fix(sdk_common): TimelineEvent::from_bundled_latest_event can remove session_id

What's more, this is saved into the event cache and sometimes it overrides another instance of the same event that actually contains the right info. This results in unresolvables UTDs.

This change tries to fetch the session id from the existing event content. It's fixed these kind of UTDs when tested in a real client.

crates/matrix-sdk-common/CHANGELOG.md

@@ -6,6 +6,10 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased] - ReleaseDate
 
+### Bug Fixes
+
+- Fix `TimelineEvent::from_bundled_latest_event` sometimes removing the `session_id` of UTDs. This broken event could later be saved to the event cache and become an unresolvable UTD. ([#5970](https://github.com/matrix-org/matrix-rust-sdk/pull/5970)).
+
 ## [0.16.0] - 2025-12-04
 
 ### Features

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

@@ -18,7 +18,7 @@ use ruma::{
     DeviceKeyAlgorithm, MilliSecondsSinceUnixEpoch, OwnedDeviceId, OwnedEventId, OwnedUserId,
     events::{
         AnySyncMessageLikeEvent, AnySyncTimelineEvent, AnyTimelineEvent, AnyToDeviceEvent,
-        MessageLikeEventType,
+        MessageLikeEventType, room::encrypted::EncryptedEventScheme,
     },
     push::Action,
     serde::{
@@ -713,14 +713,21 @@ impl TimelineEvent {
 
             Ok(Some(MessageLikeEventType::RoomEncrypted)) => {
                 // The bundled latest thread event is encrypted, but we didn't have any
-                // information about it in the unsigned map. Provide some dummy
-                // UTD info, since we can't really do much better.
+                // information about it in the unsigned map. Try to fetch the information from
+                // the content instead.
+                let session_id = if let Some(content) =
+                    latest_event.get_field::<EncryptedEventScheme>("content").ok().flatten()
+                {
+                    match content {
+                        EncryptedEventScheme::MegolmV1AesSha2(content) => Some(content.session_id),
+                        _ => None,
+                    }
+                } else {
+                    None
+                };
                 Some(Box::new(TimelineEvent::from_utd_with_max_timestamp(
                     latest_event.cast(),
-                    UnableToDecryptInfo {
-                        session_id: None,
-                        reason: UnableToDecryptReason::Unknown,
-                    },
+                    UnableToDecryptInfo { session_id, reason: UnableToDecryptReason::Unknown },
                     max_timestamp,
                 )))
             }
@@ -1457,7 +1464,9 @@ mod tests {
     use insta::{assert_json_snapshot, with_settings};
     use ruma::{
         DeviceKeyAlgorithm, MilliSecondsSinceUnixEpoch, UInt, device_id, event_id,
-        events::room::message::RoomMessageEventContent, serde::Raw, user_id,
+        events::{AnySyncTimelineEvent, room::message::RoomMessageEventContent},
+        serde::Raw,
+        user_id,
     };
     use serde::Deserialize;
     use serde_json::json;
@@ -2096,4 +2105,43 @@ mod tests {
             }
         });
     }
+
+    #[test]
+    fn test_from_bundled_latest_event_keeps_session_id() {
+        let session_id = "hgLyeSqXfb8vc5AjQLsg6TSHVu0HJ7HZ4B6jgMvxkrs";
+        let serialized = json!({
+            "content": {
+              "algorithm": "m.megolm.v1.aes-sha2",
+              "ciphertext": "AwgAEoABzL1JYhqhjW9jXrlT3M6H8mJ4qffYtOQOnPuAPNxsuG20oiD/Fnpv6jnQGhU6YbV9pNM+1mRnTvxW3CbWOPjLKqCWTJTc7Q0vDEVtYePg38ncXNcwMmfhgnNAoW9S7vNs8C003x3yUl6NeZ8bH+ci870BZL+kWM/lMl10tn6U7snNmSjnE3ckvRdO+11/R4//5VzFQpZdf4j036lNSls/WIiI67Fk9iFpinz9xdRVWJFVdrAiPFwb8L5xRZ8aX+e2JDMlc1eW8gk",
+              "device_id": "SKCGPNUWAU",
+              "sender_key": "Gim/c7uQdSXyrrUbmUOrBT6sMC0gO7QSLmOK6B7NOm0",
+              "session_id": session_id,
+            },
+            "event_id": "$xxxxx:example.org",
+            "origin_server_ts": 2189,
+            "room_id": "!someroom:example.com",
+            "sender": "@carl:example.com",
+            "type": "m.room.encrypted"
+        });
+        let json = serialized.to_string();
+        let value = Raw::<AnySyncTimelineEvent>::from_json_string(json).unwrap();
+
+        let kind = TimelineEventKind::UnableToDecrypt {
+            event: value.clone(),
+            utd_info: UnableToDecryptInfo {
+                session_id: None,
+                reason: UnableToDecryptReason::Unknown,
+            },
+        };
+        let result = TimelineEvent::from_bundled_latest_event(
+            &kind,
+            Some(value.cast_unchecked()),
+            MilliSecondsSinceUnixEpoch::now(),
+        )
+        .expect("Could not get bundled latest event");
+
+        assert_let!(TimelineEventKind::UnableToDecrypt { utd_info, .. } = result.kind);
+        assert!(utd_info.session_id.is_some());
+        assert_eq!(utd_info.session_id.unwrap(), session_id);
+    }
 }