[Java.Interop] JNI handles are now in a "control block"

Context: dotnet/runtime#114184
Context: dotnet/android#10125
Context: dotnet/android#10125 (comment)

Part of adding a GC bridge to CoreCLR are the new APIs:

	namespace System.Runtime.InteropServices.Java;
	public struct ComponentCrossReference {
	    public nint SourceGroupIndex, DestinationGroupIndex;
	}
	public unsafe struct StronglyConnectedComponent {
	    public nint Count;
	    public IntPtr* Context;
	}
	public static partial class JavaMarshal {
	    public static unsafe void Initialize(
	        delegate* unmanaged<
	            System.IntPtr,                  // sccsLen
	            StronglyConnectedComponent*,    // sccs
	            System.IntPtr,                  // ccrsLen
	            ComponentCrossReference*,       // ccrs
	            void> markCrossReferences);
	    public static GCHandle CreateReferenceTrackingHandle(object obj, IntPtr context);
	    public static IntPtr GetContext(GCHandle obj);
	}

Of note is the "data flow" of `context`:

  * `JavaMarshal.CreateReferenceTrackingHandle()` has a "`context`"
    parameter.

  * The `context` parameter to
    `JavaMarshal.CreateReferenceTrackingHandle()` is the return value
    of `JavaMarshal.GetContext()`

  * The `context` parameter to
    `JavaMarshal.CreateReferenceTrackingHandle()` is stored within
    `StronglyConnectedComponent.Context`.

  * The `markCrossReferences` parameter of `JavaMarshal.Initialize()`
    is called by the GC bridge and given a native array of
    `StronglyConnectedComponent` instances, which contains `Context`.

The short of it is that the proposed GC bridge doesn't contain direct
access to the `IJavaPeerable` instances in play.  Instead, it has
access to "context" which must contain the JNI Object Reference
information that the `markCrossReferences` callback needs access to.

Furthermore, the `context` pointer value *cannot change*, i.e. it
needs to be a native pointer value a'la **malloc**(3), ***not*** a
value which can be moved by the GC.  (The *contents* can change; the
pointer value cannot.))

While we're still prototyping this, what we currently believe we need
is the JNI object reference, JNI object reference type, and (maybe?)
the JNI Weak Global Reference value and "refs added" values.

Update `IJavaPeerable` to add a `JniObjectReferenceControlBlock`
property which can be used as the `context` parameter:

	partial interface IJavaPeerable {
	    IntPtr JniObjectReferenceControlBlock => 0;
	}

This supports usage of:

	IJavaPeerable   value   = …
	GCHandle        handle  = JavaMarshal.CreateReferenceTrackingHandle(
	    value,
	    value.JniObjectReferenceControlBlock
	);

Author: jonpryor

src/Java.Interop/Java.Interop/IJavaPeerable.cs

@@ -37,6 +37,8 @@ public interface IJavaPeerable : IDisposable
 		void    Disposed ();
 		/// <include file="../Documentation/Java.Interop/IJavaPeerable.xml" path="/docs/member[@name='M:Finalized']/*" />
 		void    Finalized ();
+
+		IntPtr  JniObjectReferenceControlBlock  => IntPtr.Zero;
 	}
 }
 

src/Java.Interop/Java.Interop/JavaObject.cs

@@ -25,13 +25,9 @@ unsafe public class JavaObject : IJavaPeerable
 		[NonSerialized] JniObjectReference  reference;
 #endif  // FEATURE_JNIOBJECTREFERENCE_SAFEHANDLES
 #if FEATURE_JNIOBJECTREFERENCE_INTPTRS
-		[NonSerialized] IntPtr                  handle;
-		[NonSerialized] JniObjectReferenceType  handle_type;
-	#pragma warning disable 0169
-		// Used by JavaInteropGCBridge
-		[NonSerialized] IntPtr                  weak_handle;
-		[NonSerialized] int                     refs_added;
-	#pragma warning restore 0169
+		IntPtr  jniObjectReferenceControlBlock;
+		unsafe  JniObjectReferenceControlBlock* JniObjectReferenceControlBlock =>
+			(JniObjectReferenceControlBlock*) jniObjectReferenceControlBlock;
 #endif  // FEATURE_JNIOBJECTREFERENCE_INTPTRS
 
