[Rgen] Compile all platforms provided in parallel.

Allow the command line to be used with more than one rsp. That allows to
pass all the rsp for the different platforms we support allowing us to
merge the compilation results later to be used by the transformation.

src/rgen/Microsoft.Macios.Transformer/CompilationResult.cs

@@ -0,0 +1,20 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+using Xamarin.Utils;
+
+namespace Microsoft.Macios.Transformer;
+
+/// <summary>
+/// Holds the result of a compilation. This is a helper record to make code cleaner in the transformer.
+/// </summary>
+/// <param name="Platform">The platform the compilation targeted.</param>
+/// <param name="Compilation">The compilation result.</param>
+/// <param name="Errors">All compilation errors.</param>
+record CompilationResult (ApplePlatform Platform, Compilation Compilation, ImmutableArray<Diagnostic> Errors) {
+
+	public (ApplePlatform Platform, Compilation compilation) ToTuple ()
+		=> (Platform, Compilation);
+}

src/rgen/Microsoft.Macios.Transformer/Main.cs

@@ -5,16 +5,17 @@
 using Serilog.Core;
 using Serilog.Events;
 using Serilog.Templates;
+using Xamarin.Utils;
 using static System.Console;
 
 public class Program {
-	static internal readonly LoggingLevelSwitch LogLevelSwitch = new (LogEventLevel.Information);
+	static internal readonly LoggingLevelSwitch LogLevelSwitch = new(LogEventLevel.Information);
 	public static ILogger logger = Log.ForContext<Program> ();
 
 	public static int Main (string [] args)
 	{
 		// Create options
-		var rspOption = new Option<string> (
+		var rspOption = new Option<string []> (
 			new [] { "--response-file", "--rsp" },
 			"Path to the RSP file"
 		);
@@ -41,9 +42,7 @@ public static int Main (string [] args)
 
 		var verbosityOption = new Option<Verbosity> (["--verbosity", "-v"],
 			getDefaultValue: () => Verbosity.Normal) {
-			IsRequired = false,
-			Arity = ArgumentArity.ZeroOrOne,
-			Description = "Set the verbosity level"
+			IsRequired = false, Arity = ArgumentArity.ZeroOrOne, Description = "Set the verbosity level"
 		};
 
 		// Create root command and add options
@@ -63,47 +62,39 @@ public static int Main (string [] args)
 		}
 
