Fix flaky AllProjectsHaveSameOutputDirectory on Windows via delete-with-retry

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/ProjectBuilder.cs

@@ -104,10 +104,7 @@ public void Save (XamarinProject project, bool doNotCleanupOnUpdate = false, boo
 
 			if (!BuiltBefore) {
 				if (project.ShouldPopulate) {
-					if (Directory.Exists (ProjectDirectory)) {
-						FileSystemUtils.SetDirectoryWriteable (ProjectDirectory);
-						Directory.Delete (ProjectDirectory, true);
-					}
+					FileSystemUtils.DeleteDirectoryWithRetry (ProjectDirectory);
 					project.Populate (ProjectDirectory, files);
 				}
 
@@ -202,10 +199,7 @@ public void Cleanup ()
 			BuiltBefore = false;
 
 			var projectDirectory = Path.Combine (XABuildPaths.TestOutputDirectory, ProjectDirectory);
-			if (Directory.Exists (projectDirectory)) {
-				FileSystemUtils.SetDirectoryWriteable (projectDirectory);
-				Directory.Delete (projectDirectory, true);
-			}
+			FileSystemUtils.DeleteDirectoryWithRetry (projectDirectory);
 		}
 
 		public struct RuntimeInfo

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/SolutionBuilder.cs

@@ -99,7 +99,7 @@ protected override void Dispose (bool disposing)
 			if (disposing)
 				if (BuildSucceeded && !string.IsNullOrEmpty (SolutionPath))
 					try {
-						Directory.Delete (SolutionPath, recursive: true);
+						FileSystemUtils.DeleteDirectoryWithRetry (SolutionPath);
 					} catch (Exception) {
 						// This happens on CI occasionally, let's not fail the test
 					}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Utilities/FileSystemUtils.cs

@@ -40,6 +40,37 @@ public static void SetDirectoryWriteable (string directory)
 			}
 		}
 
+		/// <summary>
+		/// Recursively deletes a directory, retrying on transient failures.
+		/// </summary>
+		/// <param name="directory">The directory path to delete.</param>
+		/// <param name="retries">The maximum number of retries before giving up.</param>
+		/// <remarks>
+		/// On Windows, a handle to a just-written file can still be held by another process
+		/// (e.g. the Roslyn shared-compilation server, an anti-virus scanner, or the search
+		/// indexer), causing <see cref="Directory.Delete(string, bool)"/> to throw
+		/// <see cref="UnauthorizedAccessException"/> or <see cref="IOException"/>. This method
+		/// backs off and retries to let the other process release the handle.
+		/// </remarks>
+		/// <seealso cref="SetDirectoryWriteable(string)"/>
+		public static void DeleteDirectoryWithRetry (string directory, int retries = 10)
+		{
+			if (!Directory.Exists (directory))
+				return;
+
+			for (int i = 0; ; i++) {
+				try {
+					SetDirectoryWriteable (directory);
+					Directory.Delete (directory, true);
+					return;
+				} catch (DirectoryNotFoundException) {
+					return;
+				} catch (Exception e) when ((e is UnauthorizedAccessException || e is IOException) && i < retries) {
+					Thread.Sleep (200 * (i + 1)); // back off; let AV/Roslyn release the handle
+				}
+			}
+		}
+
 		/// <summary>
 		/// Sets a single file to be writable by removing the read-only attribute if present.
 		/// </summary>