improvement: faster /syncs

src/client_server/sync.rs

@@ -245,6 +245,9 @@ async fn sync_helper(
         let insert_lock = mutex_insert.lock().unwrap();
         drop(insert_lock);
 
+        let timeline_pdus;
+        let limited;
+        if db.rooms.last_timeline_count(&sender_user, &room_id)? > since {
             let mut non_timeline_pdus = db
                 .rooms
                 .pdus_until(&sender_user, &room_id, u64::MAX)?
@@ -262,13 +265,21 @@ async fn sync_helper(
                 });
 
             // Take the last 10 events for the timeline
-        let timeline_pdus: Vec<_> = non_timeline_pdus
+            timeline_pdus = non_timeline_pdus
                 .by_ref()
                 .take(10)
                 .collect::<Vec<_>>()
                 .into_iter()
                 .rev()
-            .collect();
+                .collect::<Vec<_>>();
+
+            // They /sync response doesn't always return all messages, so we say the output is
+            // limited unless there are events in non_timeline_pdus
+            limited = non_timeline_pdus.next().is_some();
+        } else {
+            timeline_pdus = Vec::new();
+            limited = false;
+        }
 
         let send_notification_counts = !timeline_pdus.is_empty()
             || db
@@ -277,10 +288,6 @@ async fn sync_helper(
                 .last_privateread_update(&sender_user, &room_id)?
                 > since;
 
-        // They /sync response doesn't always return all messages, so we say the output is
-        // limited unless there are events in non_timeline_pdus
-        let limited = non_timeline_pdus.next().is_some();
-
         let mut timeline_users = HashSet::new();
         for (_, event) in &timeline_pdus {
             timeline_users.insert(event.sender.as_str().to_owned());

src/database.rs

@@ -263,6 +263,7 @@ impl Database {
                 stateinfo_cache: Mutex::new(LruCache::new(
                     (100.0 * config.conduit_cache_capacity_modifier) as usize,
                 )),
+                lasttimelinecount_cache: Mutex::new(HashMap::new()),
             },
             account_data: account_data::AccountData {
                 roomuserdataid_accountdata: builder.open_tree("roomuserdataid_accountdata")?,

src/database/rooms.rs

@@ -32,7 +32,7 @@ use serde::Deserialize;
 use serde_json::value::to_raw_value;
 use std::{
     borrow::Cow,
-    collections::{BTreeMap, HashMap, HashSet},
+    collections::{hash_map, BTreeMap, HashMap, HashSet},
     fmt::Debug,
     iter,
     mem::size_of,
@@ -128,6 +128,7 @@ pub struct Rooms {
             )>,
         >,
     >,
+    pub(super) lasttimelinecount_cache: Mutex<HashMap<Box<RoomId>, u64>>,
 }
 
 impl Rooms {
@@ -1331,6 +1332,10 @@ impl Rooms {
             &pdu_id,
             &serde_json::to_vec(&pdu_json).expect("CanonicalJsonObject is always a valid"),
         )?;
+        self.lasttimelinecount_cache
+            .lock()
+            .unwrap()
+            .insert(pdu.room_id.clone(), count2);
 
         self.eventid_pduid
             .insert(pdu.event_id.as_bytes(), &pdu_id)?;
@@ -1498,6 +1503,36 @@ impl Rooms {
         Ok(pdu_id)
     }
 
+    #[tracing::instrument(skip(self))]
+    pub fn last_timeline_count(&self, sender_user: &UserId, room_id: &RoomId) -> Result<u64> {
+        match self
+            .lasttimelinecount_cache
+            .lock()
+            .unwrap()
+            .entry(room_id.to_owned())
+        {
+            hash_map::Entry::Vacant(v) => {
+                if let Some(last_count) = self
+                    .pdus_until(&sender_user, &room_id, u64::MAX)?
+                    .filter_map(|r| {
+                        // Filter out buggy events
+                        if r.is_err() {
+                            error!("Bad pdu in pdus_since: {:?}", r);
+                        }
+                        r.ok()
+                    })
+                    .map(|(pduid, _)| self.pdu_count(&pduid))
+                    .next()
+                {
+                    Ok(*v.insert(last_count?))
+                } else {
+                    Ok(0)
+                }
+            }
+            hash_map::Entry::Occupied(o) => Ok(*o.get()),
+        }
+    }
+
     #[tracing::instrument(skip(self))]
     pub fn reset_notification_counts(&self, user_id: &UserId, room_id: &RoomId) -> Result<()> {
         let mut userroom_id = user_id.as_bytes().to_vec();