[vm/ffi] Allow Pointer objects to be shared across isolates

We consider `Pointer` objects as simple value types. We may unbox them
in the future to be simple integers - at which point our message sending
code wouldn't even know they are pointers anymore. There's therefore no
reason to prevent sending them across `SendPort`s.

Owners of `Pointer`s may implement `Finalizable` and sending such
finalizables across `SendPort`s is prohibited.

Closes #50457

TEST=vm/dart/isiolates/fast_object_copy*_test

Change-Id: Ia215d119c5bb0e48e2c5dc83cc82e132f46931c1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/350822
Commit-Queue: Martin Kustermann <[email protected]>
Reviewed-by: Daco Harkes <[email protected]>

Author: mkustermann

runtime/tests/vm/dart/isolates/fast_object_copy2_test.dart

@@ -78,6 +78,7 @@ final sharableObjects = [
   Float32x4(1.0, 2.0, 3.0, 4.0),
   Float64x2(1.0, 2.0),
   StackTrace.current,
+  Pointer<Int8>.fromAddress(0xdeadbeef),
 ];
 
 final copyableClosures = <dynamic>[

runtime/tests/vm/dart/isolates/fast_object_copy_test.dart

@@ -257,7 +257,6 @@ class SendReceiveTest extends SendReceiveTestBase {
     await testFinalizer();
     await testNativeFinalizer();
     await testFinalizable();
-    await testPointer();
 
     await testForbiddenClosures();
   }
@@ -791,13 +790,6 @@ class SendReceiveTest extends SendReceiveTestBase {
     Expect.throwsArgumentError(() => sendPort.send(finalizable));
   }
 
-  Future testPointer() async {
-    print('testPointer');
-
-    final pointer = Pointer<Int8>.fromAddress(0xdeadbeef);
-    Expect.throwsArgumentError(() => sendPort.send(pointer));
-  }
-
   Future testForbiddenClosures() async {
     print('testForbiddenClosures');
     for (final closure in nonCopyableClosures) {

runtime/vm/class_id.h

@@ -433,7 +433,7 @@ inline bool ShouldHaveImmutabilityBitSet(intptr_t index) {
          index == kFloat32x4Cid || index == kFloat64x2Cid ||
          index == kInt32x4Cid || index == kSendPortCid ||
          index == kCapabilityCid || index == kRegExpCid || index == kBoolCid ||
-         index == kNullCid;
+         index == kNullCid || index == kPointerCid;
 }
 
 inline bool IsFfiTypeClassId(intptr_t index) {

tests/ffi/snapshot_test.dart

@@ -10,9 +10,14 @@ import 'dart:isolate';
 
 import 'package:expect/expect.dart';
 
+class Foo implements Finalizable {
+  final Pointer<Void> value;
+  Foo(this.value);
+}
+
 main(args) async {
   try {
-    await Isolate.spawn(print, Pointer<Void>.fromAddress(1));
+    await Isolate.spawn(print, Foo(Pointer<Void>.fromAddress(1)));
   } catch (e) {
     Expect.type<ArgumentError>(e);
     return;