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

[jonathanpeppers]

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 #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.

[jonathanpeppers]

So looks like gendarme is reporting errors:

mono64 --debug=casts lib/gendarme-2.10/gendarme.exe --html gendarme.html --xml gendarme.xml --ignore gendarme-ignore.txt bin/GendarmeDebug/netstandard2.0/Java.Interop.dll
Gendarme v2.10.0.0
Copyright (C) 2005-2011 Novell, Inc. and contributors

Initialization: 0.9 seconds
Java.Interop.dll: 0.8 seconds
TearDown: <0.1 seconds

One assembly processed in 1.7 seconds.
31 defects found. Reports written to: `gendarme.xml',`gendarme.html'.

I'll check that out on macOS. I can also make gendarme.html an artifact or something so we can read it easily in the future.

[jonathanpeppers]

I fixed up the build definition, too; so we'll have these next time:

[jonpryor]

As part of this PR, please update tests/invocation-overhead/jni.cs by running:

$ make -C tests/invocation-overhead

That will implicitly update tests/invocation-overhead/jni.cs.

Additionally, please apply the following patch to tests:

diff --git a/tests/invocation-overhead/Makefile b/tests/invocation-overhead/Makefile
index 66f61d7..8f58b59 100644
--- a/tests/invocation-overhead/Makefile
+++ b/tests/invocation-overhead/Makefile
@@ -9,6 +9,7 @@ clean:
 
 include ../../build-tools/scripts/mono.mk
 include ../../build-tools/scripts/jdk.mk
+include ../../bin/BuildDebug/JdkInfo.mk
 include ../../build-tools/scripts/msbuild.mk
 
 $(JNIENV_GEN):

[jonathanpeppers]

There must be something else I need to fix here:

jni.cs(15668,19): error CS1594: Delegate `Java.Interop.XAIntPtrs.JniAction_JNIEnvPtr_jobject_IntPtr_JniArgumentValuePtr' has some invalid arguments
jni.cs(15668,83): error CS1503: Argument `#4' cannot convert `System.IntPtr' expression to type `Java.Interop.JniArgumentValue*'

The line is:

var __info = JniEnvironment.CurrentInfo;
__info.Invoker.CallStaticVoidMethodA (__info.EnvironmentPointer, type, method, (IntPtr) args);

Looking into it.