[Xamarin.Android.Build.Tasks] fix for parallel builds

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

@@ -74,10 +74,10 @@ void FixupResources (ITaskItem item, Dictionary<string, string> acwMap)
 				}
 				Log.LogDebugMessage ("  Processing: {0}   {1} > {2}", file, srcmodifiedDate, lastUpdate);
 				var tmpdest = Path.GetTempFileName ();
-				MonoAndroidHelper.CopyIfChanged (file, tmpdest);
+				File.Copy (file, tmpdest, overwrite: true);
 				MonoAndroidHelper.SetWriteable (tmpdest);
 				try {
-					AndroidResource.UpdateXmlResource (resdir, tmpdest, acwMap,
+					bool success = AndroidResource.UpdateXmlResource (resdir, tmpdest, acwMap,
 						ResourceDirectories.Where (s => s != item).Select(s => s.ItemSpec), (t, m) => {
 							string targetfile = file;
 							if (targetfile.StartsWith (resdir, StringComparison.InvariantCultureIgnoreCase)) {
@@ -98,6 +98,10 @@ void FixupResources (ITaskItem item, Dictionary<string, string> acwMap)
 									break;
 							}
 						});
+					if (!success) {
+						//If we failed to write the file, a warning is logged, we should skip to the next file
+						continue;
+					}
 
 					// We strip away an eventual UTF-8 BOM from the XML file.
 					// This is a requirement for the Android designer because the desktop Java renderer

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

@@ -55,6 +55,50 @@ public void BuildBasicApplicationReleaseFSharp ()
 			}
 		}
 
+		[Test]
+		public void BuildInParallel ()
+		{
+			var proj = new XamarinAndroidApplicationProject ();
+			proj.MainActivity = proj.DefaultMainActivity.Replace ("public class MainActivity : Activity", "public class MainActivity : Xamarin.Forms.Platform.Android.FormsAppCompatActivity");
+
+			var packages = proj.Packages;
+			packages.Add (KnownPackages.XamarinForms_3_0_0_561731);
+			packages.Add (KnownPackages.Android_Arch_Core_Common_26_1_0);
+			packages.Add (KnownPackages.Android_Arch_Lifecycle_Common_26_1_0);
+			packages.Add (KnownPackages.Android_Arch_Lifecycle_Runtime_26_1_0);
+			packages.Add (KnownPackages.AndroidSupportV4_27_0_2_1);
+			packages.Add (KnownPackages.SupportCompat_27_0_2_1);
+			packages.Add (KnownPackages.SupportCoreUI_27_0_2_1);
+			packages.Add (KnownPackages.SupportCoreUtils_27_0_2_1);
+			packages.Add (KnownPackages.SupportDesign_27_0_2_1);
+			packages.Add (KnownPackages.SupportFragment_27_0_2_1);
+			packages.Add (KnownPackages.SupportMediaCompat_27_0_2_1);
+			packages.Add (KnownPackages.SupportV7AppCompat_27_0_2_1);
+			packages.Add (KnownPackages.SupportV7CardView_27_0_2_1);
+			packages.Add (KnownPackages.SupportV7MediaRouter_27_0_2_1);
+			packages.Add (KnownPackages.SupportV7RecyclerView_27_0_2_1);
+
+			using (var b = CreateApkBuilder (Path.Combine ("temp", TestName))) {
+				//We don't want these things stepping on each other
+				b.BuildLogFile = null;
+				b.Save (proj, saveProject: true);
+				proj.NuGetRestore (Path.Combine (Root, b.ProjectDirectory), b.PackagesDirectory);
+
+				Parallel.For (0, 5, i => {
+					try {
+						//NOTE: things are going to break here
+						b.Build (proj);
+					} catch (Exception exc) {
+						TestContext.WriteLine ("Expected error in {0}: {1}", nameof (BuildInParallel), exc);
+					}
+				});
+
+				//The key here, is a build afterward should work
+				b.BuildLogFile = "after.log";
+				Assert.IsTrue (b.Build (proj), "The build after a parallel failed build should succeed!");
+			}
+		}
+
 		[Test]
 		public void CheckKeystoreIsCreated ()
 		{

src/Xamarin.Android.Build.Tasks/Utilities/AndroidResource.cs

@@ -11,7 +11,7 @@
 namespace Monodroid {
 	static class AndroidResource {
 
-		public static void UpdateXmlResource (string res, string filename, Dictionary<string, string> acwMap, IEnumerable<string> additionalDirectories = null, Action<TraceLevel, string> logMessage = null)
+		public static bool UpdateXmlResource (string res, string filename, Dictionary<string, string> acwMap, IEnumerable<string> additionalDirectories = null, Action<TraceLevel, string> logMessage = null)
 		{
 			// use a temporary file so we only update the real file if things actually changed
 			string tmpfile = filename + ".bk";
@@ -24,14 +24,13 @@ public static void UpdateXmlResource (string res, string filename, Dictionary<st
 						xw.WriteNode (doc.CreateNavigator (), false);
 				Xamarin.Android.Tasks.MonoAndroidHelper.CopyIfChanged (tmpfile, filename);
 				File.Delete (tmpfile);
-			}
-			catch (Exception e) {
+				return true;
+			} catch (Exception e) {
 				if (File.Exists (tmpfile)) {
 					File.Delete (tmpfile);
 				}
-				if (logMessage != null)
-					logMessage (TraceLevel.Warning, $"AndroidResgen: Warning while updating Resource XML '{filename}': {e.Message}");
-				return;
+				logMessage?.Invoke (TraceLevel.Warning, $"AndroidResgen: Warning while updating Resource XML '{filename}': {e.Message}");
+				return false;
 			}
 		}