From e79f83e5cc7b561fae21d79283f64c135b95fc4e Mon Sep 17 00:00:00 2001
From: Ivan Enderlin <ivan@mnt.io>
Date: Tue, 26 May 2026 14:31:18 +0200
Subject: [PATCH] fix(sdk): Re-compute the `ThreadSummary` when R2D2 redecrypts
 an event in a thread.

---
 .../src/event_cache/caches/room/mod.rs        |  2 +-
 .../src/event_cache/caches/thread/mod.rs      | 31 +++++++++-----
 .../matrix-sdk/src/event_cache/redecryptor.rs | 40 ++++++++++++++-----
 3 files changed, 52 insertions(+), 21 deletions(-)

diff --git a/crates/matrix-sdk/src/event_cache/caches/room/mod.rs b/crates/matrix-sdk/src/event_cache/caches/room/mod.rs
index 734896d64..faf59a53c 100644
--- a/crates/matrix-sdk/src/event_cache/caches/room/mod.rs
+++ b/crates/matrix-sdk/src/event_cache/caches/room/mod.rs
@@ -261,7 +261,7 @@ impl RoomEventCache {
         Ok(())
     }
 
-    pub(super) async fn update_thread_summary(
+    pub(in super::super) async fn update_thread_summary(
         &self,
         thread_id: &EventId,
         new_thread_summary: Option<ThreadSummary>,
diff --git a/crates/matrix-sdk/src/event_cache/caches/thread/mod.rs b/crates/matrix-sdk/src/event_cache/caches/thread/mod.rs
index a9347ebc1..58733c0ff 100644
--- a/crates/matrix-sdk/src/event_cache/caches/thread/mod.rs
+++ b/crates/matrix-sdk/src/event_cache/caches/thread/mod.rs
@@ -162,7 +162,7 @@ impl ThreadEventCache {
     }
 
     /// Return a reference to the state.
-    pub(super) fn state(&self) -> &LockedThreadEventCacheState {
+    pub(in super::super) fn state(&self) -> &LockedThreadEventCacheState {
         &self.inner.state
     }
 
@@ -260,20 +260,29 @@ impl ThreadEventCache {
     /// Try to locate the events in the linked chunk corresponding to the given
     /// list of decrypted events, and replace them, while alerting observers
     /// about the update.
+    ///
+    /// Return `true` if at least one event has been updated.
     #[cfg(feature = "e2e-encryption")]
-    pub(in super::super) async fn replace_utds(&self, events: &[ResolvedUtd]) -> Result<()> {
+    pub(in super::super) async fn replace_utds(&self, events: &[ResolvedUtd]) -> Result<bool> {
         let timeline_event_diffs = self.inner.state.write().await?.replace_utds(events).await?;
 
-        if let Some(timeline_event_diffs) = timeline_event_diffs
-            && !timeline_event_diffs.is_empty()
-        {
-            self.inner.update_sender.send(
-                TimelineVectorDiffs { diffs: timeline_event_diffs, origin: EventsOrigin::Cache },
-                Some(RoomEventCacheGenericUpdate { room_id: self.inner.room_id.clone() }),
-            );
-        }
+        Ok(
+            if let Some(timeline_event_diffs) = timeline_event_diffs
+                && !timeline_event_diffs.is_empty()
+            {
+                self.inner.update_sender.send(
+                    TimelineVectorDiffs {
+                        diffs: timeline_event_diffs,
+                        origin: EventsOrigin::Cache,
+                    },
+                    Some(RoomEventCacheGenericUpdate { room_id: self.inner.room_id.clone() }),
+                );
 
-        Ok(())
+                true
+            } else {
+                false
+            },
+        )
     }
 }
 
diff --git a/crates/matrix-sdk/src/event_cache/redecryptor.rs b/crates/matrix-sdk/src/event_cache/redecryptor.rs
index 746e437ed..0bd04c6d6 100644
--- a/crates/matrix-sdk/src/event_cache/redecryptor.rs
+++ b/crates/matrix-sdk/src/event_cache/redecryptor.rs
@@ -121,7 +121,11 @@ use std::{
 
 use as_variant::as_variant;
 use futures_core::Stream;
-use futures_util::{StreamExt, future::join_all, pin_mut};
+use futures_util::{
+    StreamExt,
+    future::{join_all, try_join_all},
+    pin_mut,
+};
 #[cfg(doc)]
 use matrix_sdk_base::{BaseClient, crypto::OlmMachine};
 use matrix_sdk_base::{
@@ -416,15 +420,33 @@ impl EventCache {
             // accumulate over time. Consider keeping track of which linked
             // chunk contain which event id, to avoid doing the linear searches
             // here.
-            join_all(
-                all_caches
-                    .threads
-                    .read()
-                    .await
-                    .values()
-                    .map(|thread_cache| thread_cache.replace_utds(&events)),
+
+            // Replaces UTDs in each thread, and maybe update the thread summary.
+            for (thread_id, thread_cache) in try_join_all(
+                all_caches.threads.read().await.iter().map(|(thread_id, thread_cache)| async {
+                    Result::<_, EventCacheError>::Ok(
+                        // If at least one event has been replaced, return the `thread_id` and the
+                        // `thread_cache` to update the thread summary later.
+                        thread_cache
+                            .replace_utds(&events)
+                            .await?
+                            .then(|| (thread_id.clone(), thread_cache.clone())),
+                    )
+                }),
             )
-            .await;
+            .await?
+            .into_iter()
+            // Filter out results that are `None`, i.e. a thread where no UTD has been replaced.
+            .flatten()
+            {
+                all_caches
+                    .room
+                    .update_thread_summary(
+                        &thread_id,
+                        thread_cache.state().read().await?.compute_thread_summary().await?,
+                    )
+                    .await?;
+            }
         }
 
         // Resolve on the pinned-events cache.