[Foundation] Don't dispose tagged pointers. Fixes #21425.

TODO: better description.

Fixes https://github.com/xamarin/xamarin-macios/issues/21425.

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -573,6 +573,9 @@
 			<!-- Set default ValidateObjectPointers value -->
 			<_ValidateObjectPointers Condition="'$(_ValidateObjectPointers)' == ''">false</_ValidateObjectPointers>
 
+			<!-- Set default DisposeTaggedPointers value -->
+			<_DisposeTaggedPointers Condition="'$(_DisposeTaggedPointers)' == ''">true</_DisposeTaggedPointers>
+
 			<_CustomLinkerOptions>
 				AreAnyAssembliesTrimmed=$(_AreAnyAssembliesTrimmed)
 				AssemblyName=$(AssemblyName).dll
@@ -687,6 +690,7 @@
 			<RuntimeHostConfigurationOption Include="ObjCRuntime.Runtime.IsManagedStaticRegistrar" Value="$(_IsManagedStaticRegistrarFeature)" Trim="true" />
 			<RuntimeHostConfigurationOption Include="ObjCRuntime.Runtime.IsNativeAOT" Value="$(_IsNativeAOTFeature)" Trim="true" />
 			<RuntimeHostConfigurationOption Include="ObjCRuntime.Class.ValidateObjectPointers" Value="$(_ValidateObjectPointers)" Trim="true" />
+			<RuntimeHostConfigurationOption Include="ObjCRuntime.NSObject.DisposeTaggedPointers" Value="$(_DisposeTaggedPointers)" Trim="true" />
 
 			<!-- Mark all assemblies to be copied if we're not linking any assemblies -->
 			<ResolvedFileToPublish

src/Foundation/NSObject2.cs

@@ -998,13 +998,49 @@ namespace Foundation {
 			handle = NativeHandle.Zero;
 		}
 
+		// This option is turned on by setting _DisposeTaggedPointers property to true in the project file.
+		static bool dispose_tagged_pointers;
+		static bool DisposeTaggedPointers {
+			get => dispose_tagged_pointers;
+			set => dispose_tagged_pointers = value;
+		}
+
 		protected virtual void Dispose (bool disposing)
 		{
 			if (disposed)
 				return;
 			disposed = true;
 
-			if (handle != NativeHandle.Zero) {
+			/* Tagged pointer is limited to 64bit, which is all we support anyway.
+			 *
+			 * The tagged pointer bit is:
+			 *
+			 * Arm64: most significant bit
+			 * Simulators (both on arm64 and x64 desktops): most significant bit
+			 * Desktop/x64 (macOS + Mac Catalyst): least significant bit
+			 * Ref: https://github.com/apple-oss-distributions/objc4/blob/89543e2c0f67d38ca5211cea33f42c51500287d5/runtime/objc-internal.h#L603-L672
+			 */
+#if __MACOS__ || __MACCATALYST__
+			ulong _OBJC_TAG_MASK;
+			if (Runtime.IsARM64CallingConvention) {
+				_OBJC_TAG_MASK = 1UL << 63;
+			} else {
+				_OBJC_TAG_MASK = 1UL;
+			}
+#else
+			const ulong _OBJC_TAG_MASK = 1UL << 63;
+#endif
+
+			bool isTaggedPointer;
+			unchecked {
+				var ulongHandle = (ulong) (IntPtr) handle;
+				isTaggedPointer = (ulongHandle & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
+			}
+
+			if (!DisposeTaggedPointers && isTaggedPointer) {
+				// don't dispose tagged pointers.
+				FreeData (); // still need to do this though.
+			} else if (handle != NativeHandle.Zero) {
 				if (disposing) {
 					ReleaseManagedRef ();
 				} else {

src/ILLink.Substitutions.macOS.xml

@@ -1,5 +1,9 @@
 <linker>
   <assembly fullname="Microsoft.macOS">
+    <type fullname="Foundation.NSObject">
+      <method signature="System.Boolean get_DisposeTaggedPointers()" body="stub" feature="Foundation.NSObject.DisposeTaggedPointers" featurevalue="false" value="false" />
+      <method signature="System.Boolean get_DisposeTaggedPointers()" body="stub" feature="Foundation.NSObject.DisposeTaggedPointers" featurevalue="true" value="true" />
+    </type>
     <type fullname="ObjCRuntime.Dlfcn">
       <method signature="System.Void WarnOnce()" body="stub" feature="System.Diagnostics.Debugger.IsSupported" featurevalue="false" />
     </type>

tests/monotouch-test/ObjCRuntime/TaggedPointerTest.cs

@@ -0,0 +1,54 @@
+using System;
+
+using Foundation;
+
+using NUnit.Framework;
+
+namespace MonoTouchFixtures.ObjCRuntime {
+
+	[TestFixture]
+	[Preserve (AllMembers = true)]
+	public class TaggedPointerTest {
+
+		[Test]
+		public void TaggedPointersArentDisposed ()
+		{
+			var notificationData =
+				"""
+				{
+					action = "action";
+					aps =
+					{
+						category = "ee";
+						"content-available" = dd;
+						"mutable-content" = cc;
+						sound = bb;
+						"thread-id" = "aa";
+					};
+					"em-account" = "a";
+					"em-account-id" = "b";
+					"em-body" = "c";
+					"em-date" = "d";
+					"em-from" = "e";
+					"em-from-address" = "f";
+					"em-notification" = g;
+					"em-notification-id" = h;
+					"em-subject" = "i";
+					"gcm.message_id" = j;
+					"google.c.a.e" = k;
+					"google.c.fid" = l;
+					"google.c.sender.id" = m;
+				}
+				""";
+
+			var data = NSData.FromString (notificationData);
+			var fmt = NSPropertyListFormat.OpenStep;
+			var userInfo = (NSMutableDictionary) NSPropertyListSerialization.PropertyListWithData (data, NSPropertyListReadOptions.Immutable, ref fmt, out var error);
+			var apsKey = new NSString ("aps");
+			// Iteration here should not throw ObjectDisposedExceptions
+			foreach (var kv in userInfo) {
+				apsKey.Dispose ();
+			}
+		}
+	}
+}