dart:ffi GC finalizers

[dcharkes]

Update 2022:

Available in https://api.dart.dev/stable/2.18.3/dart-ffi/NativeFinalizer-class.html.

Update 2020-06-26:
Finalizers are now available in native code (not yet exposed in Dart):

d.lookupFunction<Void Function(Handle), void Function(Object)>("PassObjectToC");

void attachFinalizer(Object o) => passObjectToC(o);

static void RunFinalizer(void* isolate_callback_data,
                         void* peer) {
  printf("Running finalizer.\n");
}

extern "c" void PassObjectToC(Dart_Handle h) {
  void* peer = 0x0;
  intptr_t size = 8;
  auto finalizable_handle = Dart_NewFinalizableHandle_DL(handle_2, peer, size, RunFinalizer);
}

Use dynamic linking (the _DL suffix) so that the symbols are available in Flutter. See documentation and samples on 7eac9f3 (C code, Dart code).

See more documentation in native_api.h and dart_api_dl.h.

Caveat:

• The dart optimiser can inline method bodies and fields, which means objects can in some cases be GCed (and their finalizer run) before all methods are done executing. When we expose finalizers in Dart, we will have a solution for this. Workaround: do an ffi call with a Dart_Handle passing that object to native, that will keep it alive until that call (like a reachability fence). Or alternatively use the Dart calling convention to keep the object alive:

  @pragma('vm:never-inline')
  Object reachabilityFence(Object obj) {
    return obj;
  }

==========================================================================================

This issue was originally intended to track exposing finalizers in Dart. (The above code exposes finalizers in C.)

A potential API would be.

/// Return a pointer object that has a finalizer attached to it. When this
/// pointer object is collected by GC the given finalizer is invoked.
///
/// Note: the pointer object passed to the finalizer is not the same as 
/// the pointer object that is returned from [finalizable] - it points
/// to the same memory region but has different identity. 
Pointer<T> finalizable<T>(Pointer<T> p, void finalizer(Pointer<T> ptr)) {

}

However, exposing FFI-specific finalizers in Dart has been superseded by adding finalizers to Dart in general https://github.com/dart-lang/sdk/issues/45455.

[dcharkes]

As discussed with @jonasfj and @mkustermann, maybe it would be better to provide a Pointer to a C function as finalizer, as the GC does currently not support running Dart code for finalization.

class Pointer {
  attachFinalizer(Pointer<NativeFunction<void Function(Pointer)>> finalizerPointer) {

  }
}

[jonasfj]

You might also want to include a sizeHint so we can signal an approximate size of the object to the GC.
Even if I don't know the exact size of the object I'm holding on to, I know if it's approximately 12 bytes, 12kb, or 12mb give or take a few orders of magnitude.

[sjindel-google]

We should implement this in the initial release since so many users seem to need it.

[sjindel-google]

I think we should estimate 3 weeks for this, given that we need:

• Support for attaching finalizers to Pointer (based on existing finalizer support).
• Maintaining a reference to the original from derived pointers.
• Maintaining a reference to the original from external typed data.

[yulingtianxia]

We should implement this in the initial release since so many users seem to need it.

Yeah, we really need it.

[ds84182]

Looking at https://dart-review.googlesource.com/c/sdk/+/123662, I don't think this solves all issues, especially with long lived structures or strings in a static method.

For example, libpng's png_create_read_struct needs the version string of libpng from link-time to ensure that ABI-incompatible changes haven't occurred (https://github.com/glennrp/libpng/blob/libpng16/png.h#L922). Yes, you could also wrap the function with another C library to forward the version correctly, but then you have to make sure that Dart, the auxiliary native library, and libpng are all in sync and are ABI compatible.

I'm just hoping that that CL isn't the final design for pointer finalizers.

[derolf]

When will this feature be available?