[Java.Interop] Add JavaPeerableRegistrationScope

Fixes: #4

Context: #426
Context: a666a6f

Alternate names?

  * JavaScope
  * JavaPeerableCleanupPool
  * JavaPeerCleanup
  * JavaReferenceCleanup
  * JniPeerRegistrationScope

Issue #426 is an idea to implement a *non*-GC-Bridged
`JniRuntime.JniValueManager` type, primarily for use with .NET Core.
This was begun in a666a6f.

What's missing is the answer to a question: what to do about
`JniRuntime.JniValueManager.CollectPeers()`?  With a Mono-style
GC bridge, `CollectPeers()` is `GC.Collect()`.  In a666a6f with
.NET Core, `CollectPeers()` calls `IJavaPeerable.Dispose()` on all
registered instances, which is "extreme".

@jonpryor thought that if there were a *scope-based* way to
selectively control which instances were disposed, that might be
"better" and more understandable.  Plus, this is issue #4!

Which brings us to the background for Issue #4, which touches upon
[bugzilla 25443][0] and [Google issue 37034307][1]: Java.Interop
attempts to provide referential identity for Java objects: if two
separate Java methods invocations return the same Java instance, then
the bindings of those methods should also return the same instance.

This is "fine" on the surface, but has a few related issues:

 1. JNI Global References are a limited resource: Android only allows
    ~52000 JNI Global References to exist, which sounds like a lot,
    but might not be.

 2. Because of (1), it is not uncommon to want to use `using` blocks
    to invoke `IJavaPeerable.Dispose()` to release
    JNI Global References.

 3. However, because of object identity, you could unexpectedly
    introduce *cross-thread sharing* of `IJavaPeerable` instances.
    This might not be at all obvious; for example, in the Android 5
    timeframe, [`Typeface.create()`][2] wouldn't necessarily return
    new Java instances, but may instead return cached instances.

Meaning that this:

	var t1 = new Thread(() => {
	    using var typeface = Typeface.Create("Family", …);
	    // use typeface…
	});
	var t2 = new Thread(() => {
	    using var typeface = Typeface.Create("Family", …);
	    // use typeface…
	});
	t1.Start();
	t2.Start();
	t1.Join();
	t2.Join();

could plausibly result in `ObjectDisposedException`s (or worse), as
the threads could be unexpectedly sharing the same bound instance.

Which *really* means that you can't reliably call `Dispose()`, unless
you *know* you created that instance:

	using var safe      = new Java.Lang.Double(42.0);       // I created this, therefore I control all access and can Dispose() it
	using var unsafe    = Java.Lang.Double.ValueOf(42.0);   // I have no idea who else may be using this instance!

Attempt to address both of these scenarios -- a modicum of .NET Core
support, and additional sanity around JNI Global Reference lifetimes --
by introducing a new `JavaPeerableRegistrationScope` type, which
introduces a scope-based mechanism to control when `IJavaPeerable`
instances are cleaned up:

        public enum JavaPeerableRegistrationScopeCleanup {
            RegisterWithManager,
            Dispose,
            Release,
        }

        public ref struct JavaPeerableRegistrationScope {
            public JavaPeerableRegistrationScope(JavaPeerableRegistrationScopeCleanup cleanup);
            public void Dispose();
        }

`JavaPeerableRegistrationScope` is a [`ref struct`][3], which means
it can only be allocated on the runtime stack, ensuring that cleanup
semantics are *scope* semantics.

TODO: is that actually a good idea?

If a `JavaPeerableRegistrationScope` is created using
`JavaPeerableRegistrationScopeCleanup.RegisterWithManager`, existing
behavior is followed.  This is useful for nested scopes, should
instances need to be registered with `JniRuntime.ValueManager`.

If a `JavaPeerableRegistrationScope` is created using
`JavaPeerableRegistrationScopeCleanup.Dispose` or
`JavaPeerableRegistrationScopeCleanup.Release`, then:

 1. `IJavaPeerable` instances created within the scope are
    "thread-local": they can be *used* by other threads, but
    `JniRuntime.JniValueManager.PeekPeer()` will only find the
    value on the creating thread.

