From 7b671f4dd65bd9b7414397198715e820b407cbbd Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 18 Mar 2026 13:27:17 +0000
Subject: [PATCH] Move sticky_ttl_ms serialization to serialize_event

---
 synapse/events/utils.py | 23 +++++++++++++++++++++++
 synapse/visibility.py   | 14 --------------
 2 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index 3919e752ac..7166341a28 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -41,6 +41,7 @@ from synapse.api.constants import (
     MAX_PDU_SIZE,
     EventContentFields,
     EventTypes,
+    EventUnsignedContentFields,
     RelationTypes,
 )
 from synapse.api.errors import Codes, SynapseError
@@ -435,6 +436,9 @@ class SerializeEventConfig:
     # only server admins can see through other configuration. For example,
     # whether an event was soft failed by the server.
     include_admin_metadata: bool = False
+    # Whether MSC4354 (sticky events) is enabled. When True, the sticky TTL
+    # will be computed and included in the unsigned section of sticky events.
+    msc4354_enabled: bool = False
 
 
 _DEFAULT_SERIALIZE_EVENT_CONFIG = SerializeEventConfig()
@@ -554,6 +558,20 @@ def _serialize_event(
         if e.internal_metadata.policy_server_spammy:
             d["unsigned"]["io.element.synapse.policy_server_spammy"] = True
 
+    if config.msc4354_enabled:
+        sticky_duration = e.sticky_duration()
+        if sticky_duration:
+            expires_at = (
+                # min() ensures that the origin server can't lie about the time and
+                # send the event 'in the future', as that would allow them to exceed
+                # the 1 hour limit on stickiness duration.
+                min(e.origin_server_ts, time_now_ms) + sticky_duration.as_millis()
+            )
+            if expires_at > time_now_ms:
+                d["unsigned"][EventUnsignedContentFields.STICKY_TTL] = (
+                    expires_at - time_now_ms
+                )
+
     only_event_fields = config.only_event_fields
     if only_event_fields:
         if not isinstance(only_event_fields, list) or not all(
@@ -575,6 +593,8 @@ class EventClientSerializer:
     def __init__(self, hs: "HomeServer") -> None:
         self._store = hs.get_datastores().main
         self._auth = hs.get_auth()
+        self._config = hs.config
+        self._clock = hs.get_clock()
         self._add_extra_fields_to_unsigned_client_event_callbacks: list[
             ADD_EXTRA_FIELDS_TO_UNSIGNED_CLIENT_EVENT_CALLBACK
         ] = []
@@ -615,6 +635,9 @@ class EventClientSerializer:
         ):
             config = make_config_for_admin(config)
 
+        if self._config.experimental.msc4354_enabled:
+            config = attr.evolve(config, msc4354_enabled=True)
+
         serialized_event = _serialize_event(event, time_now, config=config)
 
         # If the event was redacted, fetch the redaction event from the database
diff --git a/synapse/visibility.py b/synapse/visibility.py
index 5ba2a14a24..452a2d50fb 100644
--- a/synapse/visibility.py
+++ b/synapse/visibility.py
@@ -237,20 +237,6 @@ async def filter_and_transform_events_for_client(
         # to the cache!
         cloned = clone_event(filtered)
         cloned.unsigned[EventUnsignedContentFields.MEMBERSHIP] = user_membership
-        if storage.main.config.experimental.msc4354_enabled:
-            sticky_duration = cloned.sticky_duration()
-            if sticky_duration:
-                now_ms = storage.main.clock.time_msec()
-                expires_at = (
-                    # min() ensures that the origin server can't lie about the time and
-                    # send the event 'in the future', as that would allow them to exceed
-                    # the 1 hour limit on stickiness duration.
-                    min(cloned.origin_server_ts, now_ms) + sticky_duration.as_millis()
-                )
-                if expires_at > now_ms:
-                    cloned.unsigned[EventUnsignedContentFields.STICKY_TTL] = (
-                        expires_at - now_ms
-                    )
 
         return cloned