Comment why we use .START (and refactor to make it a .START)

synapse/handlers/sliding_sync/extensions.py

@@ -1005,9 +1005,9 @@ class SlidingSyncExtensionHandler:
         if not sticky_events_request.enabled:
             return None
         now = self.clock.time_msec()
-        since_token = sticky_events_request.since or SlidingSyncStickyEventsToken(
-            sticky_events_stream_id=0
-        )
+        # If there is no `since` token specified, start from the beginning of the stream
+        # to make sure the client receives all visible (unexpired) sticky events
+        since_token = sticky_events_request.since or SlidingSyncStickyEventsToken.START
         (
             sticky_events_to_id,
             room_to_event_ids,

synapse/types/rest/client/__init__.py

@@ -19,6 +19,7 @@
 #
 #
 import re
+from typing import ClassVar
 
 import pydantic_core.core_schema
 from pydantic import (
@@ -126,6 +127,7 @@ class SlidingSyncStickyEventsToken:
     """
 
     PATTERN = re.compile(r"^sticky_([0-9]+)$")
+    START: ClassVar["SlidingSyncStickyEventsToken"]
 
     def __init__(self, *, sticky_events_stream_id: int) -> None:
         self.sticky_events_stream_id = sticky_events_stream_id
@@ -160,6 +162,12 @@ class SlidingSyncStickyEventsToken:
         return self.serialise()
 
 
+# Starting reading a stream at 0 ensures all stream fact rows wlil be read
+SlidingSyncStickyEventsToken.START = SlidingSyncStickyEventsToken(
+    sticky_events_stream_id=0
+)
+
+
 class SlidingSyncBody(RequestBodyModel):
     """
     Sliding Sync API request body.