Fix GREF leak

A "funny" thing happened after 628aa39: a teardown assertion fired!

	% dotnet test bin/TestRelease-net8.0/Java.Interop-Tests.dll
	…
	# jonp: LoadJvmLibrary(/Users/runner/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.15-6/x64/Contents/Home/lib/libjli.dylib)=140707427245328
	# jonp: JNI_CreateJavaVM=7113469040; JNI_GetCreatedJavaVMs=7113469120
	# jonp: executing JNI_CreateJavaVM=1a7feec70
	# jonp: r=0 javavm=1a9dc1830 jnienv=7fe37d0422b0
	TearDown failed for test fixture Java.InteropTests.JavaObjectArray_object_ContractTest
	  JNI global references: grefStartCount=324; gref=340
	  Expected: True
	  But was:  False

	TearDown : NUnit.Framework.AssertionException :   JNI global references: grefStartCount=324; gref=340
	  Expected: True
	  But was:  False

	StackTrace:    at Java.InteropTests.JavaObjectArray_object_ContractTest.EndCheckGlobalRefCount() in /Users/runner/work/1/s/tests/Java.Interop-Tests/Java.Interop/JavaObjectArrayTest.cs:line 158

	--TearDown
	   at NUnit.Framework.Assert.ReportFailure(String message)
	   at NUnit.Framework.Assert.ReportFailure(ConstraintResult result, String message, Object[] args)
	   at NUnit.Framework.Assert.That[TActual](TActual actual, IResolveConstraint expression, String message, Object[] args)
	   at NUnit.Framework.Assert.IsTrue(Boolean condition, String message, Object[] args)
	   at Java.InteropTests.JavaObjectArray_object_ContractTest.EndCheckGlobalRefCount() in /Users/runner/work/1/s/tests/Java.Interop-Tests/Java.Interop/JavaObjectArrayTest.cs:line 158
	   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
	   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
	  Skipped References_CreatedReference_GlobalRef [3 ms]
	  Skipped References_CreatedReference_LocalRef [< 1 ms]
	  Skipped DoesTheJmethodNeedToMatchDeclaringType [5 ms]
	# jonp: LoadJvmLibrary(/Users/runner/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.15-6/x64/Contents/Home/lib/libjli.dylib)=140707427245328
	# jonp: JNI_CreateJavaVM=7113469040; JNI_GetCreatedJavaVMs=7113469120
	# jonp: executing JNI_CreateJavaVM=1a7feec70
	# jonp: r=-5 javavm=0 jnienv=0

Unfortunately, teardown assertions "don't count", i.e. CI was green,
even though the above is copied from the CI run.

The cause of the problem?  Having `JavaObject.Dispose()`
call `JniObjectReferenceControlBlock.Free()`.

The underlying problem is that
`JniRuntime.JniValueManager.DisposePeer()` uses `JavaObject.PeerReference`
*after* calling `JavaObject.Disposed()` (which calls
`JavaObject.Dispose(bool)`), yet `JavaObject.Dispose(bool)` released
the `JniObjectReferenceControlBlock` that backs `PeerReference`,
meaning we "lost" (leaked!) the GREF!

There are two possible solutions:

 1. Update `JniRuntime.JniValueManager.DisposePeer()` to use
    `JavaObject.PeerReference` *before* calling `JavaObject.Disposed()`

 2. Update `JavaObject.Dispose(bool)` to not release the
    `JniObjectReferenceControlBlock` that backs `PeerReference`.

I avoided (1) because I didn't want to have to audit and update
dotnet/android and all other potential callsites.

Which brings us to (2): if not in `Dispose(bool)`, then where?

The last thing that `JniRuntime.JniValueManager.DisposePeer()` and
`JniRuntime.JniValueManager.FinalizePeer()` do is call
`JavaObject.SetPeerReference(default)`.  *This*, then, is where
cleanup needs to happen.

Update `JavaObject` and `JavaException` to use the
`JniManagedPeerStates` field to track "have I been disposed?",
and then update `SetPeerReference()` to call
`JniObjectReferenceControlBlock.Free()` if the state is "disposed".

This fixes the leak.

Author: jonpryor

src/Java.Interop/Java.Interop/JavaException.cs

@@ -5,7 +5,7 @@
 namespace Java.Interop
 {
 	[JniTypeSignature (JniTypeName, GenerateJavaPeer=false)]
-	unsafe public class JavaException : Exception, IJavaPeerable
+	unsafe public partial class JavaException : Exception, IJavaPeerable
 	{
 		internal    const   string          JniTypeName = "java/lang/Throwable";
 		readonly    static  JniPeerMembers  _members    = new JniPeerMembers (JniTypeName, typeof (JavaException));
@@ -171,9 +171,6 @@ protected virtual void Dispose (bool disposing)
 			if (inner != null) {
 				inner.Dispose ();
 			}
-#if FEATURE_JNIOBJECTREFERENCE_INTPTRS
-			Java.Interop.JniObjectReferenceControlBlock.Free (ref jniObjectReferenceControlBlock);
-#endif  // FEATURE_JNIOBJECTREFERENCE_INTPTRS
 		}
 
 		public override bool Equals (object? obj)
@@ -267,11 +264,13 @@ protected void SetJavaStackTrace (JniObjectReference peerReferenceOverride = def
 
 		void IJavaPeerable.Disposed ()
 		{
+			JniManagedPeerState |= Disposed;
 			Dispose (disposing: true);
 		}
 
 		void IJavaPeerable.Finalized ()
 		{
+			JniManagedPeerState |= Disposed;
 			Dispose (disposing: false);
 		}
 
