[jnienv-gen] fix p/invoke usage for .NET framework

I recently attempted to use Java.Interop from a full .NET framework
console application on Windows.

We don't currently build `java-interop.dll` for Windows, so I:

* Took `C:\Program Files (x86)\Microsoft Visual
  Studio\2019\Enterprise\MSBuild\Xamarin\Android\libmono-android.release.dll`
  and just renamed it to `java-interop.dll`.
* Since this is a 64-bit binary, I made the .NET framework project
  targeting `x64` only (it was *not* `AnyCPU`).
* I added `java-interop.dll` as a `Content` build action.

My console app was attempting to run the `main` method of `r8.jar`:

    var builder = new JreRuntimeOptions {
        JvmLibraryPath = @"C:\Users\jopepper\android-toolchain\jdk\jre\bin\server\jvm.dll",
        MarshalMemberBuilder = new ProxyMarshalMemberBuilder (),
        ObjectReferenceManager = new ProxyObjectReferenceManager (),
        ValueManager = new ProxyValueManager (),
        TypeManager = new ProxyTypeManager (),
    };

    builder.ClassPath.Add (@"C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\MSBuild\Xamarin\Android\r8.jar");

    using (var jre = builder.CreateJreVM ()) {
        var @string = new JniType ("java/lang/String");
        var swissArmyKnife = new JniType ("com.android.tools.r8.SwissArmyKnife");
        var main = swissArmyKnife.GetStaticMethod ("main", "([Ljava/lang/String;)V");

        var help = JniEnvironment.Strings.NewString ("--help");
        var args = JniEnvironment.Arrays.NewObjectArray (1, @string.PeerReference, help);
        var __args = stackalloc JniArgumentValue [1];
        __args [0] = new JniArgumentValue (args);
        JniEnvironment.StaticMethods.CallStaticVoidMethod (swissArmyKnife.PeerReference, main, __args);
    }

Unfortunately this code crashes at runtime with a cryptic error on any
p/invoke using `JniArgumentValue*`:

    System.Runtime.InteropServices.MarshalDirectiveException:
        Cannot marshal 'parameter dotnet#5': Pointers cannot reference marshaled structures.  Use ByRef instead.

This seems like a limitation of .NET framework...

However, it seems to work fine if we use `IntPtr` instead and just
cast any `JniArgumentValue*` values to `IntPtr`.

So for example, the p/invoke can change to:

    [DllImport (JavaInteropLib, CallingConvention=CallingConvention.Cdecl, CharSet=CharSet.Ansi)]
    internal static extern unsafe jobject java_interop_jnienv_call_object_method_a (IntPtr jnienv, out IntPtr thrown, jobject instance, IntPtr method, IntPtr args);

`args` used to be a `JniArgumentValue*`. Other generated methods need
a cast, such as:

    public static unsafe JniObjectReference CallObjectMethod (JniObjectReference instance, JniMethodInfo method, JniArgumentValue* args)
    {
        ...
        IntPtr thrown;
        var tmp = NativeMethods.java_interop_jnienv_call_object_method_a (JniEnvironment.EnvironmentPointer, out thrown, instance.Handle, method.ID, (IntPtr) args);
        ...
    }

After this, my .NET framework console app was able to start, and it
printed `r8 --help` output.

Author: jonathanpeppers

build-tools/jnienv-gen/Generator.cs

@@ -308,11 +308,11 @@ static void GenerateNativeMethods (TextWriter o, HandleStyle style)
 				o.WriteLine ();
 				o.WriteLine ("\t\t[DllImport (JavaInteropLib, CallingConvention=CallingConvention.Cdecl, CharSet=CharSet.Ansi)]");
 				o.WriteLine ("\t\tinternal static extern unsafe {0} {1} (IntPtr jnienv{2}{3}{4});",
-					entry.ReturnType.GetMarshalType (style, isReturn: true),
+					entry.ReturnType.GetMarshalType (style, isReturn: true, isPinvoke: true),
 					GetPinvokeName (entry.Name),
 					entry.Throws ? ", out IntPtr thrown" : "",
 					entry.Parameters.Length != 0 ? ", " : "",
-					string.Join (", ", entry.Parameters.Select (p => string.Format ("{0} {1}", p.Type.GetMarshalType (style, isReturn: false), Escape (p.Name)))));
+					string.Join (", ", entry.Parameters.Select (p => string.Format ("{0} {1}", p.Type.GetMarshalType (style, isReturn: false, isPinvoke: true), Escape (p.Name)))));
 			}
 			o.WriteLine ("\t}");
 			o.WriteLine ();
@@ -653,8 +653,8 @@ protected TypeInfo (string jniType)
 			JniType = jniType;
 		}
 
