Source file `lwt_lock_file.ml`

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
(*****************************************************************************)
(*                                                                           *)
(* Open Source License                                                       *)
(* Copyright (c) 2018 Dynamic Ledger Solutions, Inc. <contact@tezos.com>     *)
(* Copyright (c) 2021 Nomadic Labs <contact@nomadic-labs.com>                *)
(*                                                                           *)
(* Permission is hereby granted, free of charge, to any person obtaining a   *)
(* copy of this software and associated documentation files (the "Software"),*)
(* to deal in the Software without restriction, including without limitation *)
(* the rights to use, copy, modify, merge, publish, distribute, sublicense,  *)
(* and/or sell copies of the Software, and to permit persons to whom the     *)
(* Software is furnished to do so, subject to the following conditions:      *)
(*                                                                           *)
(* The above copyright notice and this permission notice shall be included   *)
(* in all copies or substantial portions of the Software.                    *)
(*                                                                           *)
(* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR*)
(* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,  *)
(* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL   *)
(* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER*)
(* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING   *)
(* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER       *)
(* DEALINGS IN THE SOFTWARE.                                                 *)
(*                                                                           *)
(*****************************************************************************)

open Error_monad

let try_with_lock ~when_locked ~filename f =
  protect (fun () ->
      let flags =
        let open Unix in
        [O_CLOEXEC; O_TRUNC; O_CREAT; O_WRONLY]
      in
      Lwt_unix.openfile filename flags 0o644 >>= fun fd ->
      Lwt.finalize
        (fun () ->
          Lwt.catch
            (fun () ->
              Lwt_unix.lockf fd Unix.F_TLOCK 0 >>= fun () -> return `Free)
            (function
              | Unix.Unix_error ((EAGAIN | EACCES | EDEADLK), _, _) ->
                  return `Locked
              | exn -> fail (Exn exn))
          >>=? function
          | `Locked -> when_locked ()
          | `Free ->
              let pid_str = string_of_int (Unix.getpid ()) in
              Lwt_unix.write_string fd pid_str 0 (String.length pid_str)
              >>= fun _ ->
              Lwt.finalize
                (fun () -> f ())
                (fun () -> Lwt_unix.lockf fd Unix.F_ULOCK 0))
        (fun () -> Lwt_unix.close fd))