 2. At the end of a `using` block / when
   `JavaScope.Dispose()` is called, all collected instances will be
   `Dispose()`d (with `.Dispose`) or released (with `.Release`), and
   left to the GC to eventually finalize.

For example:

	using (new JavaPeerableRegistrationScope (JavaPeerableRegistrationScopeCleanup.Dispose)) {
	    var singleton = JavaSingleton.Singleton;
	    // use singleton

	    // If other threads attempt to access `JavaSingleton.Singleton`,
	    // they'll create their own peer wrapper
	}
	// `singleton.Dispose()` is called at the end of the `using` block

However, if e.g. the singleton instance is already accessed, then it
won't be added to the registration scope and won't be disposed:

	var singleton = JavaSingleton.Singleton;
	// singleton is registered with the ValueManager

	// later on the same thread or some other threa…
	using (new JavaPeerableRegistrationScope (JavaPeerableRegistrationScopeCleanup.Dispose)) {
	    var anotherSingleton = JavaSingleton.Singleton;
	    // use anotherSingleton…
	}

then `anotherSingleton` will *not* disposed, as it already existed.
*Only newly created instances* are added to the current scope.

By default, only `JavaPeerableRegistrationScopeCleanup.RegisterWithManager`
is supported.  To support the other cleanup modes,
`JniRuntime.JniValueManager.SupportsPeerableRegistrationScopes` must
return `true`, which in turn requires that:

  * `JniRuntime.JniValueManager.AddPeer()` calls
    `TryAddPeerToRegistrationScope()`,
  * `JniRuntime.JniValueManager.RemovePeer()` calls
    `TryRemovePeerFromRegistrationScopes()`
  * `JniRuntime.JniValueManager.PeekPeer()` calls
    `TryPeekPeerFromRegistrationScopes()`.

See `ManagedValueManager` for an example implementation.

Finally, add the following methods to `JniRuntime.JniValueManager`
to help further assist peer management:

	partial class JniRuntime.JniValueManager {
	    public virtual bool CanCollectPeers { get; }
	    public virtual bool CanReleasePeers { get; }
	    public virtual void ReleasePeers ();
	}

TODO: docs?

TODO: *nested* scopes, and "bound" vs. "unbound" instance construction
around `JniValueManager.GetValue<T>()` or `.CreateValue<T>()`,
and *why* they should be treated differently.

TODO: Should `CreateValue<T>()` be *removed*?  name implies it always
"creates" a new value, but implementation will return existing instances,
so `GetValue<T>()` alone may be better.  One related difference is that`
`CreateValue<T>()` uses `PeekBoxedObject()`, while `GetValue<T>()` doesn't.
*Should* it?

[0]: https://web.archive.org/web/20211106214514/https://bugzilla.xamarin.com/25/25443/bug.html#c63
[1]: https://issuetracker.google.com/issues/37034307
[2]: https://developer.android.com/reference/android/graphics/Typeface#create(java.lang.String,%20int)
[3]: https://docs.microsoft.com/dotnet/csharp/language-reference/builtin-types/struct#ref-struct

Author: jonpryor

src/Java.Interop/Java.Interop/JavaPeerableRegistrationScope.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+
+namespace Java.Interop {
+	public enum JavaPeerableRegistrationScopeCleanup {
+		RegisterWithManager,
+		Dispose,
+		Release,
+	}
+
+	public ref struct JavaPeerableRegistrationScope {
+		PeerableCollection? scope;
+		bool disposed;
+
+		public JavaPeerableRegistrationScope (JavaPeerableRegistrationScopeCleanup cleanup)
+		{
+			scope       = JniEnvironment.CurrentInfo.BeginRegistrationScope (cleanup);
+			disposed    = false;
+		}
+
+		public void Dispose ()
+		{
+			if (disposed) {
+				return;
+			}
+			disposed    = true;
+			JniEnvironment.CurrentInfo.EndRegistrationScope (scope);
+			scope       = null;
+		}
+	}
+}