 		protected   static  readonly    JniObjectReference*     InvalidJniObjectReference  = null;
@@ -41,13 +37,17 @@ unsafe public class JavaObject : IJavaPeerable
 			JniEnvironment.Runtime.ValueManager.FinalizePeer (this);
 		}
 
-		public          JniObjectReference          PeerReference {
+		public  unsafe  JniObjectReference          PeerReference {
 			get {
 #if FEATURE_JNIOBJECTREFERENCE_SAFEHANDLES
 				return reference;
 #endif  // FEATURE_JNIOBJECTREFERENCE_SAFEHANDLES
 #if FEATURE_JNIOBJECTREFERENCE_INTPTRS
-				return new JniObjectReference (handle, handle_type);
+				var c = JniObjectReferenceControlBlock;
+				if (c == null) {
+					return default;
+				}
+				return new JniObjectReference (c->handle, (JniObjectReferenceType) c->handle_type);
 #endif  // FEATURE_JNIOBJECTREFERENCE_INTPTRS
 			}
 		}
@@ -91,8 +91,14 @@ protected void SetPeerReference (ref JniObjectReference reference, JniObjectRefe
 			this.reference      = reference;
 #endif  // FEATURE_JNIOBJECTREFERENCE_SAFEHANDLES
 #if FEATURE_JNIOBJECTREFERENCE_INTPTRS
-			this.handle         = reference.Handle;
-			this.handle_type    = reference.Type;
+			var c   = JniObjectReferenceControlBlock;
+			if (c == null) {
+				jniObjectReferenceControlBlock    =
+					Java.Interop.JniObjectReferenceControlBlock.Alloc ();
+				c   = JniObjectReferenceControlBlock;
+			}
+			c->handle       = reference.Handle;
+			c->handle_type  = (int) reference.Type;
 #endif  // FEATURE_JNIOBJECTREFERENCE_INTPTRS
 
 			JniObjectReference.Dispose (ref reference, options);
@@ -117,6 +123,9 @@ public virtual void DisposeUnlessReferenced ()
 
 		protected virtual void Dispose (bool disposing)
 		{
+#if FEATURE_JNIOBJECTREFERENCE_INTPTRS
+			Java.Interop.JniObjectReferenceControlBlock.Free (ref jniObjectReferenceControlBlock);
+#endif  // FEATURE_JNIOBJECTREFERENCE_INTPTRS
 		}
 
 		public override bool Equals (object? obj)
@@ -169,6 +178,9 @@ void IJavaPeerable.SetPeerReference (JniObjectReference reference)
 		{
 			SetPeerReference (ref reference, JniObjectReferenceOptions.Copy);
 		}
+
+		IntPtr IJavaPeerable.JniObjectReferenceControlBlock =>
+			jniObjectReferenceControlBlock;
 	}
 }
 

src/Java.Interop/PublicAPI.Unshipped.txt

@@ -11,3 +11,4 @@ Java.Interop.JniRuntime.JniTypeManager.GetInvokerType(System.Type! type) -> Syst
 Java.Interop.JniRuntime.JniValueManager.GetPeer(Java.Interop.JniObjectReference reference, System.Type? targetType = null) -> Java.Interop.IJavaPeerable?
 Java.Interop.JniTypeSignatureAttribute.InvokerType.get -> System.Type?
 Java.Interop.JniTypeSignatureAttribute.InvokerType.set -> void
+Java.Interop.IJavaPeerable.JniObjectReferenceControlBlock.get -> nint