From 99926affef9578bdb23f0d9911eafe98c3b75c2e Mon Sep 17 00:00:00 2001
From: Michael Telatynski <7t3chguy@gmail.com>
Date: Thu, 22 Sep 2022 11:46:57 +0100
Subject: [PATCH] Sync hot-path optimisation for `filter_events_for_client`

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>
---
 synapse/handlers/sync.py |  2 ++
 synapse/visibility.py    | 15 ++++++++++-----
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 5293fa4d0e..bb348db3b9 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -539,6 +539,7 @@ class SyncHandler:
                     sync_config.user.to_string(),
                     recents,
                     always_include_ids=current_state_ids,
+                    limit=timeline_limit + 1,
                 )
                 log_kv({"recents_after_visibility_filtering": len(recents)})
             else:
@@ -616,6 +617,7 @@ class SyncHandler:
                     sync_config.user.to_string(),
                     loaded_recents,
                     always_include_ids=current_state_ids,
+                    limit=timeline_limit + 1 - len(recents),
                 )
 
                 log_kv({"loaded_recents_after_client_filtering": len(loaded_recents)})
diff --git a/synapse/visibility.py b/synapse/visibility.py
index c810a05907..733b0a7027 100644
--- a/synapse/visibility.py
+++ b/synapse/visibility.py
@@ -60,6 +60,7 @@ async def filter_events_for_client(
     is_peeking: bool = False,
     always_include_ids: FrozenSet[str] = frozenset(),
     filter_send_to_client: bool = True,
+    limit: int = -1,
 ) -> List[EventBase]:
     """
     Check which events a user is allowed to see. If the user can see the event but its
@@ -78,6 +79,7 @@ async def filter_events_for_client(
         filter_send_to_client: Whether we're checking an event that's going to be
             sent to a client. This might not always be the case since this function can
             also be called to check whether a user can see the state at a given point.
+        limit: The number of events to bail at, as a hot path optimisation.
 
     Returns:
         The filtered events.
@@ -122,12 +124,15 @@ async def filter_events_for_client(
             sender_erased=erased_senders.get(event.sender, False),
         )
 
-    # Check each event: gives an iterable of None or (a potentially modified)
-    # EventBase.
-    filtered_events = map(allowed, events)
+    filtered_events: List[EventBase] = []
+    for event in events:
+        checked = allowed(event)
+        if checked is not None:
+            filtered_events.append(checked)
+        if len(filtered_events) >= limit >= 0:
+            break
 
-    # Turn it into a list and remove None entries before returning.
-    return [ev for ev in filtered_events if ev]
+    return filtered_events
 
 
 async def filter_event_for_clients_with_state(