You kept crypto here, was that intentional? If so, it probably needs a guard for skipping the test if appropriate

That is from the blocking PR, I will rebase once that is fixed.

I'm worried that doing reference counting is too fragile.
Why not replace the for either with a .forEach for with a for (const hook of Array.from(active_hooks_array))

I'm worried that doing reference counting is too fragile.

Could you be more explicit?

edit: Just to avoid confusion, this is not a reference counting it is a depth counting.

Why not replace the for either with a .forEach

.forEach is not resistant to mutation.

const a = [1,2,3];
a.forEach(function (value) {
  process._rawDebug(value);
  a.pop();
});

prints 1 2.

with a for (const hook of Array.from(active_hooks_array))

Array copy is too slow. When async_hooks is enabled this is the hottest code path we have.

Ack. I was sure .forEach iterated over a fixed array (it just ignores newly added elements)

nit: Since we are going with depth counting, IMHO the decrement should be in a finally clause

I guess, but there is no way to escape fatalError so the code will never be executed.

The try catch finally order is:

try {
  throw new Error();
} catch (e) {
  console.log('catch');
} finally {
  console.log('finally');
}

outputs catch finally.

I guess, but there is no way to escape fatalError so the code will never be executed.

At the moment... If at some point we'll enable a fatalError trap this will break. Better to be explicit about these.
Reference/depth counting only works if it's ACID

finally

Possibly change comment above from simply

Track whether a hook [...]

to something like

Use a counter to track [...] and prevent nested calls [...]