Deferred.cancel propagation failure in Async nested deferreds

Answer

In OCaml's Async library, calling Deferred.cancel on a deferred only marks that deferred as cancelled; the cancellation signal does **not** automatically recurse into child deferreds that were created independently of the parent.

Confirmed behavior

• Deferred.cancel updates the internal state of the target deferred and raises Cancelled when the deferred is awaited.
• Propagation follows the data‑flow graph: a deferred receives the cancellation only if it is awaiting (directly or indirectly) the cancelled deferred via combinators such as bind, map, both, all, or_error, etc.
• Independent concurrent deferreds created with Deferred.create or spawned without linking to the parent remain unaffected and may continue execution.

Likely explanation

The library treats cancellation as a cooperative signal that travels along the same edges used for normal results. If a child is not linked to the parent through those edges, there is no path for the signal to follow, so the child runs to completion unless it checks the parent’s state manually.

Steps to ensure child deferreds are cancelled

1. When spawning a child, attach it to the parent using a linking combinator, e.g.:

   let parent = Deferred.create ()
   let child  = parent >>= fun _ -> Deferred.of_unit (do_work ())
   

   or use Deferred.both parent child / Deferred.all [parent; child] so that cancelling the parent forces the child to await a cancelled result.
2. If you already have a set of independent children, register a cancel handler on the parent (available in Async ≥ v0.10.0):

   Deferred.set_cancel_handler parent (fun () ->
     List.iter Deferred.cancel children)
   

3. For older Async versions lacking set_cancel_handler, manually propagate by linking each child before spawning, or keep a reference list and call Deferred.cancel on each when you decide to cancel the parent.

Missing diagnostic detail

To give a precise recommendation, please confirm the version of Async (or base_async) you are using; the availability of Deferred.set_cancel_handler changes whether you need the handler approach or must rely solely on linking combinators.