[Microsoft.Android.Build.Tasks] Enable MT for package names

src/Microsoft.Android.Build.Tasks/Tasks/GetAndroidPackageName.cs

@@ -36,7 +36,8 @@
 
 namespace Microsoft.Android.Tasks
 {
-	public class GetAndroidPackageName : AndroidTask
+	[MSBuildMultiThreadableTask]
+	public class GetAndroidPackageName : MultithreadedAndroidTask
 	{
 		public override string TaskPrefix => "GAP";
 
@@ -52,13 +53,16 @@ public class GetAndroidPackageName : AndroidTask
 
 		public override bool RunTask ()
 		{
-			if (!ManifestFile.IsNullOrEmpty () && File.Exists (ManifestFile)) {
-				using var stream = File.OpenRead (ManifestFile);
-				using var reader = XmlReader.Create (stream);
-				if (reader.MoveToContent () == XmlNodeType.Element) {
-					var package = reader.GetAttribute ("package");
-					if (!package.IsNullOrEmpty ()) {
-						PackageName = ManifestPlaceholderResolver.Replace (ManifestPlaceholders, package);
+			if (!ManifestFile.IsNullOrEmpty ()) {
+				var manifestFile = TaskEnvironment.GetAbsolutePath (ManifestFile);
+				if (File.Exists (manifestFile)) {
+					using var stream = File.OpenRead (manifestFile);
+					using var reader = XmlReader.Create (stream);
+					if (reader.MoveToContent () == XmlNodeType.Element) {
+						var package = reader.GetAttribute ("package");
+						if (!package.IsNullOrEmpty ()) {
+							PackageName = ManifestPlaceholderResolver.Replace (ManifestPlaceholders, package);
+						}
 					}
 				}
 			}

src/Microsoft.Android.Build.Tasks/Tests/Microsoft.Android.Build.Tasks.Tests/GetAndroidPackageNameTests.cs

@@ -1,4 +1,7 @@
+using System;
 using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
 using Microsoft.Build.Framework;
 using NUnit.Framework;
 using Microsoft.Android.Tasks;
@@ -9,6 +12,69 @@ namespace Xamarin.Android.Build.Tests {
 	[Parallelizable (ParallelScope.Children)]
 	public class GetAndroidPackageNameTests {
 
+		[TestCase (false, true, "com.application.id", "com.example.some_name")]
+		[TestCase (true, true, null, "com.example.some_name")]
+		[TestCase (false, false, null, "Example.Example")]
+		[TestCase (true, false, "com.application.id", "com.application.id")]
+		public void ManifestPaths (bool absolutePath, bool exists, string? packageName, string expected)
+		{
+			string directory = CreateProjectDirectory ();
+			try {
+				string path = Path.Combine (directory, "AndroidManifest.xml");
+				if (exists) {
+					File.WriteAllText (path, "<manifest package=\"${prefix}.some-name\" />");
+				}
+				string manifestFile = absolutePath ? path : "AndroidManifest.xml";
+				var task = new GetAndroidPackageName {
+					BuildEngine = new MockBuildEngine (TestContext.Out),
+					TaskEnvironment = TaskEnvironment.CreateWithProjectDirectoryAndEnvironment (directory, new Dictionary<string, string> ()),
+					ManifestFile = manifestFile,
+					AssemblyName = "Example",
+					PackageName = packageName,
+					ManifestPlaceholders = ["prefix=com.example"],
+				};
+
+				Assert.IsTrue (task.Execute ());
+				Assert.AreEqual (expected, task.PackageName);
+				Assert.AreEqual (manifestFile, task.ManifestFile, "Rooting file I/O must not change the public input.");
+			} finally {
+				Directory.Delete (directory, recursive: true);
+			}
+		}
+
+		[Test]
+		public void ConcurrentProjectDirectories ()
+		{
+			string root = CreateProjectDirectory ();
+			try {
+				Parallel.For (0, 2, i => {
+					string directory = Path.Combine (root, $"project{i}");
+					string package = $"com.project{i}";
+					Directory.CreateDirectory (directory);
+					File.WriteAllText (Path.Combine (directory, "AndroidManifest.xml"), $"<manifest package=\"{package}\" />");
+					var task = new GetAndroidPackageName {
+						BuildEngine = new MockBuildEngine (TextWriter.Null),
+						TaskEnvironment = TaskEnvironment.CreateWithProjectDirectoryAndEnvironment (directory, new Dictionary<string, string> ()),
+						ManifestFile = "AndroidManifest.xml",
+						AssemblyName = "Fallback",
+					};
+
+					Assert.IsTrue (task.Execute ());
+					Assert.AreEqual (package, task.PackageName);
+				});
+			} finally {
+				Directory.Delete (root, recursive: true);
+			}
+		}
+
+		static string CreateProjectDirectory ()
+		{
+			string directory = Path.Combine (TestContext.CurrentContext.WorkDirectory, "GetAndroidPackageName", Guid.NewGuid ().ToString ("N"));
+			Directory.CreateDirectory (directory);
+			Assert.AreNotEqual (Directory.GetCurrentDirectory (), directory);
+			return directory;
+		}
+
 		[TestCase ("com.example.for", "for")]
 		[TestCase ("com.class.example", "class")]
 		[TestCase ("com.example.true", "true")]