attempt to remove canonical alias entries when deleting local aliases

The spec says that this behavior is optional, but it's what synapse does
and we had a TODO for it.

book/changelog.md

@@ -331,3 +331,6 @@ This will be the first release of Grapevine since it was forked from Conduit
     ([!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))
+28. Attempt to remove alias from m.room.canonical_alias event when deleting a
+    local alias.
+    ([!186](https://gitlab.computer.surgery/matrix/grapevine/-/merge_requests/186))

src/api/client_server/alias.rs

@@ -8,10 +8,16 @@ use ruma::{
         },
         federation,
     },
+    events::{
+        room::canonical_alias::RoomCanonicalAliasEventContent, StateEventType,
+        TimelineEventType,
+    },
     OwnedRoomAliasId,
 };
+use serde_json::value::to_raw_value;
+use tracing::info;
 
-use crate::{services, Ar, Error, Ra, Result};
+use crate::{service::pdu::PduBuilder, services, Ar, Error, Ra, Result};
 
 /// # `PUT /_matrix/client/r0/directory/room/{roomAlias}`
 ///
@@ -75,12 +81,13 @@ pub(crate) async fn delete_alias_route(
         ));
     }
 
-    if services().rooms.alias.resolve_local_alias(&body.room_alias)?.is_none() {
-        return Err(Error::BadRequest(
-            ErrorKind::NotFound,
-            "Alias is not assigned.",
-        ));
-    }
+    let room_id = services()
+        .rooms
+        .alias
+        .resolve_local_alias(&body.room_alias)?
+        .ok_or_else(|| {
+            Error::BadRequest(ErrorKind::NotFound, "Alias is not assigned.")
+        })?;
 
     if let Some(info) = &body.appservice_info {
         if !info.aliases.is_match(body.room_alias.as_str()) {
@@ -98,7 +105,66 @@ pub(crate) async fn delete_alias_route(
 
     services().rooms.alias.remove_alias(&body.room_alias, sender_user)?;
 
-    // TODO: update alt_aliases?
+    // Attempt to remove the alias from the m.room.canonical_alias event, if it
+    // was present.
+    let canonical_alias: Option<RoomCanonicalAliasEventContent> = services()
+        .rooms
+        .state_accessor
+        .room_state_get(&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()
+        });
+    if let Some(mut canonical_alias) = canonical_alias {
+        let mut changed = false;
+        if canonical_alias.alias.as_ref() == Some(&body.room_alias) {
+            canonical_alias.alias = None;
+            changed = true;
+        }
+        canonical_alias.alt_aliases.retain(|a| {
+            let equal = a == &body.room_alias;
+            changed |= equal;
+            !equal
+        });
+
+        if changed {
+            let room_token = services()
+                .globals
+                .roomid_mutex_state
+                .lock_key(room_id.clone())
+                .await;
+
+            if let Err(error) = services()
+                .rooms
+                .timeline
+                .build_and_append_pdu(
+                    PduBuilder {
+                        event_type: TimelineEventType::RoomCanonicalAlias,
+                        content: to_raw_value(&canonical_alias)
+                            .expect("event is valid"),
+                        unsigned: None,
+                        state_key: Some(String::new()),
+                        redacts: None,
+                    },
+                    sender_user,
+                    &room_token,
+                )
+                .await
+            {
+                // > Servers which choose to update the canonical alias event
+                // > are recommended to, in addition to their other relevant
+                // > permission checks, delete the alias and return a successful
+                // > response even if the user does not have permission to
+                // > update the m.room.canonical_alias event.
+                //
+                // <https://spec.matrix.org/v1.14/client-server-api/#delete_matrixclientv3directoryroomroomalias>
+                info!(%error, "Failed to remove canonical alias");
+            }
+        }
+    }
 
     Ok(Ra(delete_alias::v3::Response::new()))
 }