-		public  abstract    string      GetMarshalType (HandleStyle style, bool isReturn);
-		public  abstract    string      GetManagedType (HandleStyle style, bool isReturn);
+		public  abstract    string      GetMarshalType (HandleStyle style, bool isReturn, bool isPinvoke = false);
+		public  abstract    string      GetManagedType (HandleStyle style, bool isReturn, bool isPinvoke = false);
 
 		public  virtual     string[]    GetHandleCreationLogStatements (HandleStyle style, string method, string variable)
 		{
@@ -681,6 +681,10 @@ public virtual string[] VerifyParameter (HandleStyle style, string variable)
 
 	class BuiltinTypeInfo : TypeInfo {
 
+		/// <summary>
+		/// NOTE: .NET framework can't marshal this
+		/// </summary>
+		const string JniArgumentValue = "JniArgumentValue*";
 		string managed;
 
 		public BuiltinTypeInfo (string jni, string managed)
@@ -689,16 +693,31 @@ public BuiltinTypeInfo (string jni, string managed)
 			this.managed    = managed;
 		}
 
-		public override string GetMarshalType (HandleStyle style, bool isReturn)
+		public override string GetMarshalType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
+			if (isPinvoke && managed == JniArgumentValue) {
+				return "IntPtr";
+			}
 			return managed;
 		}
 
-		public override string GetManagedType (HandleStyle style, bool isReturn)
+		public override string GetManagedType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
+			if (isPinvoke && managed == JniArgumentValue) {
+				return "IntPtr";
+			}
 			return managed;
 		}
 
+		public override string GetManagedToMarshalExpression (HandleStyle style, string variable)
+		{
+			var value = base.GetManagedToMarshalExpression (style, variable);
+			if (managed == JniArgumentValue) {
+				value = "(IntPtr) " + value;
+			}
+			return value;
+		}
+
 		public override string[] VerifyParameter (HandleStyle style, string variable)
 		{
 			if (managed != "IntPtr")
@@ -720,12 +739,12 @@ public BooleanTypeInfo (string jni)
 		{
 		}
 
-		public override string GetMarshalType (HandleStyle style, bool isReturn)
+		public override string GetMarshalType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			return "byte";
 		}
 
-		public override string GetManagedType (HandleStyle style, bool isReturn)
+		public override string GetManagedType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			return "bool";
 		}
@@ -750,12 +769,12 @@ public StringTypeInfo (string jni)
 		{
 		}
 
-		public override string GetMarshalType (HandleStyle style, bool isReturn)
+		public override string GetMarshalType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			return "string";
 		}
 
-		public override string GetManagedType (HandleStyle style, bool isReturn)
+		public override string GetManagedType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			return "string";
 		}
@@ -798,12 +817,12 @@ public JniReleaseArrayElementsModeTypeInfo ()
 		{
 		}
 
-		public override string GetMarshalType (HandleStyle style, bool isReturn)
+		public override string GetMarshalType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			return "int";
 		}
 
-		public override string GetManagedType (HandleStyle style, bool isReturn)
+		public override string GetManagedType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			return "JniReleaseArrayElementsMode";
 		}
@@ -839,12 +858,12 @@ public IdTypeInfo (string jni, string type)
 			this.type   = type;
 		}
 
-		public override string GetMarshalType (HandleStyle style, bool isReturn)
+		public override string GetMarshalType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			return "IntPtr";
 		}
 
-		public override string GetManagedType (HandleStyle style, bool isReturn)
+		public override string GetManagedType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			switch (style) {
 			case HandleStyle.SafeHandle:
@@ -968,7 +987,7 @@ public ObjectReferenceTypeInfo (string jni, string safeType, string refType)
 			this.refType    = refType;
 		}
 
-		public override string GetMarshalType (HandleStyle style, bool isReturn)
+		public override string GetMarshalType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			switch (style) {
 			case HandleStyle.SafeHandle:
@@ -981,7 +1000,7 @@ public override string GetMarshalType (HandleStyle style, bool isReturn)
 			return null;
 		}
 
-		public override string GetManagedType (HandleStyle style, bool isReturn)
+		public override string GetManagedType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			switch (style) {
 			case HandleStyle.SafeHandle:
@@ -1088,12 +1107,12 @@ public JavaVMPointerTypeInfo (string jni)
 		{
 		}
 
-		public override string GetMarshalType (HandleStyle style, bool isReturn)
+		public override string GetMarshalType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			return "out IntPtr";
 		}
 
-		public override string GetManagedType (HandleStyle style, bool isReturn)
+		public override string GetManagedType (HandleStyle style, bool isReturn, bool isPinvoke)
 		{
 			return "out IntPtr";
 		}

src/Java.Interop/Java.Interop/JniEnvironment.Types.cs

@@ -53,7 +53,7 @@ public static unsafe JniObjectReference FindClass (string classname)
 				__args [0]  = new JniArgumentValue (java);
 
 				IntPtr ignoreThrown;
-				c   = NativeMethods.java_interop_jnienv_call_object_method_a (info.EnvironmentPointer, out ignoreThrown, info.Runtime.ClassLoader.Handle, info.Runtime.ClassLoader_LoadClass.ID, __args);
+				c   = NativeMethods.java_interop_jnienv_call_object_method_a (info.EnvironmentPointer, out ignoreThrown, info.Runtime.ClassLoader.Handle, info.Runtime.ClassLoader_LoadClass.ID, (IntPtr) __args);
 				JniObjectReference.Dispose (ref java);
 				if (ignoreThrown == IntPtr.Zero) {
 					JniObjectReference.Dispose (ref e);