[Java.Interop, generator] Fix GetPeerMembers for interfaces (#486)

Context: dotnet/android#3533

Consider the following Java interface which contains a default
interface method:

	// Java
	package com.xamarin.android;

	public interface DefaultMethodsInterface
	{
	    default int foo () { return 0; }
	}

This is bound as:

	// C# Binding
	public partial interface IDefaultMethodsInterface : IJavaPeerable {
	    static readonly JniPeerMembers _members = new JniPeerMembers (
	            "java/DefaultMethodsInterface",
	            typeof (IDefaultMethodsInterface));

	    [Register ("foo", "()V", "…")]
	    virtual unsafe int Foo ()
	    {
	        return _members.InstanceMethods.InvokeVirtualInt32Method ("foo.()V", this, null);
	    }
	}

If we "implement" the interface from C# *without* implementing
`IDefaultMethodsInterface.Foo()`:

	// C#
	class ManagedOverrideDefault : Java.Lang.Object, IDefaultMethodsInterface
	{
	}

Then invoke the default interface method:

	var over = new ManagedOverrideDefault ();
	(over as IDefaultMethodsInterface).Foo ();

The attempt to invoke the default implementation of
`IDefaultMethodsInterface.Foo()` causes a crash:

	Java.Lang.NoSuchMethodError : no non-static method "Ljava/lang/Object;.foo()I"

The cause of the crash is as follows: `IDefaultMethodsInterface.Foo()`
calls `JniPeerMembers.JniInstanceMethods.InvokeVirtualInt32Method()`
tries to determine if the method is overridden, and if the method is
*not* overridden -- as is the case here -- then we hit the non-virtual
invocation path:

	// src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceMethods_Invoke.cs
	var j = Members.GetPeerMembers (self);
	var n = j.InstanceMethods.GetMethodInfo (encodedMember);
	return JniEnvironment.InstanceMethods.CallNonvirtualIntMethod (self.PeerReference, j.JniPeerType.PeerReference, n, parameters);

`JniInstanceMethods.Members` will be the
`IDefaultMethodsInterface._members` field, and
`Members.GetPeerMembers()` would return `self.JniPeerMembers`.
Because this is a C# class which inherits `Java.Lang.Object`,
`Java.Lang.Object._members` is returned.  We then attempt to resolve
`foo()` from `java.lang.Object`, which *does not exist*, which is why
the `NoSuchMethodError` is thrown.

The fix is to "intercept" the `Members.GetPeerMembers()` invocation
so that `j` refers to `IDefaultMethodsInterface._members`, *not*
`Java.Lang.Object._members`.  This would allow `n` to refer to
`DefaultMethodsInterface.foo()`, which *does* exist, and *can* be
invoked via `JniEnvironment.InstanceMethods.CallNonvirtualIntMethod()`.

Add the following `JniPeerMembers` constructor overload:

	partial class JniPeerMembers {
	    public JniPeerMembers (string jniPeerTypeName, Type managedPeerType, bool isInterface);
	}

Update `JniPeerMembers.GetPeerMembers()` so that if `isInterface` is
true, we return `this` (the *interface*s `JniPeerMembers` instance).

Update `generator` so that `_members` construction within interfaces
sets the `isInterface` parameter to true, resulting in:

	partial interface IDefaultMethodsInterface {
	    static readonly JniPeerMembers _members = new JniPeerMembers (
	            "java/DefaultMethodsInterface",
	            typeof (IDefaultMethodsInterface),
	            isInterface: true);
	}

This allows `(over as IDefaultMethodsInterface).Foo()` to work
without an exception being thrown.

