Allow the game root directory to be moved away from the binaries.

OpenRA.Game/ExternalMods.cs

@@ -121,7 +121,7 @@ namespace OpenRA
 				mods[key] = mod;
 		}
 
-		internal void Register(Manifest mod, string launchPath, ModRegistration registration)
+		internal void Register(Manifest mod, string launchPath, IEnumerable<string> launchArgs, ModRegistration registration)
 		{
 			if (mod.Metadata.Hidden)
 				return;
@@ -133,7 +133,7 @@ namespace OpenRA
 				new MiniYamlNode("Version", mod.Metadata.Version),
 				new MiniYamlNode("Title", mod.Metadata.Title),
 				new MiniYamlNode("LaunchPath", launchPath),
-				new MiniYamlNode("LaunchArgs", "Game.Mod=" + mod.Id)
+				new MiniYamlNode("LaunchArgs", new[] { "Game.Mod=" + mod.Id }.Concat(launchArgs).JoinWith(", "))
 			}));
 
 			using (var stream = mod.Package.GetStream("icon.png"))

OpenRA.Game/Game.cs

@@ -276,6 +276,10 @@ namespace OpenRA
 
 		static void Initialize(Arguments args)
 		{
+			var engineDirArg = args.GetValue("Engine.EngineDir", null);
+			if (!string.IsNullOrEmpty(engineDirArg))
+				Platform.OverrideEngineDir(engineDirArg);
+
 			var supportDirArg = args.GetValue("Engine.SupportDir", null);
 			if (!string.IsNullOrEmpty(supportDirArg))
 				Platform.OverrideSupportDir(supportDirArg);
@@ -324,7 +328,7 @@ namespace OpenRA
 				Settings.Game.Platform = p;
 				try
 				{
-					var rendererPath = Path.Combine(Platform.EngineDir, "OpenRA.Platforms." + p + ".dll");
+					var rendererPath = Path.Combine(Platform.BinDir, "OpenRA.Platforms." + p + ".dll");
 					var assembly = Assembly.LoadFile(rendererPath);
 
 					var platformType = assembly.GetTypes().SingleOrDefault(t => typeof(IPlatform).IsAssignableFrom(t));
@@ -367,14 +371,24 @@ namespace OpenRA
 
 			if (modID != null && Mods.TryGetValue(modID, out _))
 			{
-				var launchPath = args.GetValue("Engine.LaunchPath", Assembly.GetEntryAssembly().Location);
+				var launchPath = args.GetValue("Engine.LaunchPath", null);
+				var launchArgs = new List<string>();
 
 				// Sanitize input from platform-specific launchers
 				// Process.Start requires paths to not be quoted, even if they contain spaces
-				if (launchPath.First() == '"' && launchPath.Last() == '"')
+				if (launchPath != null && launchPath.First() == '"' && launchPath.Last() == '"')
 					launchPath = launchPath.Substring(1, launchPath.Length - 2);
 
-				ExternalMods.Register(Mods[modID], launchPath, ModRegistration.User);
+				if (launchPath == null)
+				{
+					// When launching the assembly directly we must propagate the Engine.EngineDir argument if defined
+					// Platform-specific launchers are expected to manage this internally.
+					launchPath = Assembly.GetEntryAssembly().Location;
+					if (!string.IsNullOrEmpty(engineDirArg))
+						launchArgs.Add("Engine.EngineDir=\"" + engineDirArg + "\"");
+				}
+
+				ExternalMods.Register(Mods[modID], launchPath, launchArgs, ModRegistration.User);
 
 				if (ExternalMods.TryGetValue(ExternalMod.MakeKey(Mods[modID]), out var activeMod))
 					ExternalMods.ClearInvalidRegistrations(activeMod, ModRegistration.User);

OpenRA.Game/Platform.cs

@@ -27,6 +27,9 @@ namespace OpenRA
 
 		static Lazy<PlatformType> currentPlatform = Exts.Lazy(GetCurrentPlatform);
 
