fix(event cache): give looser parameters for the deduplicator's bloom filters

crates/matrix-sdk/src/event_cache/deduplicator.rs

@@ -41,8 +41,10 @@ impl fmt::Debug for Deduplicator {
 }
 
 impl Deduplicator {
-    const APPROXIMATED_MAXIMUM_NUMBER_OF_EVENTS: usize = 800_000;
-    const DESIRED_FALSE_POSITIVE_RATE: f64 = 0.001;
+    // Note: don't use too high numbers here, or the amount of allocated memory will
+    // explode. See https://github.com/matrix-org/matrix-rust-sdk/pull/4231 for details.
+    const APPROXIMATED_MAXIMUM_NUMBER_OF_EVENTS: usize = 1_000;
+    const DESIRED_FALSE_POSITIVE_RATE: f64 = 0.01;
 
     /// Create a new `Deduplicator`.
     pub fn new() -> Self {
@@ -138,7 +140,7 @@ pub enum Decoration<I> {
 
 #[cfg(test)]
 mod tests {
-    use assert_matches2::assert_let;
+    use assert_matches2::{assert_let, assert_matches};
     use matrix_sdk_base::deserialized_responses::SyncTimelineEvent;
     use ruma::{owned_event_id, user_id, EventId};
 
@@ -267,4 +269,40 @@ mod tests {
             assert!(events.next().is_none());
         }
     }
+
+    #[test]
+    fn test_bloom_filter_growth() {
+        // This test was used as a testbed to observe, using `valgrind --tool=massive`,
+        // the total memory allocated by the deduplicator. We keep it checked in
+        // to revive this experiment in the future, if needs be.
+
+        let num_rooms = if let Ok(num_rooms) = std::env::var("ROOMS") {
+            num_rooms.parse().unwrap()
+        } else {
+            10
+        };
+
+        let num_events = if let Ok(num_events) = std::env::var("EVENTS") {
+            num_events.parse().unwrap()
+        } else {
+            100
+        };
+
+        let mut dedups = Vec::with_capacity(num_rooms);
+
+        for _ in 0..num_rooms {
+            let dedup = Deduplicator::new();
+            let existing_events = RoomEvents::new();
+
+            for i in 0..num_events {
+                let event = sync_timeline_event(&EventId::parse(format!("$event{i}")).unwrap());
+                let mut it = dedup.scan_and_learn([event].into_iter(), &existing_events);
+
+                assert_matches!(it.next(), Some(Decoration::Unique(..)));
+                assert_matches!(it.next(), None);
+            }
+
+            dedups.push(dedup);
+        }
+    }
 }