feat(ui): Create a task to listen to redecryptor reports in the timeline

This task is still necessary because the redecryptor in the event cache
might miss some room keys.

In this case the timeline can tell the redecryptor which events it
should retry to decrypt.

We're collecting all the UTDs in the timeline and telling the
redecryptor to do its best.

Author: poljar

crates/matrix-sdk-ui/src/timeline/controller/decryption_retry_task.rs

@@ -25,6 +25,7 @@ use matrix_sdk::{
     Client, Room,
     deserialized_responses::TimelineEventKind as SdkTimelineEventKind,
     encryption::backups::BackupState,
+    event_cache::{self, RedecryptorReport},
     event_handler::EventHandlerHandle,
     executor::{JoinHandle, spawn},
 };
@@ -103,6 +104,32 @@ pub(super) fn compute_redecryption_candidates(
         })
 }
 
+async fn redecryption_report_task(timeline_controller: TimelineController) {
+    let client = timeline_controller.room().client();
+    let stream = client.event_cache().subscribe_to_decryption_reports();
+
+    pin_mut!(stream);
+
+    while let Some(report) = stream.next().await {
+        match report {
+            Ok(RedecryptorReport::ResolvedUtds { events, .. }) => {
+                let state = timeline_controller.state.read().await;
+
+                if let Some(utd_hook) = &state.meta.unable_to_decrypt_hook {
+                    for event_id in events {
+                        utd_hook.on_late_decrypt(&event_id).await;
+                    }
+                }
+            }
+            Ok(RedecryptorReport::Lagging) | Err(_) => {
+                // The room key stream lagged or the OlmMachine got regenerated. Let's tell the
+                // redecryptor to attempt redecryption of our timeline items.
+                timeline_controller.retry_event_decryption(None).await;
+            }
+        }
+    }
+}
+
 /// The task that handles the room keys from backups.
 async fn room_keys_from_backups_task<S>(stream: S, timeline_controller: TimelineController)
 where