+		static bool engineDirAccessed;
+		static string engineDir;
+
 		static bool supportDirInitialized;
 		static string systemSupportPath;
 		static string legacyUserSupportPath;
@@ -170,6 +173,44 @@ namespace OpenRA
 		}
 
 		public static string EngineDir
+		{
+			get
+			{
+				// Engine directory defaults to the location of the binaries,
+				// unless OverrideGameDir is called during startup.
+				if (!engineDirAccessed)
+					engineDir = BinDir;
+
+				engineDirAccessed = true;
+				return engineDir;
+			}
+		}
+
+		/// <summary>
+		/// Specify a custom engine directory that already exists on the filesystem.
+		/// Cannot be called after Platform.EngineDir has been accessed.
+		/// </summary>
+		public static void OverrideEngineDir(string path)
+		{
+			if (engineDirAccessed)
+				throw new InvalidOperationException("Attempted to override engine directory after it has already been accessed.");
+
+			// Note: Relative paths are interpreted as being relative to BinDir, not the current working dir.
+			if (!Path.IsPathRooted(path))
+				path = Path.Combine(BinDir, path);
+
+			if (!Directory.Exists(path))
+				throw new DirectoryNotFoundException(path);
+
+			if (!path.EndsWith(Path.DirectorySeparatorChar.ToString(), StringComparison.Ordinal) &&
+			    !path.EndsWith(Path.AltDirectorySeparatorChar.ToString(), StringComparison.Ordinal))
+				path += Path.DirectorySeparatorChar;
+
+			engineDirAccessed = true;
+			engineDir = path;
+		}
+
+		public static string BinDir
 		{
 			get
 			{
@@ -194,12 +235,18 @@ namespace OpenRA
 			if (path == "^EngineDir")
 				return EngineDir;
 
+			if (path == "^BinDir")
+				return BinDir;
+
 			if (path.StartsWith("^SupportDir|", StringComparison.Ordinal))
 				path = SupportDir + path.Substring(12);
 
 			if (path.StartsWith("^EngineDir|", StringComparison.Ordinal))
 				path = EngineDir + path.Substring(11);
 
+			if (path.StartsWith("^BinDir|", StringComparison.Ordinal))
+				path = BinDir + path.Substring(8);
+
 			return path;
 		}
 	}

OpenRA.Game/UtilityCommands/RegisterModCommand.cs

@@ -31,7 +31,7 @@ namespace OpenRA.UtilityCommands
 			if (args[2] == "user" || args[2] == "both")
 				type |= ModRegistration.User;
 
-			new ExternalMods().Register(utility.ModData.Manifest, args[1], type);
+			new ExternalMods().Register(utility.ModData.Manifest, args[1], Enumerable.Empty<string>(), type);
 		}
 	}
 }

OpenRA.Mods.Common/UtilityCommands/CheckRuntimeAssembliesCommand.cs

@@ -36,7 +36,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 				.ToArray();
 
 			// Load the renderer assembly so we can check its dependencies
-			Assembly.LoadFile(Path.Combine(Platform.EngineDir, "OpenRA.Platforms.Default.dll"));
+			Assembly.LoadFile(Path.Combine(Platform.BinDir, "OpenRA.Platforms.Default.dll"));
 
 			var missing = new List<string>();
 			foreach (var a in AppDomain.CurrentDomain.GetAssemblies())

OpenRA.Server/Program.cs

@@ -23,6 +23,11 @@ namespace OpenRA.Server
 		static void Main(string[] args)
 		{
 			var arguments = new Arguments(args);
+
+			var engineDirArg = arguments.GetValue("Engine.EngineDir", null);
+			if (!string.IsNullOrEmpty(engineDirArg))
+				Platform.OverrideEngineDir(engineDirArg);
+
 			var supportDirArg = arguments.GetValue("Engine.SupportDir", null);
 			if (!string.IsNullOrEmpty(supportDirArg))
 				Platform.OverrideSupportDir(supportDirArg);

OpenRA.Utility/Program.cs

@@ -40,6 +40,10 @@ namespace OpenRA
 	{
 		static void Main(string[] args)
 		{
+			var engineDir = Environment.GetEnvironmentVariable("ENGINE_DIR");
+			if (!string.IsNullOrEmpty(engineDir))
+				Platform.OverrideEngineDir(engineDir);
+
 			Log.AddChannel("perf", null);
 			Log.AddChannel("debug", null);