fix: allow rename to overwrite read-only destination on Windows when deduplicating Dune cache

doc/changes/fixed/13713.md

@@ -0,0 +1,3 @@
+- Fix the Dune cache on Windows by correctly handling renames onto read-only
+  files. Before this change, the Dune cache would be filled but the stored
+  artifacts would not generally be usable by Dune. (#13713, @Nevor)

otherlibs/stdune/src/fpath.ml

@@ -204,6 +204,23 @@ let unlink_exn ~chmod dir fn =
     unlink_exn_ignore_missing fn
 ;;
 
+let win32_rename_exn src dst =
+  match Unix.rename src dst with
+  | () -> ()
+  | exception Unix.Unix_error (Unix.EACCES, _, _) ->
+    (* Try removing the read-only attribute.
+       Workaround a behavior discrepancy between Unix and Windows of the
+       [Unix.rename] function. It accepts readonly override on Unix but not
+       on Windows. This discrepancy will be fixed in OCaml 5.6 which will
+       include https://github.com/ocaml/ocaml/pull/14602 *)
+    Unix.chmod dst 0o666;
+    Unix.rename src dst
+;;
+
+let rename_exn =
+  if Stdlib.Sys.win32 then fun x y -> win32_rename_exn x y else fun x y -> Unix.rename x y
+;;
+
 let rec clear_dir ?(chmod = false) dir =
   match
     match Readdir.read_directory_with_kinds dir with

otherlibs/stdune/src/fpath.mli

@@ -46,6 +46,7 @@ type unlink_status =
 (** Unlink and return error, if any. *)
 val unlink : string -> unlink_status
 
+val rename_exn : string -> string -> unit
 val initial_cwd : string
 
 type clear_dir_result =

src/dune_cache/local.ml

@@ -191,7 +191,7 @@ module Artifacts = struct
                     [rename] operation has a quirk where [path_in_temp_dir] can
                     remain on disk. This is not a problem because we clean the
                     temporary directory later. *)
-                 Unix.rename
+                 Fpath.rename_exn
                    (Path.to_string path_in_temp_dir)
                    (Path.to_string path_in_build_dir)
                with