[r8] ignore custom multidex.keep files if API >= 21

Context: https://r8.googlesource.com/r8/+/0e5c4339df0207a0e38f11438db84b29f328f777%5E%21/

In R8 1.4.93, they appear to have removed support for custom
`multidex.keep` files when `minSdkVersion` >= 21. R8 decides on its
own if multidex is needed, and splits up dex files appropriately.

Since `r8.jar` will now abort with an error if you specify
`multidex.keep` files and `minSdkVersion` >= 21. I feel all we can do
in this case is add a new warning if developers specify
`@(MultiDexMainDexList)` with R8--then allow R8 to calculate multidex
on its own.

I updated the `MultiDexCustomMainDexFileList` to check for these
scenarios. R8 does appear to create multiple dex files without any
multidex settings applied.

Documentation/guides/messages/xa4306.md

@@ -0,0 +1,26 @@
+---
+title: Xamarin.Android warning XA4306
+description: XA4306 warning code
+ms.date: 05/28/2019
+---
+# Xamarin.Android warning XA4306
+
+## Issue
+
+[d8/r8][r8_source] does not support using custom `multidex.keep` files
+if `minSdkVersion` >= 21, e.g. devices where native multidex is
+supported. In theses cases, d8/r8 calculates the contents of the main
+`classes.dex` without rules specified by developers.
+
+[This change][r8_commit] in d8/r8 appeared in version 1.4.93.
+
+## Solution
+
+Verify you are not declaring any `MultiDexMainDexList` build items.
+
+Consider submitting a [bug][bug] if your application does not function
+after removing any `MultiDexMainDexList` items.
+
+[r8_source]: https://r8.googlesource.com/
+[r8_commit]: https://r8.googlesource.com/r8/+/0e5c4339df0207a0e38f11438db84b29f328f777%5E%21/
+[bug]: https://github.com/xamarin/xamarin-android/wiki/Submitting-Bugs,-Feature-Requests,-and-Pull-Requests

src/Xamarin.Android.Build.Tasks/Tasks/D8.cs

@@ -45,6 +45,8 @@ protected override string GenerateCommandLineCommands ()
 
 		protected virtual string MainClass => "com.android.tools.r8.D8";
 
+		protected int MinApiVersion { get; set; }
+
 		protected virtual CommandLineBuilder GetCommandLineBuilder ()
 		{
 			var cmd = new CommandLineBuilder ();
@@ -66,8 +68,8 @@ protected virtual CommandLineBuilder GetCommandLineBuilder ()
 			//NOTE: if this is blank, we can omit --min-api in this call
 			if (!string.IsNullOrEmpty (AndroidManifestFile)) {
 				var doc = AndroidAppManifest.Load (AndroidManifestFile, MonoAndroidHelper.SupportedVersions);
-				int minApiVersion = doc.MinSdkVersion == null ? 4 : (int)doc.MinSdkVersion;
-				cmd.AppendSwitchIfNotNull ("--min-api ", minApiVersion.ToString ());
+				MinApiVersion = doc.MinSdkVersion == null ? 4 : (int)doc.MinSdkVersion;
+				cmd.AppendSwitchIfNotNull ("--min-api ", MinApiVersion.ToString ());
 			}
 
 			if (!EnableDesugar)

src/Xamarin.Android.Build.Tasks/Tasks/R8.cs

@@ -51,7 +51,9 @@ protected override CommandLineBuilder GetCommandLineBuilder ()
 			var cmd = base.GetCommandLineBuilder ();
 
 			if (EnableMultiDex) {
-				if (string.IsNullOrEmpty (MultiDexMainDexListFile)) {
+				if (MinApiVersion >= 21) {
+					Log.LogCodedWarning ("XA4306", "R8 does not support `MultiDexMainDexList` files when android:minSdkVersion >= 21");
+				} else if (string.IsNullOrEmpty (MultiDexMainDexListFile)) {
 					Log.LogCodedWarning ("XA4305", $"MultiDex is enabled, but '{nameof (MultiDexMainDexListFile)}' was not specified.");
 				} else {
 					var content = new List<string> ();

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

@@ -965,30 +965,34 @@ public void BuildAfterMultiDexIsNotRequired ([Values (true, false)] bool useD8)
 		}
 
 		[Test]
-		public void MultiDexCustomMainDexFileList ([Values (true, false)] bool useD8)
+		public void MultiDexCustomMainDexFileList ([Values ("dx", "d8")] string dexTool, [Values ("19", "21")] string minSdkVersion)
 		{
 			var expected = new [] {
 				"android/support/multidex/ZipUtil$CentralDirectory.class",
 				"android/support/multidex/MultiDexApplication.class",
 				"android/support/multidex/MultiDex$V19.class",
-				"android/support/multidex/MultiDex$V4.class",
 				"android/support/multidex/ZipUtil.class",
 				"android/support/multidex/MultiDexExtractor$1.class",
 				"android/support/multidex/MultiDexExtractor.class",
-				"android/support/multidex/MultiDex$V14.class",
 				"android/support/multidex/MultiDex.class",
 				"MyTest.class",
 			};
 			var proj = CreateMultiDexRequiredApplication ();
-			if (useD8) {
-				proj.DexTool = "d8";
-			}
+			proj.DexTool = dexTool;
+			proj.AndroidManifest = proj.AndroidManifest.Replace ("<uses-sdk />", $"<uses-sdk android:minSdkVersion=\"{minSdkVersion}\" />");
 			proj.SetProperty ("AndroidEnableMultiDex", "True");
 			proj.OtherBuildItems.Add (new BuildItem ("MultiDexMainDexList", "mymultidex.keep") { TextContent = () => "MyTest.class", Encoding = Encoding.ASCII });
 			proj.OtherBuildItems.Add (new BuildItem ("AndroidJavaSource", "MyTest.java") { TextContent = () => "public class MyTest {}", Encoding = Encoding.ASCII });
-			using (var b = CreateApkBuilder (Path.Combine ("temp", $"{nameof (MultiDexCustomMainDexFileList)}{useD8}"))) {
+			using (var b = CreateApkBuilder (Path.Combine ("temp", TestName))) {
 				Assert.IsTrue (b.Build (proj), "build should succeed. Run will fail.");
-
+				string androidBinDir = Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath, "android", "bin");
+				Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+				FileAssert.Exists (Path.Combine (androidBinDir, "classes.dex"));
+				FileAssert.Exists (Path.Combine (androidBinDir, "classes2.dex"));
+				if (dexTool == "d8" && minSdkVersion == "21") {
+					//NOTE: d8/r8 does not support custom dex list files in this case
+					return;
+				}
 				//NOTE: d8 has the list in a different order, so we should do an unordered comparison
 				var actual = File.ReadAllLines (Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath, "multidex.keep"));
 				foreach (var item in expected) {