Remove Fork effect

`Fork_ignore` can be used to implement it just as easily.

lib_eio/eio.ml

@@ -293,7 +293,6 @@ module Private = struct
     type 'a enqueue = 'a Suspend.enqueue
     type _ eff += 
       | Suspend = Suspend.Suspend
-      | Fork = Fibre.Fork
       | Fork_ignore = Fibre.Fork_ignore
       | Get_context = Cancel.Get_context
       | Trace = Std.Trace

lib_eio/eio.mli

@@ -655,9 +655,6 @@ module Private : sig
           passing it the suspended fibre's context and a function to resume it.
           [fn] should arrange for [enqueue] to be called once the thread is ready to run again. *)
 
-      | Fork : Fibre_context.t * (unit -> 'a) -> 'a Promise.t eff
-      (** See {!Fibre.fork} *)
-
       | Fork_ignore : Fibre_context.t * (unit -> unit) -> unit eff
       (** See {!Fibre.fork_ignore} *)
 

lib_eio/fibre.ml

@@ -1,12 +1,5 @@
 open EffectHandlers
 
-type _ eff += Fork : Cancel.fibre_context * (unit -> 'a) -> 'a Promise.t eff
-
-let fork ~sw f =
-  let f () = Switch.with_op sw f in
-  let new_fibre = Cancel.Fibre_context.make ~cc:sw.cancel in
-  perform (Fork (new_fibre, f))
-
 type _ eff += Fork_ignore : Cancel.fibre_context * (unit -> unit) -> unit eff
 
 let fork_ignore ~sw f =
@@ -18,6 +11,17 @@ let fork_ignore ~sw f =
   let new_fibre = Cancel.Fibre_context.make ~cc:sw.cancel in
   perform (Fork_ignore (new_fibre, f))
 
+let fork ~sw f =
+  let new_fibre = Cancel.Fibre_context.make ~cc:sw.Switch.cancel in
+  let p, r = Promise.create_with_id (Cancel.Fibre_context.tid new_fibre) in
+  let f () =
+    match Switch.with_op sw f with
+    | x -> Promise.fulfill r x
+    | exception ex -> Promise.break r ex
+  in
+  perform (Fork_ignore (new_fibre, f));
+  p
+
 let yield () =
   let fibre = Suspend.enter (fun fibre enqueue -> enqueue (Ok fibre)) in
   Cancel.check fibre.cancel_context
@@ -30,13 +34,18 @@ let both f g = all [f; g]
 
 let pair f g =
   Cancel.sub @@ fun cancel ->
-  let f _fibre =
-    try f ()
-    with ex -> Cancel.cancel cancel ex; raise ex
-  in
   let x =
+    let p, r = Promise.create () in
+    let f _fibre =
+      match f () with
+      | x -> Promise.fulfill r x
+      | exception ex ->
+        Cancel.cancel cancel ex;
+        Promise.break r ex
+    in
     let new_fibre = Cancel.Fibre_context.make ~cc:cancel in
-    perform (Fork (new_fibre, f))
+    perform (Fork_ignore (new_fibre, f));
+    p
   in
   match g () with
   | gr -> Promise.await x, gr               (* [g] succeeds - just report [f]'s result *)
@@ -99,7 +108,13 @@ let any fs =
           | [f] -> wrap f (); []
           | f :: fs ->
             let new_fibre = Cancel.Fibre_context.make ~cc in
-            let p = perform (Fork (new_fibre, wrap f)) in
+            let p, r = Promise.create () in
+            let f () =
+              match wrap f () with
+              | x -> Promise.fulfill r x
+              | exception ex -> Promise.break r ex
+            in
+            perform (Fork_ignore (new_fibre, f));
             p :: aux fs
         in
         let ps = aux fs in

lib_eio_linux/eio_linux.ml

@@ -1052,20 +1052,6 @@ let rec run ?(queue_depth=64) ?(block_size=4096) main =
                 );
               schedule st
             )
-          | Eio.Private.Effects.Fork (new_fibre, f) -> Some (fun k -> 
-              let k = { Suspended.k; fibre } in
-              let promise, resolver = Promise.create_with_id (Fibre_context.tid new_fibre) in
-              enqueue_thread st k promise;
-              fork
-                ~new_fibre
-                (fun () ->
-                   match f () with
-                   | x -> Promise.fulfill resolver x
-                   | exception ex ->
-                     Log.debug (fun f -> f "Forked fibre failed: %a" Fmt.exn ex);
-                     Promise.break resolver ex
-                )
-            )
           | Eio.Private.Effects.Fork_ignore (new_fibre, f) -> Some (fun k -> 
               let k = { Suspended.k; fibre } in
               enqueue_thread st k ();

lib_eio_luv/eio_luv.ml

@@ -625,20 +625,6 @@ let rec run main =
             fn loop fibre (enqueue_thread st k))
         | Eio.Private.Effects.Trace ->
           Some (fun k -> continue k Eunix.Trace.default_traceln)
-        | Eio.Private.Effects.Fork (new_fibre, f) ->
-          Some (fun k -> 
-            let k = { Suspended.k; fibre } in
-            let promise, resolver = Promise.create_with_id (Fibre_context.tid new_fibre) in
-            enqueue_thread st k promise;
-            fork
-              ~new_fibre
-              (fun () ->
-                 match f () with
-                 | x -> Promise.fulfill resolver x
-                 | exception ex ->
-                   Log.debug (fun f -> f "Forked fibre failed: %a" Fmt.exn ex);
-                   Promise.break resolver ex
-              ))
         | Eio.Private.Effects.Fork_ignore (new_fibre, f) ->
           Some (fun k -> 
             let k = { Suspended.k; fibre } in