[Java.Interop.Tools.JavaCallableWrappers] Add TypeNameMapGenerator.Dispose() (#91)

When the [`<GenerateJavaStubs/>` task in `xamarin-android`][0] uses
`Java.Interop.Tools.JavaCallableWrappers.TypeNameMapGenerator`
and calls `TypeNameMapGenerator.WriteJavaToManaged()`, a
`NullReferenceException` is thrown:

	Error executing task GenerateJavaStubs: System.NullReferenceException: Object reference not set to an instance of an object
	  at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Resolve (Mono.Cecil.AssemblyNameReference reference, Mono.Cecil.ReaderParameters parameters) [0x00028] in <3f2ebd7ea1fd47bd954f448c66f03323>:0
	  at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Resolve (Mono.Cecil.AssemblyNameReference reference) [0x00001] in <3f2ebd7ea1fd47bd954f448c66f03323>:0
	  at Mono.Cecil.MetadataResolver.Resolve (Mono.Cecil.TypeReference type) [0x0004e] in <4f9b8c8361824d0ca9025449919e12be>:0
	  at Mono.Cecil.ModuleDefinition.Resolve (Mono.Cecil.TypeReference type) [0x00007] in <4f9b8c8361824d0ca9025449919e12be>:0
	  at Mono.Cecil.TypeReference.Resolve () [0x00014] in <4f9b8c8361824d0ca9025449919e12be>:0
	  at Java.Interop.Tools.Cecil.TypeDefinitionRocks.GetBaseType (Mono.Cecil.TypeDefinition type) [0x00014] in <3f2ebd7ea1fd47bd954f448c66f03323>:0
	  at Java.Interop.Tools.Cecil.TypeDefinitionRocks+<GetTypeAndBaseTypes>c__Iterator0.MoveNext () [0x00048] in <3f2ebd7ea1fd47bd954f448c66f03323>:0
	  at System.Linq.Enumerable.Any[TSource] (System.Collections.Generic.IEnumerable`1[T] source, System.Func`2[T,TResult] predicate) [0x00048] in <51a1d86c1edf494cacaba8bbf8d1334e>:0
	  at Java.Interop.Tools.Cecil.TypeDefinitionRocks.ImplementsInterface (Mono.Cecil.TypeDefinition type, System.String interfaceName) [0x00014] in <3f2ebd7ea1fd47bd954f448c66f03323>:0
	  at Java.Interop.Tools.TypeNameMappings.JniType.ToJniName (Mono.Cecil.TypeDefinition type, Java.Interop.ExportParameterKind exportKind) [0x00049] in <3076bd81225243d6b1a7e6e2e66cb5f7>:0
	  at Java.Interop.Tools.TypeNameMappings.JniType.ToJniName (Mono.Cecil.TypeDefinition type) [0x00001] in <3076bd81225243d6b1a7e6e2e66cb5f7>:0
	  at Java.Interop.Tools.JavaCallableWrappers.TypeNameMapGenerator.GetTypeMapping (System.Func`2[T,TResult] skipType, System.Func`2[T,TResult] key, System.Func`2[T,TResult] value) [0x00047] in <3076bd81225243d6b1a7e6e2e66cb5f7>:0
	  at Java.Interop.Tools.JavaCallableWrappers.TypeNameMapGenerator.WriteJavaToManaged (System.IO.Stream output) [0x00012] in <3076bd81225243d6b1a7e6e2e66cb5f7>:0
	  at Xamarin.Android.Tasks.GenerateJavaStubs.UpdateWhenChanged (System.String path, System.Action`1[T] generator) [0x00014] in <d1b286cda0c74ed8bdaf1b21993bd410>:0
	  at Xamarin.Android.Tasks.GenerateJavaStubs.WriteTypeMappings (System.Collections.Generic.List`1[T] types) [0x00078] in <d1b286cda0c74ed8bdaf1b21993bd410>:0
	  at Xamarin.Android.Tasks.GenerateJavaStubs.Run (Java.Interop.Tools.Cecil.DirectoryAssemblyResolver res) [0x00153] in <d1b286cda0c74ed8bdaf1b21993bd410>:0
	  at Xamarin.Android.Tasks.GenerateJavaStubs.Execute () [0x001f2] in <d1b286cda0c74ed8bdaf1b21993bd410>:0

The underlying problem is that because the
`TypeNameMapGenerator(IEnumerable<string>, Action<string, string>)`
constructor disposes of the intermediate `DirectoryAssemblyResolver`
isntance [^1], when e.g. `TypeNameMapGenerator.WriteJavaToManaged()`
is later called, `DirectoryAssemblyResolver.Resolve()` will
implicitly be called (because `DirectoryAssemblyResolver.Load()` sets
`ReaderParameters.AssemblyResolver` to `this`), which attempts to
access `DirectoryAssemblyResolver.cache`, which is `null`, resulting
in the aforementioned `NullReferenceException`.

