Source file semaphore.ml

(*
 * Copyright (C) Citrix Systems Inc.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published
 * by the Free Software Foundation; version 2.1 only. with the special
 * exception on linking described in file LICENSE.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *)

type t = {mutable n: int; m: Mutex.t; c: Condition.t}

let create n =
  if n <= 0 then
    invalid_arg (Printf.sprintf "Semaphore value must be positive, got %d" n) ;
  let m = Mutex.create () and c = Condition.create () in
  {n; m; c}

exception Inconsistent_state of string

let inconsistent_state fmt =
  Printf.ksprintf (fun msg -> raise (Inconsistent_state msg)) fmt

let acquire s k =
  if k <= 0 then
    invalid_arg
      (Printf.sprintf "Semaphore acquisition requires a positive value, got %d"
         k
      ) ;
  Mutex.lock s.m ;
  while s.n < k do
    Condition.wait s.c s.m
  done ;
  if not (s.n >= k) then
    inconsistent_state "Semaphore value cannot be smaller than %d, got %d" k s.n ;
  s.n <- s.n - k ;
  Condition.signal s.c ;
  Mutex.unlock s.m

let release s k =
  if k <= 0 then
    invalid_arg
      (Printf.sprintf "Semaphore release requires a positive value, got %d" k) ;
  Mutex.lock s.m ;
  s.n <- s.n + k ;
  Condition.signal s.c ;
  Mutex.unlock s.m

let execute_with_weight s k f =
  acquire s k ;
  Xapi_stdext_pervasives.Pervasiveext.finally f (fun () -> release s k)

let execute s f = execute_with_weight s 1 f