[Xamarin.Android.Build.Tasks] Enable MT library imports cache reads

src/Xamarin.Android.Build.Tasks/Tasks/ReadLibraryProjectImportsCache.cs

@@ -24,17 +24,15 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 #nullable enable
-using System;
 using System.IO;
-using System.Linq;
 using System.Xml.Linq;
-using Microsoft.Build.Utilities;
 using Microsoft.Build.Framework;
 using Microsoft.Android.Build.Tasks;
 
 namespace Xamarin.Android.Tasks
 {
-	public class ReadLibraryProjectImportsCache : AndroidTask
+	[MSBuildMultiThreadableTask]
+	public class ReadLibraryProjectImportsCache : MultithreadedAndroidTask
 	{
 		public override string TaskPrefix => "RLC";
 
@@ -66,11 +64,11 @@ public override bool RunTask ()
 		{
 			Log.LogDebugMessage ("Task ReadLibraryProjectImportsCache");
 			Log.LogDebugMessage ("  CacheFile: {0}", CacheFile);
-			if (!File.Exists (CacheFile)) {
+			if (CacheFile.IsNullOrEmpty () || !File.Exists (TaskEnvironment.GetAbsolutePath (CacheFile))) {
 				Log.LogDebugMessage ("{0} does not exist. No Project Library Imports found", CacheFile);
 				return !Log.HasLoggedErrors;
 			}
-			var doc = XDocument.Load (CacheFile);
+			var doc = XDocument.Load (TaskEnvironment.GetAbsolutePath (CacheFile));
 			Jars = doc.GetPathsAsTaskItems ("Jars", "Jar");
 			ResolvedAssetDirectories = doc.GetPathsAsTaskItems ("ResolvedAssetDirectories", "ResolvedAssetDirectory");
 			ResolvedResourceDirectories = doc.GetPathsAsTaskItems ("ResolvedResourceDirectories", "ResolvedResourceDirectory");
@@ -84,4 +82,3 @@ public override bool RunTask ()
 		}
 	}
 }
-

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

@@ -8,13 +8,48 @@
 using Microsoft.Build.Framework;
 using Mono.Cecil;
 using NUnit.Framework;
+using Xamarin.Android.Tasks;
 using Xamarin.ProjectTools;
 
 namespace Xamarin.Android.Build.Tests
 {
 	[TestFixture]
 	public class MultiThreadedMigrationTests
 	{
+		[Test]
+		public void ReadLibraryProjectImportsCacheUsesProjectDirectory ()
+		{
+			var directory = Path.Combine (Path.GetTempPath (), Path.GetRandomFileName ());
+			Directory.CreateDirectory (directory);
+			try {
+				var cacheFile = Path.GetRandomFileName ();
+				File.WriteAllText (Path.Combine (directory, cacheFile), "<Paths><Jars><Jar>a.jar</Jar></Jars></Paths>");
+				var task = new ReadLibraryProjectImportsCache {
+					BuildEngine = new MockBuildEngine (TextWriter.Null),
+					TaskEnvironment = TaskEnvironment.CreateWithProjectDirectoryAndEnvironment (directory, new Dictionary<string, string> ()),
+					CacheFile = cacheFile,
+				};
+				Assert.IsTrue (task.Execute ());
+				Assert.AreEqual ("a.jar", task.Jars?.Single ().ItemSpec);
+			} finally {
+				Directory.Delete (directory, recursive: true);
+			}
+		}
+
+		[TestCase ("")]
+		[TestCase ("missing.xml")]
+		public void ReadLibraryProjectImportsCacheMissingFile (string cacheFile)
+		{
+			var task = new ReadLibraryProjectImportsCache {
+				BuildEngine = new MockBuildEngine (TextWriter.Null),
+				TaskEnvironment = TaskEnvironment.CreateWithProjectDirectoryAndEnvironment (
+					Path.Combine (Path.GetTempPath (), Path.GetRandomFileName ()), new Dictionary<string, string> ()),
+				CacheFile = cacheFile,
+			};
+			Assert.IsTrue (task.Execute ());
+			Assert.IsNull (task.Jars);
+		}
+
 		[Test]
 		public void MigratedTasksHaveDirectAttribute ()
 		{