[xabuild] Use a temp file for MSBuild config (#969)

Context: https://github.com/xamarin/xamarin-android/pull/954
Context: https://github.com/xamarin/xamarin-android/pull/964
Context: https://github.com/xamarin/xamarin-android/pull/967

The `Xamarin.Android.Build.Tests` unit tests run `xabuild.exe` in
parallel, which causes all manner of "weirdness" as the multiple
`xabuild.exe` process all attempt to overwrite the *same* config file
used by the `MSBuildApp` class.

Fix this by creating *unique* per-process config files, stored in
`Path.GetTempFileName()`, and overriding the `$MSBUILD_EXE_PATH`
environment variable to refer to the per-process config file -- which
is what the [MSBuild Unit Tests use][msbuild-tests] to override the
`msbuild.exe.config` file used at runtime -- allowing each
`xabuild.exe` process to sanely provide separate config files
without conflicting with each other.

[msbuild-tests]: https://github.com/Microsoft/msbuild/blob/080ef97/src/Build.UnitTests/BuildEnvironmentHelper_Tests.cs#L47

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs

@@ -1,8 +1,10 @@
 using System;
+using System.Diagnostics;
 using System.IO;
 using System.Linq;
 using System.Text;
 using System.Text.RegularExpressions;
+using System.Threading.Tasks;
 using System.Xml.Linq;
 using Microsoft.Build.Framework;
 using NUnit.Framework;
@@ -2078,6 +2080,26 @@ namespace UnnamedProject {
 				StringAssert.Contains ($"warning XA4", builder.LastBuildOutput, "warning XA4212");
 			}
 		}
+
+		[Test]
+		public void RunXABuildInParallel ()
+		{
+			var xabuild = new ProjectBuilder ("temp/RunXABuildInParallel").XABuildExe;
+			var psi     = new ProcessStartInfo (xabuild, "/version") {
+				CreateNoWindow         = true,
+				RedirectStandardOutput = true,
+				RedirectStandardError  = true,
+				WindowStyle            = ProcessWindowStyle.Hidden,
+				UseShellExecute        = false,
+			};
+
+			Parallel.For (0, 10, i => {
+				using (var p = Process.Start (psi)) {
+					p.WaitForExit ();
+					Assert.AreEqual (0, p.ExitCode);
+				}
+			});
+		}
 	}
 }
 

tools/xabuild/XABuild.cs

@@ -11,23 +11,32 @@ namespace Xamarin.Android.Build
 		static int Main ()
 		{
 			var paths = new XABuildPaths ();
-			if (!Directory.Exists (paths.XamarinAndroidBuildOutput)) {
-				Console.WriteLine ($"Unable to find Xamarin.Android build output at {paths.XamarinAndroidBuildOutput}");
-				return 1;
-			}
-
-			//Create a custom xabuild.exe.config
-			CreateConfig (paths);
-
-			//Create link to .NETFramework and .NETPortable directory
-			foreach (var dir in Directory.GetDirectories(paths.SystemProfiles)) {
-				var name = Path.GetFileName(dir);
-				if (!SymbolicLink.Create(Path.Combine(paths.FrameworksDirectory, name), dir)) {
+			try {
+				if (!Directory.Exists (paths.XamarinAndroidBuildOutput)) {
+					Console.WriteLine ($"Unable to find Xamarin.Android build output at {paths.XamarinAndroidBuildOutput}");
 					return 1;
 				}
-			}
 
-			return MSBuildApp.Main ();
+				//Create a custom xabuild.exe.config
+				CreateConfig (paths);
+
+				//Create link to .NETFramework and .NETPortable directory
+				foreach (var dir in Directory.GetDirectories (paths.SystemProfiles)) {
+					var name = Path.GetFileName (dir);
+					if (!SymbolicLink.Create (Path.Combine (paths.FrameworksDirectory, name), dir)) {
+						return 1;
+					}
+				}
+
+				return MSBuildApp.Main ();
+			} finally {
+				//NOTE: these are temporary files
+				foreach (var file in new [] { paths.MSBuildExeTempPath, paths.XABuildConfig }) {
+					if (File.Exists (file)) {
+						File.Delete (file);
+					}
+				}
+			}
 		}
 
 		static void CreateConfig (XABuildPaths paths)
@@ -79,6 +88,8 @@ namespace Xamarin.Android.Build
 			}
 
 			xml.Save (paths.XABuildConfig);
+
+			Environment.SetEnvironmentVariable ("MSBUILD_EXE_PATH", paths.MSBuildExeTempPath, EnvironmentVariableTarget.Process);
 		}
 
 		/// <summary>

tools/xabuild/XABuildPaths.cs

@@ -21,7 +21,7 @@ namespace Xamarin.Android.Build
 		public string XABuildDirectory { get; private set; }
 
 		/// <summary>
-		/// Path to xabuild.exe.config, on Unix it seems to use MSBuild.dll.config instead
+		/// Path to xabuild.exe's config file, this is now a temporary file based on MSBuildExeTempPath
 		/// </summary>
 		public string XABuildConfig { get; private set; }
 
@@ -45,6 +45,11 @@ namespace Xamarin.Android.Build
 		/// </summary>
 		public string MSBuildBin { get; private set; }
 
+		/// <summary>
+		/// Temporary file used for MSBUILD_EXE_PATH
+		/// </summary>
+		public string MSBuildExeTempPath { get; private set; }
+
 		/// <summary>
 		/// Path to MSBuild's App.config file
 		/// </summary>
@@ -110,7 +115,6 @@ namespace Xamarin.Android.Build
 				MSBuildConfig            = Path.Combine (MSBuildBin, "MSBuild.exe.config");
 				ProjectImportSearchPaths = new [] { MSBuildPath, "$(MSBuildProgramFiles32)\\MSBuild" };
 				SystemProfiles           = Path.Combine (programFiles, "Reference Assemblies", "Microsoft", "Framework");
-				XABuildConfig            = Path.Combine (XABuildDirectory, "xabuild.exe.config");
 				SearchPathsOS            = "windows";
 			} else {
 				string mono              = IsMacOS ? "/Library/Frameworks/Mono.framework/Versions/Current/lib/mono" : "/usr/lib/mono";
@@ -120,7 +124,6 @@ namespace Xamarin.Android.Build
 				MSBuildConfig            = Path.Combine (MSBuildBin, "MSBuild.dll.config");
 				ProjectImportSearchPaths = new [] { MSBuildPath, Path.Combine (mono, "xbuild"), Path.Combine (monoExternal, "xbuild") };
 				SystemProfiles           = Path.Combine (mono, "xbuild-frameworks");
-				XABuildConfig            = Path.Combine (XABuildDirectory, "MSBuild.dll.config");
 				SearchPathsOS            = IsMacOS ? "osx" : "unix";
 			}
 
@@ -129,6 +132,8 @@ namespace Xamarin.Android.Build
 			MonoAndroidToolsDirectory = Path.Combine (prefix, "xbuild", "Xamarin", "Android");
 			AndroidSdkDirectory       = Path.Combine (userProfile, "android-toolchain", "sdk");
 			AndroidNdkDirectory       = Path.Combine (userProfile, "android-toolchain", "ndk");
+			MSBuildExeTempPath        = Path.GetTempFileName ();
+			XABuildConfig             = MSBuildExeTempPath + ".config";
 		}
 
 		[DllImport ("libc")]