Separate editing and adding playlist items

osu.Game/Online/Multiplayer/IMultiplayerRoomServer.cs

@@ -83,6 +83,12 @@ namespace osu.Game.Online.Multiplayer
         /// <param name="item">The item to add.</param>
         Task AddPlaylistItem(MultiplayerPlaylistItem item);
 
+        /// <summary>
+        /// Edits an existing playlist item with new values.
+        /// </summary>
+        /// <param name="item">The item to edit, containing new properties. Must have an ID.</param>
+        Task EditPlaylistItem(MultiplayerPlaylistItem item);
+
         /// <summary>
         /// Removes an item from the playlist.
         /// </summary>

osu.Game/Online/Multiplayer/MultiplayerClient.cs

@@ -335,6 +335,8 @@ namespace osu.Game.Online.Multiplayer
 
         public abstract Task AddPlaylistItem(MultiplayerPlaylistItem item);
 
+        public abstract Task EditPlaylistItem(MultiplayerPlaylistItem item);
+
         public abstract Task RemovePlaylistItem(long playlistItemId);
 
         Task IMultiplayerClient.RoomStateChanged(MultiplayerRoomState state)

osu.Game/Online/Multiplayer/OnlineMultiplayerClient.cs

@@ -162,6 +162,14 @@ namespace osu.Game.Online.Multiplayer
             return connection.InvokeAsync(nameof(IMultiplayerServer.AddPlaylistItem), item);
         }
 
+        public override Task EditPlaylistItem(MultiplayerPlaylistItem item)
+        {
+            if (!IsConnected.Value)
+                return Task.CompletedTask;
+
+            return connection.InvokeAsync(nameof(IMultiplayerServer.EditPlaylistItem), item);
+        }
+
         public override Task RemovePlaylistItem(long playlistItemId)
         {
             if (!IsConnected.Value)

osu.Game/Screens/OnlinePlay/Multiplayer/MultiplayerMatchSongSelect.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Diagnostics;
 using System.Linq;
+using System.Threading.Tasks;
 using Microsoft.AspNetCore.SignalR;
 using osu.Framework.Allocation;
 using osu.Framework.Logging;
@@ -64,7 +65,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
             {
                 loadingLayer.Show();
 
-                client.AddPlaylistItem(new MultiplayerPlaylistItem
+                var multiplayerItem = new MultiplayerPlaylistItem
                 {
                     ID = itemToEdit?.ID ?? 0,
                     BeatmapID = item.BeatmapID,
@@ -72,7 +73,11 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
                     RulesetID = item.RulesetID,
                     RequiredMods = item.RequiredMods.Select(m => new APIMod(m)).ToArray(),
                     AllowedMods = item.AllowedMods.Select(m => new APIMod(m)).ToArray()
-                }).ContinueWith(t =>
+                };
+
+                Task task = itemToEdit != null ? client.EditPlaylistItem(multiplayerItem) : client.AddPlaylistItem(multiplayerItem);
+
+                task.ContinueWith(t =>
                 {
                     Schedule(() =>
                     {

osu.Game/Tests/Visual/Multiplayer/TestMultiplayerClient.cs

@@ -309,20 +309,25 @@ namespace osu.Game.Tests.Visual.Multiplayer
             Debug.Assert(APIRoom != null);
             Debug.Assert(currentItem != null);
 
-            bool isNewAddition = item.ID == 0;
-
             if (Room.Settings.QueueMode == QueueMode.HostOnly && Room.Host?.UserID != LocalUser?.UserID)
                 throw new InvalidOperationException("Local user is not the room host.");
 
             item.OwnerID = userId;
 
-            if (isNewAddition)
-            {
             await addItem(item).ConfigureAwait(false);
             await updateCurrentItem(Room).ConfigureAwait(false);
         }
-            else
+
+        public override Task AddPlaylistItem(MultiplayerPlaylistItem item) => AddUserPlaylistItem(api.LocalUser.Value.OnlineID, item);
+
+        public async Task EditUserPlaylistItem(int userId, MultiplayerPlaylistItem item)
         {
+            Debug.Assert(Room != null);
+            Debug.Assert(APIRoom != null);
+            Debug.Assert(currentItem != null);
+
+            item.OwnerID = userId;
+
             var existingItem = serverSidePlaylist.SingleOrDefault(i => i.ID == item.ID);
 
             if (existingItem == null)
@@ -338,20 +343,17 @@ namespace osu.Game.Tests.Visual.Multiplayer
             item.PlaylistOrder = existingItem.PlaylistOrder;
 
             serverSidePlaylist[serverSidePlaylist.IndexOf(existingItem)] = item;
+
             await ((IMultiplayerClient)this).PlaylistItemChanged(item).ConfigureAwait(false);
         }
-        }
 
-        public override Task AddPlaylistItem(MultiplayerPlaylistItem item) => AddUserPlaylistItem(api.LocalUser.Value.OnlineID, item);
+        public override Task EditPlaylistItem(MultiplayerPlaylistItem item) => EditUserPlaylistItem(api.LocalUser.Value.OnlineID, item);
 
         public async Task RemoveUserPlaylistItem(int userId, long playlistItemId)
         {
             Debug.Assert(Room != null);
             Debug.Assert(APIRoom != null);
 
-            if (Room.Settings.QueueMode == QueueMode.HostOnly)
-                throw new InvalidOperationException("Items cannot be removed in host-only mode.");
-
             var item = serverSidePlaylist.Find(i => i.ID == playlistItemId);
 
             if (item == null)