Portably delete files on Windows

Part of diskuv/dkml-installer-ocaml#67

src/runtimescripts/dkml_runtimescripts.ml

@@ -1,5 +1,22 @@
 open Bos
 
+let portable_delete_file target_fp =
+  let ( let* ) = Result.bind in
+  (* [doc from diskuvbox]
+     [tracks https://github.com/dbuenzli/bos/issues/98]
+     For Windows, can't write without turning off read-only flag.
+     In fact, you can still get Permission Denied even after turning
+     off read-only flag, perhaps because Windows has a richer
+     permissions model than POSIX. So we remove the file
+     after turning off read-only *)
+  if Sys.win32 then
+    let* exists = OS.File.exists target_fp in
+    if exists then
+      let* () = OS.Path.Mode.set target_fp 0o644 in
+      OS.File.delete target_fp
+    else Ok ()
+  else OS.File.delete target_fp
+
 let extract_dkml_scripts ~dkmlversion dir_fp =
   let ( let* ) = Result.bind in
   let file_list_helper file_list read subdir_fp =

src/runtimescripts/dkml_runtimescripts.mli

@@ -1,2 +1,6 @@
 val extract_dkml_scripts :
   dkmlversion:string -> Fpath.t -> (unit, Rresult.R.msg) result
+
+(** [portable_delete_file] deletes the file even it is read-only on Windows.
+    Works around https://github.com/dbuenzli/bos/issues/98 *)
+val portable_delete_file : Fpath.t -> (unit, Rresult.R.msg) result