Expose on_{read,write} to let the user to suspend on events

This API is experimental but can be useful (mainly if we want to
read/write on bigstring via Miou without an extension of the core API).

lib/miou_unix.ml

@@ -50,7 +50,7 @@ module Table = Hashtbl.Make (struct
   type t = Unix.file_descr
 
   let equal a b = Int.equal (Obj.magic a) (Obj.magic b)
-  let hash v = (Obj.magic v) land max_int
+  let hash v = Obj.magic v land max_int
 end)
 
 type unix_scheduler = {
@@ -138,8 +138,8 @@ let rec smallest_sleeper ~now sleepers =
         let delta = time -. now in
         if delta <= 0. then None else Some delta
 
-let blocking_read fd =
-  let syscall = Miou.make (Fun.const ()) in
+let on_read fd fn =
+  let syscall = Miou.make fn in
   let uid = Miou.uid syscall in
   let unix = get () in
   Hashtbl.add unix.revert uid fd;
@@ -152,8 +152,10 @@ let blocking_read fd =
   set unix;
   Miou.suspend syscall
 
-let blocking_write fd =
-  let syscall = Miou.make (Fun.const ()) in
+let blocking_read fd = on_read fd (Fun.const ())
+
+let on_write fd fn =
+  let syscall = Miou.make fn in
   let uid = Miou.uid syscall in
   let unix = get () in
   Hashtbl.add unix.revert uid fd;
@@ -166,6 +168,8 @@ let blocking_write fd =
   set unix;
   Miou.suspend syscall
 
+let blocking_write fd = on_write fd (Fun.const ())
+
 let with_lock ~lock fn =
   Mutex.lock lock;
   Fun.protect ~finally:(fun () -> Mutex.unlock lock) fn
@@ -374,7 +378,8 @@ let no_events unix =
 
 let[@inline always] keys_of_table tbl =
   let res = ref [] in
-  Table.iter (fun k _ -> res := k :: !res) tbl; !res
+  Table.iter (fun k _ -> res := k :: !res) tbl;
+  !res
 
 let select _domain interrupt ~poll (cancelled_syscalls : Miou.uid list) =
   let unix = get () in

lib/miou_unix.mli

@@ -135,3 +135,8 @@ val bind_and_listen : ?backlog:int -> file_descr -> Unix.sockaddr -> unit
     [backlog] is the maximal number of pending requests. *)
 
 val run : ?g:Random.State.t -> ?domains:int -> (unit -> 'a) -> 'a
+
+(**/**)
+
+val on_read : Unix.file_descr -> (unit -> unit) -> unit
+val on_write : Unix.file_descr -> (unit -> unit) -> unit