[vm/ffi] Disallow adding finalizers to Pointer

[dcharkes]

We should disallow Pointer in Dart_NewWeakPersistentHandle and Dart_NewFinalizableHandle.

1. Pointers do not have identity. We might want to optimize them to just be integers later.

2. Also the nested structs and inline arrays representation does not keep original pointers alive:

class Foo extends Struct {
  external Bar bar;
}
class Bar extends Struct {
  Int32()
  external int a;
}

void main(){
  Pointer<Foo> fooPointer = calloc();
  // Attach native finalizer to `fooPointer` using `Dart_NewWeakPersistentHandle` or `Dart_NewFinalizableHandle`.
  Foo foo = fooPointer.ref;
  Bar bar = foo.bar;
  // GC
  bar.a; // undefined behavior.
}

This is a breaking change.

[mkustermann]

Maybe we should make a breaking change request following docs/process/breaking-changes.md and getting it approved?

Furthermore: One reason for doing this breaking change is to avoid use-after-free if pointer gets finalized before derived objects. The same argument can be made for struct objects which might get finalized before derived objects (e.g. inner structs / arrays). So I think we should include that in this breaking change request.

[simolus3]

Pointers do not have identity. We might want to optimize them to just be integers later.

This might have to be another (breaking?) issue, but I think that an Expando should also reject pointers if they have no identity? Should they also be disallowed in identical and identityHashCode?

sdk/sdk/lib/_internal/vm/lib/expando_patch.dart

Lines 139 to 147 in 588fadc

	static _checkType(object) {
	if ((object == null) ||
	(object is bool) ||
	(object is num) ||
	(object is String)) {
	throw new ArgumentError.value(object,
	"Expandos are not allowed on strings, numbers, booleans or null");
	}
	}

[dcharkes]

This might have to be another (breaking?) issue, but I think that an Expando should also reject pointers if they have no identity?

After some internal discussion, we've added this to the breaking change request.

Should they also be disallowed in identical and identityHashCode?

After some internal discussion, we'll keep these as they are for now. (See https://dart-review.googlesource.com/c/sdk/+/186280/11..16/CHANGELOG.md#b11 on telling yourself apart from identical twins. 😄 )

Thanks for your suggestions @simolus3!