Implement message forward pagination from start when no from is given, fixes #12383 (#14149)

Fixes https://github.com/matrix-org/synapse/issues/12383

2 files changed, 19 insertions, 0 deletions

diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
index a4ca9cb8b4..c572508a02 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -448,6 +448,12 @@ class PaginationHandler:
 
         if pagin_config.from_token:
             from_token = pagin_config.from_token
+        elif pagin_config.direction == "f":
+            from_token = (
+                await self.hs.get_event_sources().get_start_token_for_pagination(
+                    room_id
+                )
+            )
         else:
             from_token = (
                 await self.hs.get_event_sources().get_current_token_for_pagination(
diff --git a/synapse/streams/events.py b/synapse/streams/events.py
index f331e1af16..619eb7f601 100644
--- a/synapse/streams/events.py
+++ b/synapse/streams/events.py
@@ -74,6 +74,19 @@ class EventSources:
         return token
 
     @trace
+    async def get_start_token_for_pagination(self, room_id: str) -> StreamToken:
+        """Get the start token for a given room to be used to paginate
+        events.
+
+        The returned token does not have the current values for fields other
+        than `room`, since they are not used during pagination.
+
+        Returns:
+            The start token for pagination.
+        """
+        return StreamToken.START
+
+    @trace
     async def get_current_token_for_pagination(self, room_id: str) -> StreamToken:
         """Get the current token for a given room to be used to paginate
         events.