src/Java.Interop/Java.Interop/JavaProxyObject.cs

@@ -13,7 +13,6 @@ sealed class JavaProxyObject : JavaObject, IEquatable<JavaProxyObject>
 		internal const string JniTypeName = "net/dot/jni/internal/JavaProxyObject";
 
 		static  readonly    JniPeerMembers                                  _members        = new JniPeerMembers (JniTypeName, typeof (JavaProxyObject));
-		static  readonly    ConditionalWeakTable<object, JavaProxyObject>   CachedValues    = new ConditionalWeakTable<object, JavaProxyObject> ();
 
 		[JniAddNativeMethodRegistrationAttribute]
 		static void RegisterNativeMembers (JniNativeMethodRegistrationArguments args)
@@ -29,10 +28,8 @@ public override JniPeerMembers JniPeerMembers {
 			}
 		}
 
-		JavaProxyObject (object value)
+		internal JavaProxyObject (object value)
 		{
-			if (value == null)
-				throw new ArgumentNullException (nameof (value));
 			Value = value;
 		}
 
@@ -57,19 +54,10 @@ public override bool Equals (object? obj)
 			return Value.ToString ();
 		}
 
-		[return: NotNullIfNotNull ("object")]
-		public static JavaProxyObject? GetProxy (object value)
+		protected override void Dispose (bool disposing)
 		{
-			if (value == null)
-				return null;
-
-			lock (CachedValues) {
-				if (CachedValues.TryGetValue (value, out var proxy))
-					return proxy;
-				proxy = new JavaProxyObject (value);
-				CachedValues.Add (value, proxy);
-				return proxy;
-			}
+			base.Dispose (disposing);
+			JniEnvironment.Runtime.ValueManager.RemoveProxy (Value);
 		}
 
 		// TODO: Keep in sync with the code generated by ExportedMemberBuilder

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

@@ -2,10 +2,13 @@
 
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Text;
 using System.Threading;
 
 namespace Java.Interop {
@@ -209,6 +212,8 @@ sealed class JniEnvironmentInfo : IDisposable {
 		bool                    disposed;
 		JniRuntime?             runtime;
 
+		List<PeerableCollection?>?          scopes;
+
 		public      int                     LocalReferenceCount     {get; internal set;}
 		public      bool                    WithinNewObjectScope    {get; set;}
 		public      JniRuntime              Runtime {
@@ -243,6 +248,11 @@ public      bool                    IsValid {
 			get {return Runtime != null && environmentPointer != IntPtr.Zero;}
 		}
 
+		public      List<PeerableCollection?>?
+		                                    Scopes => scopes;
+		public      PeerableCollection?     CurrentScope =>
+			scopes == null ? null : scopes [scopes.Count-1];
+
 		public JniEnvironmentInfo ()
 		{
 			Runtime             = JniRuntime.CurrentRuntime;
@@ -293,6 +303,42 @@ public void Dispose ()
 			disposed                = true;
 		}
 