@@ -288,6 +287,11 @@ void IJavaPeerable.SetJniManagedPeerState (JniManagedPeerStates value)
 		void IJavaPeerable.SetPeerReference (JniObjectReference reference)
 		{
 			SetPeerReference (ref reference, JniObjectReferenceOptions.Copy);
+#if FEATURE_JNIOBJECTREFERENCE_INTPTRS
+			if (!reference.IsValid && JniManagedPeerState.HasFlag (Disposed)) {
+				Java.Interop.JniObjectReferenceControlBlock.Free (ref jniObjectReferenceControlBlock);
+			}
+#endif  // FEATURE_JNIOBJECTREFERENCE_INTPTRS
 		}
 
 		IntPtr IJavaPeerable.JniObjectReferenceControlBlock =>

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

@@ -8,7 +8,7 @@ namespace Java.Interop
 {
 	[JniTypeSignature ("java/lang/Object", GenerateJavaPeer=false)]
 	[Serializable]
-	unsafe public class JavaObject : IJavaPeerable
+	unsafe public partial class JavaObject : IJavaPeerable
 	{
 		internal const DynamicallyAccessedMemberTypes Constructors = DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.NonPublicConstructors;
 
@@ -122,9 +122,6 @@ 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)
@@ -155,11 +152,13 @@ public override unsafe int GetHashCode ()
 
 		void IJavaPeerable.Disposed ()
 		{
+			managedPeerState    |= Disposed;
 			Dispose (disposing: true);
 		}
 
 		void IJavaPeerable.Finalized ()
 		{
+			managedPeerState    |= Disposed;
 			Dispose (disposing: false);
 		}
 
@@ -176,6 +175,12 @@ void IJavaPeerable.SetJniManagedPeerState (JniManagedPeerStates value)
 		void IJavaPeerable.SetPeerReference (JniObjectReference reference)
 		{
 			SetPeerReference (ref reference, JniObjectReferenceOptions.Copy);
+
+#if FEATURE_JNIOBJECTREFERENCE_INTPTRS
+			if (!reference.IsValid && managedPeerState.HasFlag (Disposed)) {
+				Java.Interop.JniObjectReferenceControlBlock.Free (ref jniObjectReferenceControlBlock);
+			}
+#endif  // FEATURE_JNIOBJECTREFERENCE_INTPTRS
 		}
 
 		IntPtr IJavaPeerable.JniObjectReferenceControlBlock =>

src/Java.Interop/Java.Interop/JniManagedPeerStates.cs

@@ -11,4 +11,12 @@ public enum JniManagedPeerStates {
 		/// <include file="../Documentation/Java.Interop/JniManagedPeerStates.xml" path="/docs/member[@name='F:Replaceable']/*" />
 		Replaceable         = (1 << 1),
 	}
+
+	partial class JavaObject {
+		const       JniManagedPeerStates        Disposed            = (JniManagedPeerStates) (1 << 2);
+	}
+
+	partial class JavaException {
+		const       JniManagedPeerStates        Disposed            = (JniManagedPeerStates) (1 << 2);
+	}
 }

tests/Java.Interop-Tests/Java.Interop/JavaObjectArrayTest.cs

@@ -39,6 +39,30 @@ protected override bool SequenceEqual (IEnumerable<T> a, IEnumerable<T> b)
 		{
 			return JniMarshal.RecursiveEquals (a, b);
 		}
+
+		int beforeTestGrefCount;
+
+		[SetUp]
+		public void LogBeginCurrentTestGrefCount ()
+		{
+			beforeTestGrefCount = JniEnvironment.Runtime.GlobalReferenceCount;
+			JniEnvironment.Runtime.ObjectReferenceManager.WriteGlobalReferenceLine(
+					"{0}",
+					$"Begin {TestContext.CurrentContext.Test.FullName}; " +
+					$"GREFs={beforeTestGrefCount}; "
+			);
+		}
+
+		[TearDown]
+		public void LogEndCurrentTestToGrefCount ()
+		{
+			int afterTestGrefCount  = JniEnvironment.Runtime.GlobalReferenceCount;
+			JniEnvironment.Runtime.ObjectReferenceManager.WriteGlobalReferenceLine(
+					"{0}",
+					$"End {TestContext.CurrentContext.Test.FullName}; " +
+					$"GREFs={afterTestGrefCount}; diff={afterTestGrefCount - beforeTestGrefCount}"
+			);
+		}
 	}
 
 	[TestFixture]