 		// Set handler for parsing and executing
-		rootCmd.SetHandler (async (rspPath, destPath, workingDirectory, sdkPath, force, verbosity) => {
-			WriteLine ($"Microsoft.Macios.Transformer v{typeof (Program).Assembly.GetName ().Version}, (c) Microsoft Corporation. All rights reserved.\n");
-
-			// Convert local to absolute, expand ~
-			rspPath = ToAbsolutePath (rspPath);
-			workingDirectory = ToAbsolutePath (workingDirectory);
-			destPath = ToAbsolutePath (destPath);
-			sdkPath = ToAbsolutePath (sdkPath);
-
-			ValidateRsp (rspPath);
-			ValidateSdk (sdkPath);
-			ValidateWorkingDirectory (workingDirectory);
-			ValidateVerbosity (verbosity);
-			PrepareDestination (destPath, force);
-
-			// logging options
-			Log.Logger = new LoggerConfiguration ()
-				.MinimumLevel.ControlledBy (LogLevelSwitch)
-				.Enrich.WithThreadName ()
-				.Enrich.WithThreadId ()
-				.Enrich.FromLogContext ()
-				.WriteTo.Console (new ExpressionTemplate (
-					"[{@t:HH:mm:ss} {@l:u3} {Substring(SourceContext, LastIndexOf(SourceContext, '.') + 1)} (Thread: {ThreadId})] {@m}\n"))
-				.CreateLogger ();
-
-			// Parse the .rsp file with Roslyn's CSharpCommandLineParser
-			var args = new [] { $"@{rspPath}" };
-			var parseResult = CSharpCommandLineParser.Default.Parse (args, workingDirectory, null);
-			logger.Information ("RSP parsed. Errors: {ParserErrorLength}", parseResult.Errors.Length);
-			foreach (var resultError in parseResult.Errors) {
-				logger.Error ("{Error}", resultError);
-				WriteLine (resultError);
-			}
-
-			await Transformer.Execute (
-				destinationDirectory: destPath,
-				rspFile: rspPath,
-				workingDirectory: workingDirectory,
-				sdkDirectory: sdkPath
-			);
-		},
+		rootCmd.SetHandler (async (rspPlatformPaths, destPath, workingDirectory, sdkPath, force, verbosity) => {
+				WriteLine (
+					$"Microsoft.Macios.Transformer v{typeof(Program).Assembly.GetName ().Version}, (c) Microsoft Corporation. All rights reserved.\n");
+
+				// Convert local to absolute, expand ~
+				//rspPath = ToAbsolutePath (rspPath);
+				workingDirectory = ToAbsolutePath (workingDirectory);
+				destPath = ToAbsolutePath (destPath);
+				sdkPath = ToAbsolutePath (sdkPath);
+
+				ValidateRsp (rspPlatformPaths, out var rspFiles);
+				ValidateSdk (sdkPath);
+				ValidateWorkingDirectory (workingDirectory);
+				ValidateVerbosity (verbosity);
+				PrepareDestination (destPath, force);
+
+				// logging options
+				Log.Logger = new LoggerConfiguration ()
+					.MinimumLevel.ControlledBy (LogLevelSwitch)
+					.Enrich.WithThreadName ()
+					.Enrich.WithThreadId ()
+					.Enrich.FromLogContext ()
+					.WriteTo.Console (new ExpressionTemplate (
+						"[{@t:HH:mm:ss} {@l:u3} {Substring(SourceContext, LastIndexOf(SourceContext, '.') + 1)} (Thread: {ThreadId})] {@m}\n"))
+					.CreateLogger ();
+
+				await Transformer.Execute (
+					destinationDirectory: destPath,
+					rspFiles: rspFiles,
+					workingDirectory: workingDirectory,
+					sdkDirectory: sdkPath
+				);
+			},
 			rspOption, destinationOption, workingDirectoryOption, sdkPathOption, forceOption, verbosityOption
 		);
 
@@ -119,13 +110,46 @@ static string ToAbsolutePath (string path)
 		return absolutePath;
 	}
 
-	static void ValidateRsp (string path)
+	static void ValidateRsp (string [] paths, out List<(ApplePlatform Platform, string RspPath)> rspFiles)
 	{
-		if (string.IsNullOrWhiteSpace (path) || !File.Exists (path))
-			throw new FileNotFoundException ("RSP file not found.");
-		if ((File.GetAttributes (path) & FileAttributes.Directory) == FileAttributes.Directory)
-			throw new Exception ("RSP path is a directory.");
-		using var stream = File.OpenRead (path); // validate readability
+		rspFiles = [];
+		// loop over all the strings, split them by the ':' and retrieve the platform and path. Then 
+		// validate the path
+		foreach (var cmdPath in paths) {
+			var parts = cmdPath.Split (':');
+			if (parts.Length != 2)
+				throw new Exception ("Invalid RSP format. Expected platform:path");
+			ApplePlatform platform;
+			var rspPath = ToAbsolutePath (parts [1]);
+			switch (parts [0]) {
+			case "ios":
+				platform = ApplePlatform.iOS;
+				break;
+			case "tvos":
+				platform = ApplePlatform.TVOS;
+				break;
+			case "macos":
+				platform = ApplePlatform.MacOSX;
+				break;
+			case "maccatalys":
+				platform = ApplePlatform.MacCatalyst;
+				break;
+			default:
+				platform = ApplePlatform.None;
+				break;
+			}
+
+			if (platform == ApplePlatform.None)
+				throw new Exception ("Invalid platform in RSP file.");
+
+			if (string.IsNullOrWhiteSpace (rspPath) || !File.Exists (rspPath))
+				throw new FileNotFoundException ($"RSP '{rspPath}' file not found.");
+			if ((File.GetAttributes (rspPath) & FileAttributes.Directory) == FileAttributes.Directory)
+				throw new Exception ($"RSP {rspPath} is a directory.");
+			using var stream = File.OpenRead (rspPath); // validate readability
+			logger.Debug ("Adding RSP file {RspPath} for platform {Platform}", rspPath, platform);
+			rspFiles.Add ((platform, rspPath));
+		}
 	}
 
 	static void ValidateWorkingDirectory (string path)