timeline: store UTDs in decrypt_room_event

When `decrypt_room_event` fails to decrypt an event, return the UTD as a
`TimelineEvent` instead of an Error.

crates/matrix-sdk-common/src/deserialized_responses.rs

@@ -673,6 +673,14 @@ pub enum UnableToDecryptReason {
     SenderIdentityNotTrusted(VerificationLevel),
 }
 
+impl UnableToDecryptReason {
+    /// Returns true if this UTD is due to a missing room key (and hence might
+    /// resolve itself if we wait a bit.)
+    pub fn is_missing_room_key(&self) -> bool {
+        matches!(self, Self::MissingMegolmSession | Self::UnknownMegolmMessageIndex)
+    }
+}
+
 /// Deserialization helper for [`SyncTimelineEvent`], for the modern format.
 ///
 /// This has the exact same fields as [`SyncTimelineEvent`] itself, but has a

crates/matrix-sdk-ui/src/notification_client.rs

@@ -20,10 +20,7 @@ use std::{
 use futures_util::{pin_mut, StreamExt as _};
 use matrix_sdk::{room::Room, Client, ClientBuildError, SlidingSyncList, SlidingSyncMode};
 use matrix_sdk_base::{
-    crypto::{vodozemac, MegolmError},
-    deserialized_responses::TimelineEvent,
-    sliding_sync::http,
-    RoomState, StoreError,
+    deserialized_responses::TimelineEvent, sliding_sync::http, RoomState, StoreError,
 };
 use ruma::{
     assign,
@@ -216,24 +213,24 @@ impl NotificationClient {
 
                         tokio::time::sleep(Duration::from_millis(wait)).await;
 
-                        match room.decrypt_event(raw_event.cast_ref()).await {
-                            Ok(new_event) => {
+                        let new_event = room.decrypt_event(raw_event.cast_ref()).await?;
+
+                        match new_event.kind {
+                            matrix_sdk::deserialized_responses::TimelineEventKind::UnableToDecrypt (
+                                utd_event) => {
+                                if utd_event.utd_info.reason.is_missing_room_key() {
+                                    // Decryption error that could be caused by a missing room
+                                    // key; retry in a few.
+                                    wait *= 2;
+                                } else {
+                                    trace!("Event could not be decrypted, but waiting longer is unlikely to help: {:?}", utd_event.utd_info.reason);
+                                    return Ok(None);
+                                }
+                            }
+                            _ => {
                                 trace!("Waiting succeeded and event could be decrypted!");
                                 return Ok(Some(new_event));
                             }
-                            Err(matrix_sdk::Error::MegolmError(
-                                MegolmError::MissingRoomKey(_)
-                                | MegolmError::Decryption(
-                                    vodozemac::megolm::DecryptionError::UnknownMessageIndex(_, _),
-                                ),
-                            )) => {
-                                // Decryption error that could be caused by a missing room key;
-                                // retry in a few.
-                                wait *= 2;
-                            }
-                            Err(err) => {
-                                return Err(err.into());
-                            }
                         }
                     }
 
@@ -259,10 +256,21 @@ impl NotificationClient {
         match encryption_sync {
             Ok(sync) => match sync.run_fixed_iterations(2, sync_permit_guard).await {
                 Ok(()) => match room.decrypt_event(raw_event.cast_ref()).await {
-                    Ok(new_event) => {
-                        trace!("Encryption sync managed to decrypt the event.");
-                        Ok(Some(new_event))
-                    }
+                    Ok(new_event) => match new_event.kind {
+                        matrix_sdk::deserialized_responses::TimelineEventKind::UnableToDecrypt(
+                            utd_event,
+                        ) => {
+                            trace!(
+                                "Encryption sync failed to decrypt the event: {:?}",
+                                utd_event.utd_info.reason
+                            );
+                            Ok(None)
+                        }
+                        _ => {
+                            trace!("Encryption sync managed to decrypt the event.");
+                            Ok(Some(new_event))
+                        }
+                    },
                     Err(err) => {
                         trace!("Encryption sync failed to decrypt the event: {err}");
                         Ok(None)

crates/matrix-sdk/src/room/mod.rs

@@ -19,7 +19,7 @@ use futures_util::{
 #[cfg(all(feature = "e2e-encryption", not(target_arch = "wasm32")))]
 pub use identity_status_changes::IdentityStatusChanges;
 #[cfg(feature = "e2e-encryption")]
-use matrix_sdk_base::crypto::DecryptionSettings;
+use matrix_sdk_base::crypto::{DecryptionSettings, RoomEventDecryptionResult};
 #[cfg(all(feature = "e2e-encryption", not(target_arch = "wasm32")))]
 use matrix_sdk_base::crypto::{IdentityStatusChange, RoomIdentityProvider, UserIdentity};
 use matrix_sdk_base::{
@@ -1210,7 +1210,8 @@ impl Room {
     /// # Arguments
     /// * `event` - The room event to be decrypted.
     ///
-    /// Returns the decrypted event.
+    /// Returns the decrypted event. In the case of a decryption error, returns
+    /// a `TimelineEvent` representing the decryption error.
     #[cfg(feature = "e2e-encryption")]
     pub async fn decrypt_event(
         &self,
@@ -1222,24 +1223,21 @@ impl Room {
         let decryption_settings = DecryptionSettings {
             sender_device_trust_requirement: self.client.base_client().decryption_trust_requirement,
         };
-        let decrypted = match machine
-            .decrypt_room_event(event.cast_ref(), self.inner.room_id(), &decryption_settings)
-            .await
+        let mut event: TimelineEvent = match machine
+            .try_decrypt_room_event(event.cast_ref(), self.inner.room_id(), &decryption_settings)
+            .await?
         {
-            Ok(event) => event,
-            Err(e) => {
+            RoomEventDecryptionResult::Decrypted(decrypted) => decrypted.into(),
+            RoomEventDecryptionResult::UnableToDecrypt(utd_info) => {
                 self.client
                     .encryption()
                     .backups()
                     .maybe_download_room_key(self.room_id().to_owned(), event.clone());
-
-                return Err(e.into());
+                TimelineEvent::new_utd_event(event.clone().cast(), utd_info)
             }
         };
 
-        let mut event: TimelineEvent = decrypted.into();
         event.push_actions = self.event_push_actions(event.raw()).await?;
-
         Ok(event)
     }