To fix this, update `TypeNameMapGenerator` to:

1. Hold the `DirectoryAssemblyResolver` as a member, instead of
    retaining it as a local variable
2. Implement `IDisposable`
3. Have `TypeNameMapGenerator.Dispose()` dispose of the
    `DirectoryAssemblyResolver` instance, if present.

[0]: https://github.com/xamarin/xamarin-android/blob/16d65fa/src/Xamarin.Android.Build.Tasks/Tasks/GenerateJavaStubs.cs#L267-L274

[^1]: `Dispose()` location:

      at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Dispose (System.Boolean disposing) [0x00000] in <3f2ebd7ea1fd47bd954f448c66f03323>:0
      at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Dispose () [0x00000] in <3f2ebd7ea1fd47bd954f448c66f03323>:0
      at Java.Interop.Tools.JavaCallableWrappers.TypeNameMapGenerator..ctor (System.Collections.Generic.IEnumerable`1[T] assemblies, System.Action`2[T1,T2] logMessage) [0x00000] in <3076bd81225243d6b1a7e6e2e66cb5f7>:0
      at Xamarin.Android.Tasks.GenerateJavaStubs.WriteTypeMappings (System.Collections.Generic.List`1[T] types) [0x00000] in <d1b286cda0c74ed8bdaf1b21993bd410>:0
      at Xamarin.Android.Tasks.GenerateJavaStubs.Run (Java.Interop.Tools.Cecil.DirectoryAssemblyResolver res) [0x00000] in <d1b286cda0c74ed8bdaf1b21993bd410>:0
      at Xamarin.Android.Tasks.GenerateJavaStubs.Execute () [0x00000] in <d1b286cda0c74ed8bdaf1b21993bd410>:0

Author: radekdoulik

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers/TypeNameMapGenerator.cs

@@ -54,10 +54,11 @@ namespace Java.Interop.Tools.JavaCallableWrappers {
 	 * The rows MUST be sorted so that strcmp(3) can be used to compare keys
 	 * values between rows
 	 */
-	public class TypeNameMapGenerator {
+	public class TypeNameMapGenerator : IDisposable {
 
 		Action<string, object[]>        Log;
 		List<TypeDefinition>            Types;
+		DirectoryAssemblyResolver       Resolver;
 
 		public TypeNameMapGenerator (IEnumerable<string> assemblies, Action<string, object []> logMessage)
 		{
@@ -70,20 +71,19 @@ public TypeNameMapGenerator (IEnumerable<string> assemblies, Action<string, obje
 			var Assemblies  = assemblies.ToList ();
 			var rp          = new ReaderParameters (ReadingMode.Immediate);
 
-			using (var resolver = new DirectoryAssemblyResolver (Log, loadDebugSymbols: true, loadReaderParameters: rp)) {
-				foreach (var assembly in Assemblies) {
-					var directory   = Path.GetDirectoryName (assembly);
-					if (string.IsNullOrEmpty (directory))
-						continue;
-					if (!resolver.SearchDirectories.Contains (directory))
-						resolver.SearchDirectories.Add (directory);
-				}
-				foreach (var assembly in Assemblies) {
-					resolver.Load (Path.GetFullPath (assembly));
-				}
-
-				Types       = JavaTypeScanner.GetJavaTypes (Assemblies, resolver, logMessage);
+			Resolver = new DirectoryAssemblyResolver (Log, loadDebugSymbols: true, loadReaderParameters: rp);
+			foreach (var assembly in Assemblies) {
+				var directory   = Path.GetDirectoryName (assembly);
+				if (string.IsNullOrEmpty (directory))
+					continue;
+				if (!Resolver.SearchDirectories.Contains (directory))
+					Resolver.SearchDirectories.Add (directory);
 			}
+			foreach (var assembly in Assemblies) {
+				Resolver.Load (Path.GetFullPath (assembly));
+			}
+
+			Types       = JavaTypeScanner.GetJavaTypes (Assemblies, Resolver, logMessage);
 		}
 
 		public TypeNameMapGenerator (IEnumerable<TypeDefinition> types, Action<string, object[]> logMessage)
@@ -97,6 +97,21 @@ public TypeNameMapGenerator (IEnumerable<TypeDefinition> types, Action<string, o
 			Types       = types.ToList ();
 		}
 
+		public void Dispose ()
+		{
+			Dispose (true);
+			GC.SuppressFinalize (this);
+		}
+
+		protected virtual void Dispose (bool disposing)
+		{
+			if (!disposing || Resolver == null)
+				return;
+
+			Resolver.Dispose ();
+			Resolver = null;
+		}
+
 		void LogWarning (string format, params object[] args)
 		{
 			Log (format, args);