+		public PeerableCollection? BeginRegistrationScope (JavaPeerableRegistrationScopeCleanup cleanup)
+		{
+			if (cleanup != JavaPeerableRegistrationScopeCleanup.RegisterWithManager &&
+					!Runtime.ValueManager.SupportsPeerableRegistrationScopes) {
+				throw new NotSupportedException ("Peerable registration scopes are not supported by this runtime.");
+			}
+			scopes ??= new List<PeerableCollection?> ();
+			if (cleanup == JavaPeerableRegistrationScopeCleanup.RegisterWithManager) {
+				scopes.Add (null);
+				return null;
+			}
+			var scope = new PeerableCollection (cleanup);
+			scopes.Add (scope);
+			return scope;
+		}
+
+		public void EndRegistrationScope (PeerableCollection? scope)
+		{
+			Debug.Assert (scopes != null);
+			if (scopes == null) {
+				return;
+			}
+
+			for (int i = scopes.Count; i > 0; --i) {
+				var s = scopes [i - 1];
+				if (s == scope) {
+					scopes.RemoveAt (i - 1);
+					break;
+				}
+			}
+			if (scopes.Count == 0) {
+				scopes = null;
+			}
+			scope?.DisposeScope ();
+		}
+
 #if FEATURE_JNIENVIRONMENT_SAFEHANDLES
 		internal    List<List<JniLocalReference>>   LocalReferences = new List<List<JniLocalReference>> () {
 			new List<JniLocalReference> (),
@@ -309,5 +355,80 @@ static unsafe JniEnvironmentInvoker CreateInvoker (IntPtr handle)
 		}
 #endif  // !FEATURE_JNIENVIRONMENT_JI_PINVOKES
 	}
+
+	sealed class PeerableCollection : KeyedCollection<int, IJavaPeerable> {
+		public JavaPeerableRegistrationScopeCleanup Cleanup { get; }
+
+		public PeerableCollection (JavaPeerableRegistrationScopeCleanup cleanup)
+		{
+			Cleanup = cleanup;
+		}
+
+		protected override int GetKeyForItem (IJavaPeerable item) => item.JniIdentityHashCode;
+
+		public IJavaPeerable? GetPeerable (JniObjectReference reference, int identityHashCode)
+		{
+			if (!reference.IsValid) {
+				return null;
+			}
+			if (TryGetValue (identityHashCode, out var p) &&
+					JniEnvironment.Types.IsSameObject (reference, p.PeerReference)) {
+				return p;
+			}
+			return null;
+		}
+
+		public void DisposeScope ()
+		{
+			Console.Error.WriteLine ($"# jonp: DisposeScope: {Cleanup}");
+			Debug.Assert (Cleanup != JavaPeerableRegistrationScopeCleanup.RegisterWithManager);
+			switch (Cleanup) {
+				case JavaPeerableRegistrationScopeCleanup.Dispose:
+					List<Exception>?    exceptions = null;
+					foreach (var p in this) {
+						DisposePeer (p, ref exceptions);
+					}
+					Clear ();
+					if (exceptions != null) {
+						throw new AggregateException ("Exceptions while disposing peers.", exceptions);
+					}
+					break;
+				case JavaPeerableRegistrationScopeCleanup.Release:
+					Clear ();
+					break;
+				case JavaPeerableRegistrationScopeCleanup.RegisterWithManager:
+				default:
+					throw new NotSupportedException ($"Unsupported scope cleanup value: {Cleanup}");
+			}
+
+			[SuppressMessage ("Design", "CA1031:Do not catch general exception types",
+					Justification = "Exceptions are bundled into an AggregateException and rethrown")]
+			static void DisposePeer (IJavaPeerable peer, ref List<Exception>? exceptions)
+			{
+				try {
+					Console.Error.WriteLine ($"# jonp: DisposeScope: disposing of: {peer} {peer.PeerReference}");
+					peer.Dispose ();
+				} catch (Exception e) {
+					exceptions ??= new ();
+					exceptions.Add (e);
+					Trace.WriteLine (e);
+				}
+			}
+		}
+
+		public override string ToString ()
+		{
+			var c   = (Collection<IJavaPeerable>) this;
+			var s   = new StringBuilder ();
+			s.Append ("PeerableCollection[").Append (Count).Append ("](");
+			for (int i  = 0; i < Count; ++i ) {
+				s.AppendLine ();
+				var e   = c [i];
+				s.Append ($"  [{i}] hash={e.JniIdentityHashCode} ref={e.PeerReference} type={e.GetType ().ToString ()} value=`{e.ToString ()}`");
+			}
+			s.Append (")");
+			return s.ToString ();
+		}
+	}
 }