fix: truncate lock file

Truncate it after successful locks and before an unlock.

This makes sure that we aren't left with garbage from previous runs.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

<!-- ps-id: 21d117de-b760-42a5-82b9-8b60517dce90 -->

src/dune_util/global_lock.ml

@@ -52,11 +52,16 @@ module Lock = struct
      | `Failure -> ()
      | `Success ->
        let fd = Flock.fd t in
+       Unix.ftruncate fd 0;
        write_pid fd);
     res
   ;;
 
-  let unlock () = Lazy.force t |> Flock.unlock |> or_raise_unix ~name:"unlock"
+  let unlock () =
+    let lock = Lazy.force t in
+    Unix.ftruncate (Flock.fd lock) 0;
+    Flock.unlock lock |> or_raise_unix ~name:"unlock"
+  ;;
 end
 
 let locked = ref false
@@ -103,3 +108,5 @@ let unlock () =
       Lock.unlock ();
       locked := false)
 ;;
+
+let () = at_exit unlock