allow searching in left rooms

The spec states that

> The search API allows clients to perform full text search across
> events in all rooms that the user has been in, including those that they
> have left. Only events that the user is allowed to see will be
> searched, e.g. it won’t include events in rooms that happened after
> you left.

<https://spec.matrix.org/v1.10/client-server-api/#server-side-search>

src/api/client_server/search.rs

@@ -35,7 +35,7 @@ pub(crate) async fn search_events_route(
         services()
             .rooms
             .state_cache
-            .rooms_joined(sender_user)
+            .rooms_once_joined(sender_user)
             .filter_map(Result::ok)
             .collect()
     });
@@ -51,7 +51,7 @@ pub(crate) async fn search_events_route(
     let mut searches = Vec::new();
 
     for room_id in room_ids {
-        if !services().rooms.state_cache.is_joined(sender_user, &room_id)? {
+        if !services().rooms.state_cache.once_joined(sender_user, &room_id)? {
             return Err(Error::BadRequest(
                 ErrorKind::forbidden(),
                 "You don't have permission to view this room.",

src/database/key_value/rooms/state_cache.rs

@@ -615,6 +615,39 @@ impl service::rooms::state_cache::Data for KeyValueDatabase {
         ))
     }
 
+    /// Returns an iterator over all rooms a user has been in.
+    #[tracing::instrument(skip(self))]
+    fn rooms_once_joined<'a>(
+        &'a self,
+        user_id: &UserId,
+    ) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a> {
+        let mut prefix = user_id.as_bytes().to_vec();
+        prefix.push(0xFF);
+
+        Box::new(self.roomuseroncejoinedids.scan_prefix(prefix).map(
+            |(key, _)| {
+                RoomId::parse(
+                    utils::string_from_bytes(
+                        key.rsplit(|&b| b == 0xFF)
+                            .next()
+                            .expect("rsplit always returns an element"),
+                    )
+                    .map_err(|_| {
+                        Error::bad_database(
+                            "Room ID in roomuseroncejoinedids is invalid \
+                             unicode.",
+                        )
+                    })?,
+                )
+                .map_err(|_| {
+                    Error::bad_database(
+                        "Room ID in roomuseroncejoinedids is invalid.",
+                    )
+                })
+            },
+        ))
+    }
+
     #[tracing::instrument(skip(self))]
     fn once_joined(&self, user_id: &UserId, room_id: &RoomId) -> Result<bool> {
         let mut userroom_id = user_id.as_bytes().to_vec();

src/service/rooms/state_accessor.rs

@@ -222,8 +222,8 @@ impl Service {
             return Ok(*visibility);
         }
 
-        let currently_member =
-            services().rooms.state_cache.is_joined(user_id, room_id)?;
+        let once_member =
+            services().rooms.state_cache.once_joined(user_id, room_id)?;
 
         let history_visibility = self
             .state_get(
@@ -245,7 +245,7 @@ impl Service {
 
         let visibility = match history_visibility {
             HistoryVisibility::WorldReadable => true,
-            HistoryVisibility::Shared => currently_member,
+            HistoryVisibility::Shared => once_member,
             HistoryVisibility::Invited => {
                 // Allow if any member on requesting server was AT LEAST
                 // invited, else deny
@@ -280,6 +280,8 @@ impl Service {
     ) -> Result<bool> {
         let currently_member =
             services().rooms.state_cache.is_joined(user_id, room_id)?;
+        let once_member =
+            services().rooms.state_cache.once_joined(user_id, room_id)?;
 
         let history_visibility = self
             .room_state_get(
@@ -299,8 +301,11 @@ impl Service {
                     })
             })?;
 
-        Ok(currently_member
-            || history_visibility == HistoryVisibility::WorldReadable)
+        Ok(match history_visibility {
+            HistoryVisibility::WorldReadable => true,
+            HistoryVisibility::Shared => once_member,
+            _ => currently_member,
+        })
     }
 
     /// Returns the state hash for this pdu.

src/service/rooms/state_cache.rs

@@ -387,6 +387,15 @@ impl Service {
         self.db.rooms_left(user_id)
     }
 
+    /// Returns an iterator over all rooms a user has been in.
+    #[tracing::instrument(skip(self))]
+    pub(crate) fn rooms_once_joined<'a>(
+        &'a self,
+        user_id: &UserId,
+    ) -> impl Iterator<Item = Result<OwnedRoomId>> + 'a {
+        self.db.rooms_once_joined(user_id)
+    }
+
     #[tracing::instrument(skip(self))]
     pub(crate) fn once_joined(
         &self,

src/service/rooms/state_cache/data.rs

@@ -125,6 +125,12 @@ pub(crate) trait Data: Send + Sync {
             + 'a,
     >;
 
+    /// Returns an iterator over all rooms a user has been in.
+    fn rooms_once_joined<'a>(
+        &'a self,
+        user_id: &UserId,
+    ) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a>;
+
     fn once_joined(&self, user_id: &UserId, room_id: &RoomId) -> Result<bool>;
 
     fn is_joined(&self, user_id: &UserId, room_id: &RoomId) -> Result<bool>;