[Harness] Make sure we do not copy/generate the app sources more than once.

tests/xharness/BCLTestImporter/Templates/Managed/XamariniOSTemplate.cs

@@ -91,6 +91,9 @@ public class XamariniOSTemplate : ITemplatedProject {
 		string WatchAppTemplatePath => Path.Combine (OutputDirectoryPath, "templates", "watchOS", "App").Replace ("/", "\\");
 		string WatchExtensionTemplatePath => Path.Combine (OutputDirectoryPath, "templates", "watchOS", "Extension").Replace ("/", "\\");
 
+		bool srcGenerated = false;
+		object srcGeneratedLock = new object ();
+
 		Stream GetTemplateStream (string templateName)
 		{
 			var resources = GetType ().Assembly.GetManifestResourceNames ();
@@ -229,24 +232,39 @@ internal static string GetPListPath (string rootDir, WatchAppType appType)
 			}
 		}
 
-		public async Task GenerateSource (string srcOuputPath)
+		// this method could be async, since we could be using async IO. The problem is that it might be called
+		// several times. The File and the Directory classes are not smart, and there is a possibility that we
+		// stop the thread between the Directory.Exist and the Directory.Delete. A nice way to solve this would be
+		// to set the src to get a AsyncLazy<bool> and generate the source only once on the first run, but AsyncLazy
+		// got move to target .netcore 5.0 (https://github.com/dotnet/runtime/issues/27510) yet we want to fix issue
+		// https://github.com/xamarin/xamarin-macios/issues/8240 so for the time being we lock, check, and generate
+		// if needed, which is better than:
+		//
+		// * Have a thread issue.
+		// * Generate the src EVERY SINGLE TIME for a list of projects.
+		public void GenerateSource (string srcOuputPath)
 		{
-			// mk the expected directories
-			if (Directory.Exists (srcOuputPath))
-				Directory.Delete (srcOuputPath, true); // delete, we always want to add the embeded src
-			BuildSrcTree (srcOuputPath);
-			// the code is simple, we are going to look for all the resources that we know are src and will write a
-			// copy of the stream in the designated output path
-			var resources = GetType ().Assembly.GetManifestResourceNames ().Where (a => a.StartsWith (srcResourcePrefix));
-
-			// we need to be smart, since the resource name != the path
-			foreach (var r in resources) {
-				var path = CalculateDestinationPath (srcOuputPath, r);
-				// copy the stream
-				using (var sourceReader = GetType ().Assembly.GetManifestResourceStream (r))
-				using (var destination = File.Create (path)) {
-					await sourceReader.CopyToAsync (destination);
+			lock (srcGeneratedLock) {
+				if (srcGenerated)
+					return;
+				// mk the expected directories
+				if (Directory.Exists (srcOuputPath))
+					Directory.Delete (srcOuputPath, true); // delete, we always want to add the embedded src
+				BuildSrcTree (srcOuputPath);
+				// the code is simple, we are going to look for all the resources that we know are src and will write a
+				// copy of the stream in the designated output path
+				var resources = GetType ().Assembly.GetManifestResourceNames ().Where (a => a.StartsWith (srcResourcePrefix));
+
+				// we need to be smart, since the resource name != the path
+				foreach (var r in resources) {
+					var path = CalculateDestinationPath (srcOuputPath, r);
+					// copy the stream
+					using (var sourceReader = GetType ().Assembly.GetManifestResourceStream (r))
+					using (var destination = File.Create (path)) {
+						sourceReader.CopyTo (destination);
+					}
 				}
+				srcGenerated = true;
 			}
 		}
 
@@ -314,7 +332,7 @@ string GenerateIncludeFilesNode (string projectName, (string FailureMessage, Lis
 		public async Task<GeneratedProjects> GenerateTestProjectsAsync (IEnumerable<(string Name, string [] Assemblies, string ExtraArgs, double TimeoutMultiplier)> projects, Platform platform)
 		{
 			// generate the template c# code before we create the diff projects
-			await GenerateSource (Path.Combine (OutputDirectoryPath, "templates"));
+			GenerateSource (Path.Combine (OutputDirectoryPath, "templates"));
 			var result = new GeneratedProjects ();
 			switch (platform) {
 			case Platform.WatchOS: