[BaseTasks] add ABI detection for RIDs (#121)

Context: dotnet/android#5432

Two cases currently do not work in .NET 6:

 1. When an Android App project includes native libraries which are
    in directory names consisting of .NET runtime identifiers instead
    of Android ABI names, e.g.

        android-arm/libfoo.so
        android-arm64/libfoo.so
        android-x86/libfoo.so
        android-x64/libfoo.so

 2. When a NuGet package places native libraries into a `native`
    directory *between* the `$(RuntimeIdentifier)` directory and the
    native library, a'la [`SQLitePCLRaw.lib.e_sqlite3.linux`][0]:

        runtimes/linux-arm/native/libe_sqlite3.so

Fix case (1) by checking using
`AndroidRidAbiHelper.RuntimeIdentifierToAbi()` on the directory name
to determine the Android ABI of the library.

Fix case (2) by also checking the native library's parent parent
directory name against Android ABI names and Runtime Identifiers.
This allows us to correctly associate

	runtimes/android-arm64/native/libe_sqlite3.so

as an arm64-v8a native library.

I implemented these two cases as fallbacks to the existing logic.

I think this will be fine for the behavior to be in "legacy"
Xamarin.Android as well as .NET 6.

I added tests for `AndroidRidAbiHelper`, since we had none before.

[0]: https://www.nuget.org/packages/SQLitePCLRaw.lib.e_sqlite3.linux/1.1.14

src/Microsoft.Android.Build.BaseTasks/AndroidRidAbiHelper.cs

@@ -19,13 +19,27 @@ public static class AndroidRidAbiHelper
 		public static string GetNativeLibraryAbi (string lib)
 		{
 			// The topmost directory the .so file is contained within
-			var dir = Path.GetFileName (Path.GetDirectoryName (lib)).ToLowerInvariant ();
-			if (dir.StartsWith ("interpreter-", StringComparison.Ordinal)) {
-				dir = dir.Substring (12);
+			var dir = Directory.GetParent (lib);
+			var dirName = dir.Name.ToLowerInvariant ();
+			if (dirName.StartsWith ("interpreter-", StringComparison.Ordinal)) {
+				dirName = dirName.Substring ("interpreter-".Length);
 			}
-			if (ValidAbis.Contains (dir)) {
-				return dir;
+			if (ValidAbis.Contains (dirName)) {
+				return dirName;
 			}
+
+			// Look for a directory with a RID as a name, such as:
+			// android-arm64/libfoo.so
+			var abi = RuntimeIdentifierToAbi (dirName);
+			if (!string.IsNullOrEmpty (abi))
+				return abi;
+
+			// Try one directory higher, such as:
+			// packages/sqlitepclraw.lib.e_sqlite3.android/1.1.11/runtimes/android-arm64/native/libe_sqlite3.so
+			abi = RuntimeIdentifierToAbi (dir.Parent.Name.ToLowerInvariant ());
+			if (!string.IsNullOrEmpty (abi))
+				return abi;
+
 			return null;
 		}
 
@@ -41,8 +55,7 @@ public static string GetNativeLibraryAbi (ITaskItem lib)
 			// First, try nominal "Link" path.
 			var link = lib.GetMetadata ("Link");
 			if (!string.IsNullOrWhiteSpace (link)) {
-				var linkdirs = link.ToLowerInvariant ().Split ('/', '\\');
-				lib_abi = ValidAbis.Where (p => linkdirs.Contains (p)).FirstOrDefault ();
+				lib_abi = GetNativeLibraryAbi (link);
 			}
 
