test: update readme and the mdx test

it's complicated and hard to use in practice, because it's not obvious
if a piece of code is running under another fiber or not, so
`Fiber.spawn` might fail because it has no parent.

So in practice we've been using `Fiber.spawn_top`… which has no
interest over just using `Fut.spawn`.

src/core/chan.ml

@@ -21,6 +21,7 @@ let create ~max_size () : _ t =
   }
 
 let try_push (self : _ t) x : bool =
+  let res = ref false in
   if Mutex.try_lock self.mutex then (
     if self.closed then (
       Mutex.unlock self.mutex;
@@ -32,46 +33,42 @@ let try_push (self : _ t) x : bool =
       let to_awake = Queue.create () in
       Queue.push x self.q;
       Queue.transfer self.pop_waiters to_awake;
+      res := true;
       Mutex.unlock self.mutex;
       (* wake up pop triggers if needed. Be careful to do that
          outside the critical section*)
-      Queue.iter Trigger.signal to_awake;
-      true
+      Queue.iter Trigger.signal to_awake
     | n when n < self.max_size ->
       Queue.push x self.q;
-      Mutex.unlock self.mutex;
-      true
-    | _ ->
-      Mutex.unlock self.mutex;
-      false
-  ) else
-    false
+      Mutex.unlock self.mutex
+    | _ -> Mutex.unlock self.mutex
+  );
+  !res
 
 let try_pop (type elt) self : elt option =
+  let res = ref None in
   if Mutex.try_lock self.mutex then (
-    match Queue.pop self.q with
+    (match Queue.pop self.q with
     | exception Queue.Empty ->
-      Mutex.unlock self.mutex;
-      if self.closed then
+      if self.closed then (
+        Mutex.unlock self.mutex;
         raise Closed
-      else
-        None
-    | x ->
-      Mutex.unlock self.mutex;
-      Some x
-  ) else
-    None
+      )
+    | x -> res := Some x);
+    Mutex.unlock self.mutex
+  );
+  !res
 
 let close (self : _ t) : unit =
-  let triggers_to_signal = Queue.create () in
+  let q = Queue.create () in
   Mutex.lock self.mutex;
   if not self.closed then (
     self.closed <- true;
-    Queue.transfer self.pop_waiters triggers_to_signal;
-    Queue.transfer self.push_waiters triggers_to_signal
+    Queue.transfer self.pop_waiters q;
+    Queue.transfer self.push_waiters q
   );
   Mutex.unlock self.mutex;
-  Queue.iter Trigger.signal triggers_to_signal
+  Queue.iter Trigger.signal q
 
 let rec push (self : _ t) x : unit =
   Mutex.lock self.mutex;

src/core/chan.mli

@@ -1,8 +1,7 @@
 (** Channels.
 
-    The channels have bounded size. They use effects/await to provide
-    a direct style implementation. Pushing into a full channel,
-    or popping from an empty one, will suspend the current task.
+    The channels have bounded size. Push/pop return futures or can use effects
+    to provide an [await]-friendly version.
 
     The channels became bounded since @0.7 .
 *)

src/core/fut.ml

@@ -431,7 +431,8 @@ let await (self : 'a t) : 'a =
   | exception C.Running ->
     let trigger = Trigger.create () in
     (* suspend until the future is resolved *)
-    if C.try_attach self trigger then Trigger.await_exn trigger;
+    if C.try_attach self trigger then
+      Option.iter Exn_bt.raise @@ Trigger.await trigger;
 
     (* un-suspended: we should have a result! *)
     get_or_fail_exn self

src/forkjoin/moonpool_forkjoin.ml

@@ -64,7 +64,7 @@ module State_ = struct
       done;
 
       (* wait for the other computation to be done *)
-      if !must_await then Trigger.await_exn trigger
+      if !must_await then Trigger.await trigger |> Option.iter Exn_bt.raise
     | Right_solved _ | Both_solved _ -> assert false
 end
 
@@ -144,7 +144,7 @@ let for_ ?chunk_size n (f : int -> int -> unit) : unit =
       i := !i + len_range
     done;
 
-    Trigger.await_exn trigger;
+    Trigger.await trigger |> Option.iter Exn_bt.raise;
     Option.iter Exn_bt.raise @@ A.get failure;
     ()
   )