Incorrect refcounting with async exceptions / finalize

[timbertson]

I noticed this because in my uv branch, the close(handle) function acts as an "assert-unique-reference". This seemed to be working correctly in the successful case, but failing (with a dangling refcount of 2) in the case where an exception is thrown, and the handle is attempted to be closed in a finally block.

This is a similar issue to koka-lang/koka#854, however the koka compiler fix in koka-lang/koka#863 is not sufficient on its own.

The only combination of koka & async.kk which I've found to work correctly is:

• koka: TimWhiting:improve-continuation-refcount
• std: timbertson:async-cancel

output:

async finally: 0
async error: failed...
sync finally: 0
sync error: failed...

Using any other branch of koka or std results in the following incorrect output:

async finally: 2
async error: failed...
sync finally: 0
sync error: failed...

This implies that the refcount can leak either via lack of the improve-continuation-refcount, or by using the version of async.kk in both libuv-basics and koka-community/std/main (they're identical). It's not obvious what changes in my async-cancel branch cause this change in behaviour.

Test case:

import std/num/int32
import std/async/async

extern create-box(): io-noexn any
  c inline "kk_cptr_raw_box(&kk_free_fun, kk_malloc(sizeof(int), _ctx), _ctx)"

extern get-refcount(^box: any): io-noexn int32
  c inline "kk_block_refcount(kk_box_to_ptr(box, _ctx))"

fun main()
  try (finalizer-async) fn(e)
    println("async error: " ++ e.show)
    0.int32
  ()

  try (finalizer-sync) fn(e)
    println("sync error: " ++ e.show)
    0.int32
  ()

fun finalizer-async()
  val box = create-box()
  with finally {
    println("async finally: " ++ box.get-refcount().show)
  }
  box-fail-async()
  box.get-refcount()

fun finalizer-sync()
  val box = create-box()
  with finally {
    println("sync finally: " ++ box.get-refcount().show)
  }
  box-fail-sync()
  box.get-refcount()

fun failure()
  Exception("failed...", ExnAssert)

fun box-fail-async()
  val _: () = await1(fn(cb) -> cb(Error(failure()))).untry
  ()

fun box-fail-sync()
  throw-exn(failure())