 			// Check for a RuntimeIdentifier

tests/Microsoft.Android.Build.BaseTasks-Tests/AndroidRidAbiHelperTests.cs

@@ -0,0 +1,142 @@
+using System.Collections.Generic;
+using Microsoft.Android.Build.Tasks;
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+using NUnit.Framework;
+
+namespace Microsoft.Android.Build.BaseTasks.Tests
+{
+	[TestFixture]
+	public class AndroidRidAbiHelperTests
+	{
+		static object [] StringValueSource = new object [] {
+			new[] {
+				/* input */    "armeabi-v7a/libfoo.so",
+				/* expected */ "armeabi-v7a"
+			},
+			new[] {
+				/* input */    "arm64-v8a/libfoo.so",
+				/* expected */ "arm64-v8a"
+			},
+			new[] {
+				/* input */    "x86/libfoo.so",
+				/* expected */ "x86"
+			},
+			new[] {
+				/* input */    "x86_64/libfoo.so",
+				/* expected */ "x86_64"
+			},
+			new[] {
+				/* input */    "android-arm/libfoo.so",
+				/* expected */ "armeabi-v7a"
+			},
+			new[] {
+				/* input */    "android-arm64/libfoo.so",
+				/* expected */ "arm64-v8a"
+			},
+			new[] {
+				/* input */    "android-x86/libfoo.so",
+				/* expected */ "x86"
+			},
+			new[] {
+				/* input */    "android-x64/libfoo.so",
+				/* expected */ "x86_64"
+			},
+			new[] {
+				/* input */    "android-arm/native/libfoo.so",
+				/* expected */ "armeabi-v7a"
+			},
+			new[] {
+				/* input */    "android-arm64/native/libfoo.so",
+				/* expected */ "arm64-v8a"
+			},
+			new[] {
+				/* input */    "android-x86/native/libfoo.so",
+				/* expected */ "x86"
+			},
+			new[] {
+				/* input */    "android-x64/native/libfoo.so",
+				/* expected */ "x86_64"
+			},
+			new[] {
+				/* input */    "android.21-x64/native/libfoo.so",
+				/* expected */ "x86_64"
+			},
+			new[] {
+				/* input */    "packages/sqlitepclraw.lib.e_sqlite3.android/1.1.11/runtimes/android-arm64/native/libe_sqlite3.so",
+				/* expected */ "arm64-v8a"
+			}
+		};
+
+		[Test]
+		[TestCaseSource (nameof (StringValueSource))]
+		public void StringValue (string input, string expected)
+		{
+			Assert.AreEqual (expected, AndroidRidAbiHelper.GetNativeLibraryAbi (input));
+		}
+
+		static object [] ITaskItemValueSource = new object [] {
+			new object [] {
+				/* input */
+				new TaskItem("armeabi-v7a/libfoo.so"),
+				/* expected */
+				"armeabi-v7a"
+			},
+			new object [] {
+				/* input */
+				new TaskItem("libabi.so", new Dictionary<string,string> {
+					{ "Abi", "armeabi-v7a" }
+				}),
+				/* expected */
+				"armeabi-v7a"
+			},
+			new object [] {
+				/* input */
+				new TaskItem("librid.so", new Dictionary<string,string> {
+					{ "RuntimeIdentifier", "android-arm" }
+				}),
+				/* expected */
+				"armeabi-v7a"
+			},
+			new object [] {
+				/* input */
+				new TaskItem("liblink.so", new Dictionary<string,string> {
+					{ "Link", "armeabi-v7a/libfoo.so" }
+				}),
+				/* expected */
+				"armeabi-v7a"
+			},
+			new object [] {
+				/* input */
+				new TaskItem("liblink.so", new Dictionary<string,string> {
+					{ "Link", "x86/libfoo.so" }
+				}),
+				/* expected */
+				"x86"
+			},
+			new object [] {
+				/* input */
+				new TaskItem("liblink.so", new Dictionary<string,string> {
+					{ "Link", "x86_64/libfoo.so" }
+				}),
+				/* expected */
+				"x86_64"
+			},
+			new object [] {
+				/* input */
+				new TaskItem("libridlink.so", new Dictionary<string,string> {
+					{ "Link", "android-arm/libfoo.so" }
+				}),
+				/* expected */
+				"armeabi-v7a"
+			},
+		};
+
+		[Test]
+		[TestCaseSource (nameof (ITaskItemValueSource))]
+		public void ITaskItemValue (ITaskItem input, string expected)
+		{
+			Assert.AreEqual (expected, AndroidRidAbiHelper.GetNativeLibraryAbi (input));
+		}
+	}
+}