestServlet):
 
         return serialized_rooms
 
+    @trace_with_opname("sliding_sync.encode_extensions")
     async def encode_extensions(
         self, requester: Requester, extensions: SlidingSyncResult.Extensions
     ) -> JsonDict: