Add umask get/set. Fixes: #2840

src/core/builtins/builtins_files.ml

@@ -558,3 +558,29 @@ let _ =
       with exn ->
         let bt = Printexc.get_raw_backtrace () in
         Lang.raise_as_runtime ~bt ~kind:"file" exn)
+
+let () =
+  if not Sys.win32 then (
+    let umask_m = Mutex.create () in
+    let get_umask =
+      Tutils.mutexify umask_m (fun () ->
+          let umask = Unix.umask 0 in
+          ignore (Unix.umask umask);
+          umask)
+    in
+    let set_umask =
+      Tutils.mutexify umask_m (fun umask -> ignore (Unix.umask umask))
+    in
+    let umask =
+      Lang.add_builtin ~base:file "umask" ~category:`File
+        ~descr:"Get the process's file mode creation mask." [] Lang.int_t
+        (fun _ -> Lang.int (get_umask ()))
+    in
+    ignore
+      (Lang.add_builtin ~base:umask "set" ~category:`File
+         ~descr:"Set process's file mode creation mask."
+         [("", Lang.int_t, None, None)]
+         Lang.unit_t
+         (fun p ->
+           set_umask (Lang.to_int (List.assoc "" p));
+           Lang.unit)))