gendarme-ignore.txt

@@ -351,6 +351,7 @@ T: Java.Interop.JniInt32ArrayElements
 T: Java.Interop.JniInt64ArrayElements
 T: Java.Interop.JniLocationException
 T: Java.Interop.JniMethodInfo
+T: Java.Interop.JniPeerMembers
 T: Java.Interop.JniSByteArrayElements
 T: Java.Interop.JniSingleArrayElements
 T: Java.Interop.JniType

src/Java.Interop/Java.Interop/JniPeerMembers.cs

@@ -7,8 +7,15 @@ namespace Java.Interop {
 
 	public partial class JniPeerMembers {
 
+		private bool isInterface;
+
+		public JniPeerMembers (string jniPeerTypeName, Type managedPeerType, bool isInterface)
+			: this (jniPeerTypeName, managedPeerType, checkManagedPeerType: true, isInterface: isInterface)
+		{
+		}
+
 		public JniPeerMembers (string jniPeerTypeName, Type managedPeerType)
-			: this (jniPeerTypeName, managedPeerType, checkManagedPeerType: true)
+			: this (jniPeerTypeName, managedPeerType, checkManagedPeerType: true, isInterface: false)
 		{
 			if (managedPeerType == null)
 				throw new ArgumentNullException ("managedPeerType");
@@ -27,7 +34,7 @@ public JniPeerMembers (string jniPeerTypeName, Type managedPeerType)
 			ManagedPeerType = managedPeerType;
 		}
 
-		JniPeerMembers (string jniPeerTypeName, Type managedPeerType, bool checkManagedPeerType)
+		JniPeerMembers (string jniPeerTypeName, Type managedPeerType, bool checkManagedPeerType, bool isInterface = false)
 		{
 			if (jniPeerTypeName == null)
 				throw new ArgumentNullException (nameof (jniPeerTypeName));
@@ -51,6 +58,8 @@ public JniPeerMembers (string jniPeerTypeName, Type managedPeerType)
 			JniPeerTypeName = jniPeerTypeName;
 			ManagedPeerType = managedPeerType;
 
+			this.isInterface = isInterface;
+
 			instanceMethods = new JniInstanceMethods (this);
 			instanceFields  = new JniInstanceFields (this);
 			staticMethods   = new JniStaticMethods (this);
@@ -140,7 +149,7 @@ protected virtual bool UsesVirtualDispatch (IJavaPeerable value, Type declaringT
 
 		protected virtual JniPeerMembers GetPeerMembers (IJavaPeerable value)
 		{
-			return value.JniPeerMembers;
+			return isInterface ? this : value.JniPeerMembers;
 		}
 
 		internal static void AssertSelf (IJavaPeerable self)

tools/generator/Java.Interop.Tools.Generator.CodeGeneration/JavaInteropCodeGenerator.cs

@@ -29,7 +29,7 @@ static string GetInvokeType (string type)
 
 		internal override void WriteClassHandle (ClassGen type, string indent, bool requireNew)
 		{
-			WritePeerMembers (indent + '\t', true, requireNew, type.RawJniName, type.Name);
+			WritePeerMembers (indent + '\t', true, requireNew, type.RawJniName, type.Name, false);
 
 			writer.WriteLine ("{0}\tinternal static {1}IntPtr class_ref {{", indent, requireNew ? "new " : string.Empty);
 			writer.WriteLine ("{0}\t\tget {{", indent);
@@ -55,12 +55,12 @@ internal override void WriteClassHandle (ClassGen type, string indent, bool requ
 
 		internal override void WriteClassHandle (InterfaceGen type, string indent, string declaringType)
 		{
-			WritePeerMembers (indent, false, true, type.RawJniName, declaringType);
+			WritePeerMembers (indent, false, true, type.RawJniName, declaringType, type.Name == declaringType);
 		}
 
 		internal override void WriteClassInvokerHandle (ClassGen type, string indent, string declaringType)
 		{
-			WritePeerMembers (indent, true, true, type.RawJniName, declaringType);
+			WritePeerMembers (indent, true, true, type.RawJniName, declaringType, false);
 
 			writer.WriteLine ();
 			writer.WriteLine ("{0}public override global::Java.Interop.JniPeerMembers JniPeerMembers {{", indent);
@@ -75,7 +75,7 @@ internal override void WriteClassInvokerHandle (ClassGen type, string indent, st
 
 		internal override void WriteInterfaceInvokerHandle (InterfaceGen type, string indent, string declaringType)
 		{
-			WritePeerMembers (indent, true, true, type.RawJniName, declaringType);
+			WritePeerMembers (indent, true, true, type.RawJniName, declaringType, false);
 
 			writer.WriteLine ();
 			writer.WriteLine ("{0}static IntPtr java_class_ref {{", indent);
@@ -262,10 +262,10 @@ internal override void WriteFieldSetBody (Field field, string indent, GenBase ty
 			writer.WriteLine ("{0}}}", indent);
 		}
 
