Add reminder multiplayer countdown type

.gitignore

@@ -1,6 +1,9 @@
 ## Ignore Visual Studio temporary files, build results, and
 ## files generated by popular Visual Studio add-ons.
 
+# C# Dev Kit cache
+*.lscache
+
 # User-specific files
 *.suo
 *.user

osu.Game/Online/Multiplayer/Countdown/CountdownTickEvent.cs

@@ -0,0 +1,35 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using MessagePack;
+using Newtonsoft.Json;
+
+namespace osu.Game.Online.Multiplayer.Countdown
+{
+    /// <summary>
+    /// Indicates a countdown tick (periodic reminder) in a <see cref="MultiplayerRoom"/>.
+    /// </summary>
+    [MessagePackObject]
+    public class CountdownTickEvent : MatchServerEvent
+    {
+        /// <summary>
+        /// The identifier of the countdown this tick pertains to.
+        /// </summary>
+        [Key(0)]
+        public readonly int CountdownId;
+
+        /// <summary>
+        /// Number of seconds (fractional) until the completion of the countdown.
+        /// </summary>
+        [Key(1)]
+        public readonly double Seconds;
+
+        [JsonConstructor]
+        [SerializationConstructor]
+        public CountdownTickEvent(int countdownId, double seconds)
+        {
+            CountdownId = countdownId;
+            Seconds = seconds;
+        }
+    }
+}

osu.Game/Online/Multiplayer/Countdown/ReminderCountdown.cs

@@ -0,0 +1,16 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using MessagePack;
+
+namespace osu.Game.Online.Multiplayer.Countdown
+{
+    /// <summary>
+    /// A countdown for match notifications/reminders, does not start the match.
+    /// </summary>
+    [MessagePackObject]
+    public class ReminderCountdown : MultiplayerCountdown
+    {
+        public override bool IsExclusive => true;
+    }
+}

osu.Game/Online/Multiplayer/MultiplayerCountdown.cs

@@ -18,6 +18,7 @@ namespace osu.Game.Online.Multiplayer
     [Union(2, typeof(ServerShuttingDownCountdown))]
     [Union(3, typeof(MatchmakingStageCountdown))]
     [Union(4, typeof(RankedPlayStageCountdown))]
+    [Union(5, typeof(ReminderCountdown))]
     public abstract class MultiplayerCountdown
     {
         /// <summary>

osu.Game/Online/SignalRWorkaroundTypes.cs

@@ -37,6 +37,7 @@ internal static class SignalRWorkaroundTypes
             (typeof(MatchStartCountdown), typeof(MultiplayerCountdown)),
             (typeof(ForceGameplayStartCountdown), typeof(MultiplayerCountdown)),
             (typeof(ServerShuttingDownCountdown), typeof(MultiplayerCountdown)),
+            (typeof(ReminderCountdown), typeof(MultiplayerCountdown)),
 
             // metadata
             (typeof(UserActivity.ChoosingBeatmap), typeof(UserActivity)),