[Xamarin.Android.Build.Tasks] <FilterAssemblies/> shouldn't throw FNFE (

dotnet#3720)

A few places during a Xamarin.Android build, we have the following
pattern:

    if (DesignTimeBuild && !File.Exists (assemblyPath)) {
        Log.LogDebugMessage ($"Skipping non-existent dependency '{assemblyPath}' during a design-time build.");
        continue;
    }

In the case of a `<ProjectReference/>` not building, there starts to
be a problem here.

What happens during a regular build, is we end up throwing a
`FileNotFoundException` in `<FilterAssemblies/>`.

If we remove the `if (DesignTimeBuild)` check, we would instead see
the behavior you would get for a .NET Console app referencing a .NET
Framework library project:

    (ResolveProjectReferences target) ->
        C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(1875,5):
        warning : The referenced project '..\MyLibrary\MyLibrary.csproj' does not exist.
    (CoreCompile target) ->
        Foo.cs(1,20,1,23):
        error CS0246: The type or namespace name 'Bar' could not be found (are you missing a using directive or an assembly reference?)
        1 Warning(s)
        1 Error(s)

We get a warning about a missing `<ProjectReference/>` and any C#
compiler errors that would be encountered.

I think this makes more sense and developers will less likely point
fingers at `<FilterAssemblies/>`?

I added a test around this scenario, too.

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

@@ -36,8 +36,8 @@ public override bool RunTask ()
 
 			var output = new List<ITaskItem> (InputAssemblies.Length);
 			foreach (var assemblyItem in InputAssemblies) {
-				if (DesignTimeBuild && !File.Exists (assemblyItem.ItemSpec)) {
-					Log.LogDebugMessage ($"Skipping non-existent dependency '{assemblyItem.ItemSpec}' during a design-time build.");
+				if (!File.Exists (assemblyItem.ItemSpec)) {
+					Log.LogDebugMessage ($"Skipping non-existent dependency '{assemblyItem.ItemSpec}'.");
 					continue;
 				}
 				using (var pe = new PEReader (File.OpenRead (assemblyItem.ItemSpec))) {

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

@@ -126,8 +126,8 @@ public override bool RunTask ()
 							Log.LogDebugMessage ($"Skipping framework assembly '{fileName}'.");
 							continue;
 						}
-						if (DesignTimeBuild && !File.Exists (assemblyPath)) {
-							Log.LogDebugMessage ($"Skipping non-existent dependency '{assemblyPath}' during a design-time build.");
+						if (!File.Exists (assemblyPath)) {
+							Log.LogDebugMessage ($"Skipping non-existent dependency '{assemblyPath}'.");
 							continue;
 						}
 						assemblies.Add (resolver.Load (assemblyPath));

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

@@ -163,8 +163,8 @@ void Extract (
 					Log.LogDebugMessage ($"Skipping framework assembly '{fileName}'.");
 					continue;
 				}
-				if (DesignTimeBuild && !File.Exists (assemblyPath)) {
-					Log.LogDebugMessage ($"Skipping non-existent dependency '{assemblyPath}' during a design-time build.");
+				if (!File.Exists (assemblyPath)) {
+					Log.LogDebugMessage ($"Skipping non-existent dependency '{assemblyPath}'.");
 					continue;
 				}
 				if (bool.TryParse (assemblyItem.GetMetadata (GetAdditionalResourcesFromAssemblies.AndroidSkipResourceExtraction), out skip) && skip) {

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

@@ -972,5 +972,54 @@ public void ChangePackageNamingPolicy ()
 				Assert.IsTrue (DexUtils.ContainsClass (className, dexFile, AndroidSdkPath), $"`{dexFile}` should include `{className}`!");
 			}
 		}
+
+		[Test]
+		public void MissingProjectReference ()
+		{
+			var path = Path.Combine ("temp", TestName);
+
+			var bar = "public class Bar { }";
+			var lib = new XamarinAndroidLibraryProject {
+				ProjectName = "MyLibrary",
+				Sources = {
+					new BuildItem.Source ("Bar.cs") {
+						TextContent = () => bar
+					},
+				}
+			};
+			var app = new XamarinAndroidApplicationProject {
+				ProjectName = "MyApp",
+				Sources = {
+					new BuildItem.Source ("Foo.cs") {
+						TextContent = () => "public class Foo : Bar { }"
+					},
+				}
+			};
+			var reference = $"..\\{lib.ProjectName}\\{lib.ProjectName}.csproj";
+			app.References.Add (new BuildItem.ProjectReference (reference, lib.ProjectName, lib.ProjectGuid));
+
+			using (var libBuilder = CreateDllBuilder (Path.Combine (path, lib.ProjectName)))
+			using (var appBuilder = CreateApkBuilder (Path.Combine (path, app.ProjectName))) {
+				libBuilder.ThrowOnBuildFailure =
+					appBuilder.ThrowOnBuildFailure = false;
+
+				// Build app before library is built
+				Assert.IsFalse (appBuilder.Build (app), "app build should have failed.");
+				Assert.IsTrue (StringAssertEx.ContainsText (appBuilder.LastBuildOutput, $"The referenced project '{reference.Replace ('\\', Path.DirectorySeparatorChar)}' does not exist."));
+				Assert.IsTrue (StringAssertEx.ContainsText (appBuilder.LastBuildOutput, " 1 Warning(s)"), "Should receive 1 Warning");
+				Assert.IsTrue (StringAssertEx.ContainsText (appBuilder.LastBuildOutput, "error CS0246"), "Should receive CS0246");
+				Assert.IsTrue (StringAssertEx.ContainsText (appBuilder.LastBuildOutput, " 1 Error(s)"), "Should receive 1 Error");
+
+				// Successful build
+				Assert.IsTrue (libBuilder.Build (lib), "lib build should have succeeded.");
+				Assert.IsTrue (appBuilder.Build (app), "app build should have succeeded.");
+
+				// Create compiler error in library, the app will still be able to build
+				bar += "}";
+				lib.Touch ("Bar.cs");
+				Assert.IsFalse (libBuilder.Build (lib), "lib build should have failed.");
+				Assert.IsTrue (appBuilder.Build (app), "app build should have succeeded.");
+			}
+		}
 	}
 }