Add API versioning

osu.Game/Online/API/APIAccess.cs

@@ -36,6 +36,8 @@ public class APIAccess : Component, IAPIProvider
 
         public string WebsiteRootUrl { get; }
 
+        public int APIVersion => 20220217; // We may want to pull this from the game version eventually.
+
         public Exception LastLoginError { get; private set; }
 
         public string ProvidedUsername { get; private set; }

osu.Game/Online/API/APIRequest.cs

@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Globalization;
 using JetBrains.Annotations;
 using Newtonsoft.Json;
 using osu.Framework.IO.Network;
@@ -112,6 +113,9 @@ public void Perform(IAPIProvider api)
             WebRequest = CreateWebRequest();
             WebRequest.Failed += Fail;
             WebRequest.AllowRetryOnTimeout = false;
+
+            WebRequest.AddHeader("x-api-version", API.APIVersion.ToString(CultureInfo.InvariantCulture));
+
             if (!string.IsNullOrEmpty(API.AccessToken))
                 WebRequest.AddHeader("Authorization", $"Bearer {API.AccessToken}");
 

osu.Game/Online/API/DummyAPIAccess.cs

@@ -33,6 +33,8 @@ public class DummyAPIAccess : Component, IAPIProvider
 
         public string WebsiteRootUrl => "http://localhost";
 
+        public int APIVersion => int.Parse(DateTime.Now.ToString("yyyyMMdd"));
+
         public Exception LastLoginError { get; private set; }
 
         /// <summary>

osu.Game/Online/API/IAPIProvider.cs

@@ -57,6 +57,11 @@ public interface IAPIProvider
         /// </summary>
         string WebsiteRootUrl { get; }
 
+        /// <summary>
+        /// The version of the API.
+        /// </summary>
+        int APIVersion { get; }
+
         /// <summary>
         /// The last login error that occurred, if any.
         /// </summary>