From f59419377d4ef34952ee7ab898668ca94cd5d6d4 Mon Sep 17 00:00:00 2001
From: Devon Hudson <devonhudson@librem.one>
Date: Sun, 9 Nov 2025 09:35:11 -0700
Subject: [PATCH] Refactor for clarity

---
 synapse/handlers/sliding_sync/extensions.py | 79 +++++++++++++--------
 1 file changed, 49 insertions(+), 30 deletions(-)

diff --git a/synapse/handlers/sliding_sync/extensions.py b/synapse/handlers/sliding_sync/extensions.py
index de50560583..c2dffa4bf6 100644
--- a/synapse/handlers/sliding_sync/extensions.py
+++ b/synapse/handlers/sliding_sync/extensions.py
@@ -1044,6 +1044,43 @@ class SlidingSyncExtensionHandler:
                         threads_in_timeline.add(thread_id)
         return threads_in_timeline
 
+    def _merge_prev_batch_token(
+        self,
+        current_token: Optional[StreamToken],
+        new_token: Optional[StreamToken],
+    ) -> Optional[StreamToken]:
+        """Merge two prev_batch tokens, taking the maximum (latest) for backwards pagination.
+
+        Args:
+            current_token: The current prev_batch token (may be None)
+            new_token: The new prev_batch token to merge (may be None)
+
+        Returns:
+            The merged token (maximum of the two, or None if both are None)
+        """
+        if new_token is None:
+            return current_token
+        if current_token is None:
+            return new_token
+        # Take the maximum (latest) prev_batch token for backwards pagination
+        if new_token.room_key.stream > current_token.room_key.stream:
+            return new_token
+        return current_token
+
+    def _merge_thread_updates(
+        self,
+        target: Dict[str, List[ThreadUpdateInfo]],
+        source: Dict[str, List[ThreadUpdateInfo]],
+    ) -> None:
+        """Merge thread updates from source into target.
+
+        Args:
+            target: The target dict to merge into (modified in place)
+            source: The source dict to merge from
+        """
+        for thread_id, updates in source.items():
+            target.setdefault(thread_id, []).extend(updates)
+
     async def get_threads_extension_response(
         self,
         sync_config: SlidingSyncConfig,
@@ -1136,21 +1173,12 @@ class SlidingSyncExtensionHandler:
                 remaining_limit -= num_updates
 
                 # Merge results
-                for thread_id, updates in room_thread_updates.items():
-                    all_thread_updates.setdefault(thread_id, []).extend(updates)
+                self._merge_thread_updates(all_thread_updates, room_thread_updates)
 
-                # If any room has a prev_batch, we should set the global prev_batch.
-                # We use the maximum (latest) prev_batch token for backwards pagination.
-                if room_prev_batch is not None:
-                    if prev_batch_token is None:
-                        prev_batch_token = room_prev_batch
-                    else:
-                        # Take the maximum (latest) prev_batch token for backwards pagination
-                        if (
-                            room_prev_batch.room_key.stream
-                            > prev_batch_token.room_key.stream
-                        ):
-                            prev_batch_token = room_prev_batch
+                # Merge prev_batch tokens
+                prev_batch_token = self._merge_prev_batch_token(
+                    prev_batch_token, room_prev_batch
+                )
 
         # Query for rooms where the user is joined, invited, or knocking, using the
         # normal to_token as the upper bound.
@@ -1167,20 +1195,12 @@ class SlidingSyncExtensionHandler:
             )
 
             # Merge results
-            for thread_id, updates in other_thread_updates.items():
-                all_thread_updates.setdefault(thread_id, []).extend(updates)
+            self._merge_thread_updates(all_thread_updates, other_thread_updates)
 
             # Merge prev_batch tokens
-            if other_prev_batch is not None:
-                if prev_batch_token is None:
-                    prev_batch_token = other_prev_batch
-                else:
-                    # Take the maximum (latest) prev_batch token for backwards pagination
-                    if (
-                        other_prev_batch.room_key.stream
-                        > prev_batch_token.room_key.stream
-                    ):
-                        prev_batch_token = other_prev_batch
+            prev_batch_token = self._merge_prev_batch_token(
+                prev_batch_token, other_prev_batch
+            )
 
         # Early return: no thread updates found
         if len(all_thread_updates) == 0:
@@ -1210,10 +1230,9 @@ class SlidingSyncExtensionHandler:
         if not filtered_updates:
             return None
 
-        # Sort updates for each thread by stream_ordering DESC to ensure updates[0] is the latest.
-        # This is critical because the prev_batch token generation below assumes DESC order.
-        for updates in filtered_updates.values():
-            updates.sort(key=lambda u: u.stream_ordering, reverse=True)
+        # Note: Updates are already sorted by stream_ordering DESC from the database query,
+        # and filter_events_for_client preserves order, so updates[0] is guaranteed to be
+        # the latest event for each thread.
 
         # Optionally fetch thread root events and their bundled aggregations
         thread_root_event_map = {}