-		void WritePeerMembers (string indent, bool isInternal, bool isNew, string rawJniType, string declaringType)
+		void WritePeerMembers (string indent, bool isInternal, bool isNew, string rawJniType, string declaringType, bool isInterface)
 		{
 			var signature = $"{(isInternal ? "internal " : "")}static {(isNew ? "new " : "")}readonly JniPeerMembers _members = ";
-			var type = $"new {GetPeerMembersType ()} (\"{rawJniType}\", typeof ({declaringType}));";
+			var type = $"new {GetPeerMembersType ()} (\"{rawJniType}\", typeof ({declaringType}){(isInterface ? ", isInterface: true" : string.Empty)});";
 
 			writer.WriteLine ($"{indent}{signature}{type}");
 		}

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteInterfaceDefaultMethod.txt

@@ -1,7 +1,7 @@
 // Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
 [Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
 public partial interface IMyInterface : IJavaObject, IJavaPeerable {
-	static new readonly JniPeerMembers _members = new JniPeerMembers ("java/code/IMyInterface", typeof (IMyInterface));
+	static new readonly JniPeerMembers _members = new JniPeerMembers ("java/code/IMyInterface", typeof (IMyInterface), isInterface: true);
 
 	static Delegate cb_DoSomething;
 #pragma warning disable 0169

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteInterfaceDefaultProperty.txt

@@ -1,7 +1,7 @@
 // Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
 [Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
 public partial interface IMyInterface : IJavaObject, IJavaPeerable {
-	static new readonly JniPeerMembers _members = new JniPeerMembers ("java/code/IMyInterface", typeof (IMyInterface));
+	static new readonly JniPeerMembers _members = new JniPeerMembers ("java/code/IMyInterface", typeof (IMyInterface), isInterface: true);
 
 	static Delegate cb_get_Value;
 #pragma warning disable 0169

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteInterfaceDefaultPropertyGetterOnly.txt

@@ -1,7 +1,7 @@
 // Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
 [Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
 public partial interface IMyInterface : IJavaObject, IJavaPeerable {
-	static new readonly JniPeerMembers _members = new JniPeerMembers ("java/code/IMyInterface", typeof (IMyInterface));
+	static new readonly JniPeerMembers _members = new JniPeerMembers ("java/code/IMyInterface", typeof (IMyInterface), isInterface: true);
 
 	static Delegate cb_get_Value;
 #pragma warning disable 0169

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1/WriteInterfaceDefaultMethod.txt

@@ -1,7 +1,7 @@
 // Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
 [Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
 public partial interface IMyInterface : IJavaObject, IJavaPeerable {
-	static new readonly JniPeerMembers _members = new XAPeerMembers ("java/code/IMyInterface", typeof (IMyInterface));
+	static new readonly JniPeerMembers _members = new XAPeerMembers ("java/code/IMyInterface", typeof (IMyInterface), isInterface: true);
 
 	static Delegate cb_DoSomething;
 #pragma warning disable 0169

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1/WriteInterfaceDefaultProperty.txt

@@ -1,7 +1,7 @@
 // Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
 [Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
 public partial interface IMyInterface : IJavaObject, IJavaPeerable {
-	static new readonly JniPeerMembers _members = new XAPeerMembers ("java/code/IMyInterface", typeof (IMyInterface));
+	static new readonly JniPeerMembers _members = new XAPeerMembers ("java/code/IMyInterface", typeof (IMyInterface), isInterface: true);
 
 	static Delegate cb_get_Value;
 #pragma warning disable 0169

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1/WriteInterfaceDefaultPropertyGetterOnly.txt

@@ -1,7 +1,7 @@
 // Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
 [Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
 public partial interface IMyInterface : IJavaObject, IJavaPeerable {
-	static new readonly JniPeerMembers _members = new XAPeerMembers ("java/code/IMyInterface", typeof (IMyInterface));
+	static new readonly JniPeerMembers _members = new XAPeerMembers ("java/code/IMyInterface", typeof (IMyInterface), isInterface: true);
 
 	static Delegate cb_get_Value;
 #pragma warning disable 0169