[Mono.Android] preserve public methods for GetJniHandleConverterForType

[jonathanpeppers]

Fixes: #6895

In some cases an app with a single HttpClient request can crash with:

android.runtime.JavaProxyThrowable: System.ArgumentNullException: ArgumentNull_Generic Arg_ParamName_Name, method
at System.Delegate.CreateDelegate(Type , Object , MethodInfo , Boolean , Boolean )
at System.Delegate.CreateDelegate(Type , MethodInfo , Boolean )
at System.Delegate.CreateDelegate(Type , MethodInfo )
at Java.Interop.JavaConvert.GetJniHandleConverterForType(Type )
at Java.Interop.JavaConvert.GetJniHandleConverter(Type )
at Java.Interop.JavaConvert.FromJniHandle[IList`1](IntPtr , JniHandleOwnership , Boolean& )
at Java.Interop.JavaConvert.FromJniHandle[IList`1](IntPtr , JniHandleOwnership )
at Android.Runtime.JavaDictionary`2[[System.String, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e],[System.Collections.Generic.IList`1[[System.String, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].Get(String )
at Android.Runtime.JavaDictionary`2[[System.String, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e],[System.Collections.Generic.IList`1[[System.String, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].get_Item(String )
at Xamarin.Android.Net.AndroidMessageHandler.CopyHeaders(HttpURLConnection , HttpResponseMessage )
at Xamarin.Android.Net.AndroidMessageHandler.DoProcessRequest(HttpRequestMessage , URL , HttpURLConnection , CancellationToken , RequestRedirectionState )
at Xamarin.Android.Net.AndroidMessageHandler.SendAsync(HttpRequestMessage , CancellationToken )
at System.Net.Http.HttpClient.GetStringAsyncCore(HttpRequestMessage , CancellationToken )
at XamarinAndroid.MainActivity.OnCreate(Bundle savedInstanceState)
at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_0(Object )
at Android.App.SyncContext.<>c__DisplayClass2_0.<Post>b__0()
at Java.Lang.Thread.RunnableImplementor.Run()
at Java.Lang.IRunnableInvoker.n_Run(IntPtr , IntPtr )
at Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PP_V(_JniMarshal_PP_V , IntPtr , IntPtr )
at mono.java.lang.RunnableImplementor.n_run(Native Method)
at mono.java.lang.RunnableImplementor.run(RunnableImplementor.java:30)
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loopOnce(Looper.java:201)
at android.os.Looper.loop(Looper.java:288)
at android.app.ActivityThread.main(ActivityThread.java:7870)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)

In this case, JavaDictionary<,>.FromJniHandle() was linked away, and
we access it via reflection:

xamarin-android/src/Mono.Android/Java.Interop/JavaConvert.cs

Lines 87 to 92 in 9f3ee58

	static Func<IntPtr, JniHandleOwnership, object> GetJniHandleConverterForType (Type t)
	{
	MethodInfo m = t.GetMethod ("FromJniHandle", BindingFlags.Static | BindingFlags.Public)!;
	return (Func<IntPtr, JniHandleOwnership, object>) Delegate.CreateDelegate (
	typeof (Func<IntPtr, JniHandleOwnership, object>), m);
	}

This works in Xamarin.Android due to:

xamarin-android/src/Xamarin.Android.Build.Tasks/Linker/PreserveLists/Mono.Android.xml

Lines 25 to 28 in 0ab3db0

	<type fullname="Android.Runtime.JavaDictionary" />
	<type fullname="Android.Runtime.JavaDictionary`2" />
	<type fullname="Android.Runtime.JavaList" />
	<type fullname="Android.Runtime.JavaList`1" />

We actually get a linker warning for this:

src\Mono.Android\Java.Interop\JavaConvert.cs(89,4):
warning IL2070: Java.Interop.JavaConvert.GetJniHandleConverterForType(Type): 'this' argument does not satisfy 'DynamicallyAccessedMemberTypes.PublicMethods' in call to 'System.Type.GetMethod(String,BindingFlags)'. The parameter 't' of method 'Java.Interop.JavaConvert.GetJniHandleConverterForType(Type)' does not have matching annotations. The source value must declare at least the same requirements as those declared on the target location it is assigned to.

Adding [DynamicallyAccessedMembers] preserves these methods, and the
warning goes away. This seems like a safe option, as we shouldn't need
to keep updating a list of types like we did in Xamarin.Android.

I tried updating InstallAndRunTests.CustomLinkDescriptionPreserve(),
but some usage of types is preserving FromJniHandle(). I couldn't
get the test to fail...

Long term, we should get the linker warnings to zero and make our
build fail if new warnings are ever introduced. Tracking this in:

#5652

[jonpryor]

@jonathanpeppers : what I'd like is an elaboration on:

Adding [DynamicallyAccessedMembers] preserves these methods

How does [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)] cause JavaDictionary<,>.FromJniHandle() to be preserved? Does it cause the linker to look at the Type values for all calls to GetJniHandleConverterForType(), thus finding typeof(JavaDictionary<,>).MakeGenericType(target.GetGenericArguments())? (This sounds like it's the case, based on the docs, but that feels like some form of wizardry.)

[jonathanpeppers]

@jonpryor yes, it's my understanding that in .NET 6 the linker has some static analysis that can tell which System.Type are passed into a method. There could be bugs there, but I've heard that if you got a warning and then solved the warning, that you're in good shape?

[eerhardt]

This sounds like it's the case, based on the docs, but that feels like some form of wizardry.

It sort of is (at least to me). But if you want to read deeper about it, check out:

• https://github.com/dotnet/linker/blob/main/docs/design/reflection-flow.md
  • Specifically https://github.com/dotnet/linker/blob/main/docs/design/reflection-flow.md#cross-method-annotations
• https://github.com/dotnet/designs/blob/main/accepted/2020/linking-libraries.md

Which has the basic designs for how these things work.