feat fiber: expose add_on_cancel/remove_on_cancel

also make it more robust by using a map

src/fib/fiber.ml

@@ -1,5 +1,6 @@
 module A = Atomic
 module FM = Handle.Map
+module Int_map = Map.Make (Int)
 
 type 'a callback = 'a Exn_bt.result -> unit
 (** Callbacks that are called when a fiber is done. *)
@@ -20,7 +21,8 @@ module Private_ = struct
   and 'a state =
     | Alive of {
         children: children;
-        on_cancel: cancel_callback list;
+        on_cancel: cancel_callback Int_map.t;
+        cancel_id: int;
       }
     | Terminating_or_done of 'a Exn_bt.result A.t
 
@@ -71,12 +73,12 @@ let rec resolve_as_failed_ : type a. a t -> Exn_bt.t -> unit =
   let promise = prom_of_fut self.res in
   while
     match A.get self.state with
-    | Alive { children; on_cancel } as old ->
+    | Alive { children; cancel_id = _; on_cancel } as old ->
       let new_st = Terminating_or_done (A.make @@ Error ebt) in
       if A.compare_and_set self.state old new_st then (
         (* here, unlike in {!resolve_fiber}, we immediately cancel children *)
         cancel_children_ ~children ebt;
-        List.iter (fun cb -> cb ebt) on_cancel;
+        Int_map.iter (fun _ cb -> cb ebt) on_cancel;
         resolve_once_children_are_done_ ~children ~promise (A.make @@ Error ebt);
         false
       ) else
@@ -96,7 +98,7 @@ let resolve_ok_ (self : 'a t) (r : 'a) : unit =
   let promise = prom_of_fut self.res in
   while
     match A.get self.state with
-    | Alive { children; on_cancel = _ } as old ->
+    | Alive { children; _ } as old ->
       let new_st = Terminating_or_done r in
       if A.compare_and_set self.state old new_st then (
         resolve_once_children_are_done_ ~children ~promise r;
@@ -111,9 +113,9 @@ let resolve_ok_ (self : 'a t) (r : 'a) : unit =
 let remove_child_ (self : _ t) (child : _ t) =
   while
     match A.get self.state with
-    | Alive { children; on_cancel } as old ->
+    | Alive ({ children; _ } as alive) as old ->
       let new_st =
-        Alive { children = FM.remove child.id children; on_cancel }
+        Alive { alive with children = FM.remove child.id children }
       in
       not (A.compare_and_set self.state old new_st)
     | _ -> false
@@ -126,9 +128,9 @@ let remove_child_ (self : _ t) (child : _ t) =
 let add_child_ ~protect (self : _ t) (child : _ t) =
   while
     match A.get self.state with
-    | Alive { children; on_cancel } as old ->
+    | Alive ({ children; _ } as alive) as old ->
       let new_st =
-        Alive { children = FM.add child.id (Any child) children; on_cancel }
+        Alive { alive with children = FM.add child.id (Any child) children }
       in
 
       if A.compare_and_set self.state old new_st then (
@@ -159,7 +161,10 @@ let spawn_ ~ls ~on (f : _ -> 'a) : 'a t =
   let res, _promise = Fut.make () in
   let fib =
     {
-      state = A.make @@ Alive { children = FM.empty; on_cancel = [] };
+      state =
+        A.make
+        @@ Alive
+             { children = FM.empty; on_cancel = Int_map.empty; cancel_id = 0 };
       id;
       res;
       runner = on;
@@ -199,12 +204,26 @@ let spawn_link ~protect f : _ t =
     add_child_ ~protect parent child;
     child
 
-let add_cancel_cb_ (self : _ t) cb =
+type cancel_handle = int
+
+let add_on_cancel (self : _ t) cb : cancel_handle =
+  let h = ref 0 in
   while
     match A.get self.state with
-    | Alive { children; on_cancel } as old ->
-      let new_st = Alive { children; on_cancel = cb :: on_cancel } in
-      not (A.compare_and_set self.state old new_st)
+    | Alive { children; cancel_id; on_cancel } as old ->
+      let new_st =
+        Alive
+          {
+            children;
+            cancel_id = cancel_id + 1;
+            on_cancel = Int_map.add cancel_id cb on_cancel;
+          }
+      in
+      if A.compare_and_set self.state old new_st then (
+        h := cancel_id;
+        false
+      ) else
+        true
     | Terminating_or_done r ->
       (match A.get r with
       | Error ebt -> cb ebt
@@ -212,14 +231,16 @@ let add_cancel_cb_ (self : _ t) cb =
       false
   do
     ()
-  done
+  done;
+  !h
 
-let remove_top_cancel_cb_ (self : _ t) =
+let remove_on_cancel (self : _ t) h =
   while
     match A.get self.state with
-    | Alive { on_cancel = []; _ } -> assert false
-    | Alive { children; on_cancel = _ :: tl } as old ->
-      let new_st = Alive { children; on_cancel = tl } in
+    | Alive ({ on_cancel; _ } as alive) as old ->
+      let new_st =
+        Alive { alive with on_cancel = Int_map.remove h on_cancel }
+      in
       not (A.compare_and_set self.state old new_st)
     | Terminating_or_done _ -> false
   do
@@ -227,13 +248,13 @@ let remove_top_cancel_cb_ (self : _ t) =
   done
 
 let with_cancel_callback (self : _ t) cb (k : unit -> 'a) : 'a =
-  add_cancel_cb_ self cb;
-  Fun.protect k ~finally:(fun () -> remove_top_cancel_cb_ self)
+  let h = add_on_cancel self cb in
+  Fun.protect k ~finally:(fun () -> remove_on_cancel self h)
 
 let with_self_cancel_callback cb (k : unit -> 'a) : 'a =
   let (Any self) = self () in
-  add_cancel_cb_ self cb;
-  Fun.protect k ~finally:(fun () -> remove_top_cancel_cb_ self)
+  let h = add_on_cancel self cb in
+  Fun.protect k ~finally:(fun () -> remove_on_cancel self h)
 
 let[@inline] await self = Fut.await self.res
 let[@inline] wait_block self = Fut.wait_block self.res

src/fib/fiber.mli

@@ -81,6 +81,17 @@ val yield : unit -> unit
 (** Yield control to the scheduler from the current fiber.
     @raise Failure if not run from inside a fiber. *)
 
+type cancel_handle
+(** An opaque handle for a single cancel callback in a fiber *)
+
+val add_on_cancel : _ t -> cancel_callback -> cancel_handle
+(** [add_on_cancel fib cb] adds [cb] to the list of cancel callbacks
+    for [fib]. If [fib] is already cancelled, [cb] is called immediately. *)
+
+val remove_on_cancel : _ t -> cancel_handle -> unit
+(** [remove_on_cancel fib h] removes the cancel callback
+    associated with handle [h]. *)
+
 val with_cancel_callback : _ t -> cancel_callback -> (unit -> 'a) -> 'a
 (** [with_cancel_callback fib cb (fun () -> <e>)] evaluates [e]
     in a scope in which, if the fiber [fib] is cancelled,

src/fib/fls.mli

@@ -3,6 +3,14 @@
     This storage is associated to the current fiber,
     just like thread-local storage is associated with
     the current thread.
+
+    See {!Moonpool.Task_local_storage} for more general information, as
+    this is based on it.
+
+    {b NOTE}: it's important to note that, while each fiber
+    has its own storage, spawning a sub-fiber [f2] from a fiber [f1]
+    will only do a shallow copy of the storage.
+    Values inside [f1]'s storage will be physically shared with [f2].
 *)
 
 include module type of struct