[Mono] Implement eager finalization of WeakReference

src/libraries/System.Private.CoreLib/src/System/WeakReference.T.cs

@@ -130,8 +130,6 @@ private T? Target
             }
         }
 
-// eager finalization is NYI on Mono
-#if !MONO
         // Note: While WeakReference<T> is formally a finalizable type, the finalizer does not actually run.
         //       Instead the instances are treated specially in GC when scanning for no longer strongly-reachable
         //       finalizable objects.
@@ -142,21 +140,6 @@ private T? Target
         }
 #pragma warning restore CA1821 // Remove empty Finalizers
 
-#else
-        // Free all system resources associated with this reference.
-        ~WeakReference()
-        {
-            IntPtr handle = Handle;
-            if (handle != default(IntPtr))
-            {
-                GCHandle.InternalFree(handle);
-
-                // keep the bit that indicates whether this reference was tracking resurrection
-                _handleAndKind &= TracksResurrectionBit;
-            }
-        }
-#endif
-
 #endif
     }
 }

src/libraries/System.Private.CoreLib/src/System/WeakReference.cs

@@ -150,10 +150,7 @@ public virtual object? Target
             // Unlike WeakReference<T> case, the instance could be of a derived type and
             //       in such case it is finalized via a finalizer.
 
-// eager finalization is NYI on Mono
-#if !MONO
             Debug.Assert(this.GetType() != typeof(WeakReference));
-#endif
 
             IntPtr handle = Handle;
             if (handle != default(IntPtr))

src/mono/mono/metadata/class-internals.h

@@ -921,6 +921,8 @@ typedef struct {
 	MonoClass *generic_ienumerator_class;
 	MonoClass *alc_class;
 	MonoClass *appcontext_class;
+	MonoClass *weakreference_class;
+	MonoClass *generic_weakreference_class;
 } MonoDefaults;
 
 /* If you need a MonoType, use one of the mono_get_*_type () functions in class-inlines.h */

src/mono/mono/metadata/domain.c

@@ -290,6 +290,11 @@ mono_init_internal (const char *root_domain_name)
 	mono_defaults.alc_class = mono_class_get_assembly_load_context_class ();
 	mono_defaults.appcontext_class = mono_class_try_load_from_name (mono_defaults.corlib, "System", "AppContext");
 
+	mono_defaults.weakreference_class = mono_class_try_load_from_name (
+	        mono_defaults.corlib, "System", "WeakReference");
+	mono_defaults.generic_weakreference_class = mono_class_try_load_from_name (
+	        mono_defaults.corlib, "System", "WeakReference`1");
+
 	// in the past we got a filename as the root_domain_name so try to get the basename
 	domain->friendly_name = g_path_get_basename (root_domain_name);
 

src/mono/mono/metadata/gc.c

@@ -249,6 +249,15 @@ mono_gc_run_finalize (void *obj, void *data)
 			return;
 	}
 
+	if (o->vtable->klass == mono_defaults.weakreference_class ||
+	    o->vtable->klass == mono_defaults.generic_weakreference_class) {
+		MonoWeakReference *wr = (MonoWeakReference*)o;
+		MonoGCHandle gc_handle = (MonoGCHandle)(wr->handleAndKind & ~(gsize)1);
+		mono_gchandle_free_internal (gc_handle);
+		wr->handleAndKind &= (gsize)1;
+		return;
+	}
+
 	if (m_class_get_image (mono_object_class (o)) == mono_defaults.corlib && !strcmp (o_name, "DynamicMethod") && finalizing_root_domain) {
 		/*
 		 * These can't be finalized during unloading/shutdown, since that would

src/mono/mono/metadata/object-internals.h

@@ -651,6 +651,11 @@ typedef struct {
 	guint32 intType;
 } MonoClassInterfaceAttribute;
 
+typedef struct {
+	MonoObject object;
+	gsize handleAndKind;
+} MonoWeakReference;
+
 /* Safely access System.Delegate from native code */
 TYPED_HANDLE_DECL (MonoDelegate);