[Xamarin.Android.Build.Tasks] CopyResource should fail with an error code

Documentation/guides/messages/xa0116.md

@@ -0,0 +1,12 @@
+# Compiler Error XA0116
+
+Xamarin.Android's build process extracts various `EmbeddedResource`
+files from its assemblies during a build. These include internal Java
+source files that are required for a Xamarin.Android application to
+start successfully. A `XA0116` error would only be encountered if one
+of these required files is missing.
+
+Consider submitting a [bug][bug] if you are getting this error under
+normal circumstances.
+
+[bug]: https://github.com/xamarin/xamarin-android/wiki/Submitting-Bugs,-Feature-Requests,-and-Pull-Requests

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

@@ -34,7 +34,11 @@ public bool Run (Assembly assm, string ResourceName, string OutputPath, TaskLogg
 
 			// Copy out one of our embedded resources to a path
 			using (var from = GetManifestResourceStream (ResourceName)) {
-
+				if (from == null) {
+					Log.LogCodedError ("XA0116", $"Unable to find `EmbeddedResource` of name `{ResourceName}`.");
+					return false;
+				}
+
 				// If the resource already exists, only overwrite if it's changed
 				if (File.Exists (OutputPath)) {
 					var hash1 = MonoAndroidHelper.HashFile (OutputPath);

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

@@ -0,0 +1,73 @@
+using Microsoft.Build.Framework;
+using NUnit.Framework;
+using System.Collections.Generic;
+using System.IO;
+using Xamarin.Android.Tasks;
+
+namespace Xamarin.Android.Build.Tests
+{
+	[TestFixture]
+	public class CopyResourceTests
+	{
+		string tempFile;
+		List<BuildErrorEventArgs> errors;
+		List<BuildMessageEventArgs> messages;
+		MockBuildEngine engine;
+
+		[SetUp]
+		public void Setup ()
+		{
+			tempFile = Path.GetTempFileName ();
+			engine = new MockBuildEngine (TestContext.Out,
+				errors: errors = new List<BuildErrorEventArgs> (),
+				messages: messages = new List<BuildMessageEventArgs> ());
+		}
+
+		[TearDown]
+		public void TearDown ()
+		{
+			File.Delete (tempFile);
+		}
+
+		// If we remove one of these, this test should fail
+		static object [] EmbeddedResources = new object [] {
+			new object[] { "machine.config" },
+			new object[] { "MonoRuntimeProvider.Bundled.java" },
+			new object[] { "NotifyTimeZoneChanges.java" },
+			new object[] { "Seppuku.java" },
+			new object[] { "IncrementalClassLoader.java" },
+			new object[] { "MultiDexLoader.java" },
+			new object[] { "Placeholder.java" },
+		};
+
+		[Test]
+		[TestCaseSource ("EmbeddedResources")]
+		public void FilesThatAreExpected (string resourceName)
+		{
+			var task = new CopyResource {
+				BuildEngine = engine,
+				ResourceName = resourceName,
+				OutputPath = tempFile,
+			};
+			Assert.IsTrue (task.Execute (), "task should succeed!");
+			FileAssert.Exists (tempFile);
+			Assert.AreNotEqual (0, new FileInfo (tempFile).Length, "file should be non-empty!");
+		}
+
+		[Test]
+		public void FileThatDoesNotExist ()
+		{
+			var resourceName = "thisdoesnotexist";
+			var task = new CopyResource {
+				BuildEngine = engine,
+				ResourceName = resourceName,
+				OutputPath = tempFile,
+			};
+			Assert.IsFalse (task.Execute (), "task should fail!");
+			Assert.AreEqual (1, errors.Count);
+			var error = errors [0];
+			Assert.AreEqual ("XA0116", error.Code);
+			StringAssert.Contains (resourceName, error.Message);
+		}
+	}
+}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Xamarin.Android.Build.Tests.csproj

@@ -82,6 +82,7 @@
     <Compile Include="ResolveSdksTaskTests.cs" />
     <Compile Include="AndroidResourceTests.cs" />
     <Compile Include="ManagedResourceParserTests.cs" />
+    <Compile Include="Tasks\CopyResourceTests.cs" />
     <Compile Include="Tasks\KeyToolTests.cs" />
     <Compile Include="Aapt2Tests.cs" />
     <Compile Include="Tasks\ValidateJavaVersionTests.cs" />