add CCLock.update_map

src/threads/CCFuture.mli

@@ -146,3 +146,4 @@ val (>|=) : 'a t -> ('a -> 'b) -> 'b t
 
 val stop_pool : unit -> unit
 (** Stop the thread pool *)
+

src/threads/CCLock.ml

@@ -104,6 +104,17 @@ let update l f =
   let l = create 5 in update l (fun x->x+1); get l = 6
   *)
 
+let update_map l f =
+  with_lock l
+    (fun x ->
+      let x', y = f x in
+      l.content <- x';
+      y)
+
+(*$T
+  let l = create 5 in update_map l (fun x->x+1, string_of_int x) = "5" && get l = 6
+  *)
+
 let get l =
   Mutex.lock l.mutex;
   let x = l.content in

src/threads/CCLock.mli

@@ -60,6 +60,11 @@ val with_lock_as_ref : 'a t -> f:('a LockRef.t -> 'b) -> 'b
 val update : 'a t -> ('a -> 'a) -> unit
 (** [update l f] replaces the content [x] of [l] with [f x], atomically *)
 
+val update_map : 'a t -> ('a -> 'a * 'b) -> 'b
+(** [update_map l f] computes [x', y = f (get l)], then puts [x'] in [l]
+    and returns [y]
+    @since NEXT_RELEASE *)
+
 val mutex : _ t -> Mutex.t
 (** Underlying mutex *)