transfer local canonical aliases to new room on upgrade

When upgrading rooms, we reassign any local aliases from the old room to
the new room. This commit updates the m.room.canonical_alias events in
the old and new rooms to reflect which aliases were moved. The spec is
unclear on whether the server should do this[1], but it's consistent
with synapse's behavior.

I went with putting the canonical alias update logic inline, rather than
something like add_canonical_alias and remove_canonical_alias helper
functions to the alias service, because it's desirable to have the alias
updates be sent as a single event than a separate event for each change.

[1]: https://github.com/matrix-org/matrix-spec/issues/2142

book/changelog.md

@@ -327,3 +327,5 @@ This will be the first release of Grapevine since it was forked from Conduit
     ([!124](https://gitlab.computer.surgery/matrix/grapevine/-/merge_requests/124))
 26. Allow adding canonical aliases from remote servers.
     ([!158](https://gitlab.computer.surgery/matrix/grapevine/-/merge_requests/158))
+27. Transfer local canonical aliases to the new room when upgrading a room.
+    ([!186](https://gitlab.computer.surgery/matrix/grapevine/-/merge_requests/186))

src/api/client_server/room.rs

@@ -777,6 +777,20 @@ pub(crate) async fn upgrade_room_route(
     }
 
     // Moves any local aliases to the new room
+    let mut old_canonical_alias: RoomCanonicalAliasEventContent = services()
+        .rooms
+        .state_accessor
+        .room_state_get(&body.room_id, &StateEventType::RoomCanonicalAlias, "")?
+        .and_then(|event| {
+            serde_json::from_str(event.content.get())
+                .inspect_err(|_| {
+                    Error::bad_database("invalid m.room.canonical_alias event");
+                })
+                .ok()
+        })
+        .unwrap_or_default();
+    let mut new_canonical_alias = RoomCanonicalAliasEventContent::default();
+
     for alias in services()
         .rooms
         .alias
@@ -788,6 +802,67 @@ pub(crate) async fn upgrade_room_route(
             &replacement_room,
             sender_user,
         )?;
+
+        if old_canonical_alias.alias.as_ref() == Some(&alias) {
+            new_canonical_alias.alias = Some(alias.clone());
+            old_canonical_alias.alias = None;
+        }
+        let mut is_alt_alias = false;
+        old_canonical_alias.alt_aliases.retain(|a| {
+            let equal = a == &alias;
+            is_alt_alias |= equal;
+            !equal
+        });
+        if is_alt_alias {
+            new_canonical_alias.alt_aliases.push(alias);
+        }
+    }
+
+    // Send updated events to both rooms if we moved any canonical aliases
+    let new_canonical_alias_empty = new_canonical_alias.alias.is_none()
+        && new_canonical_alias.alt_aliases.is_empty();
+    if !new_canonical_alias_empty {
+        if let Err(error) = services()
+            .rooms
+            .timeline
+            .build_and_append_pdu(
+                PduBuilder {
+                    event_type: TimelineEventType::RoomCanonicalAlias,
+                    content: to_raw_value(&old_canonical_alias)
+                        .expect("event is valid"),
+                    unsigned: None,
+                    state_key: Some(String::new()),
+                    redacts: None,
+                },
+                sender_user,
+                &original_room_token,
+            )
+            .await
+        {
+            // The sender may not have had permission to send
+            // m.room.canonical_alias in the old room. Don't treat
+            // this as fatal.
+            warn!(
+                %error,
+                "Failed to remove local canonical aliases from old room"
+            );
+        }
+        services()
+            .rooms
+            .timeline
+            .build_and_append_pdu(
+                PduBuilder {
+                    event_type: TimelineEventType::RoomCanonicalAlias,
+                    content: to_raw_value(&new_canonical_alias)
+                        .expect("event is valid"),
+                    unsigned: None,
+                    state_key: Some(String::new()),
+                    redacts: None,
+                },
+                sender_user,
+                &replacement_room_token,
+            )
+            .await?;
     }
 
     // Get the old room power levels