fix Cmi_format error in voodoo-do, by checking in prep that the magic…

… numbers match

src/voodoo-prep/prep.ml

@@ -12,10 +12,32 @@ let process_package : Fpath.t -> Package.t -> Fpath.t list -> unit =
  fun root package files ->
   let dest = Package.prep_path package in
 
+  let format_matches_compiler_version path =
+    let open Unix in
+    let cmd = "ocamlobjinfo " ^ Fpath.to_string path in
+    let ic, oc = open_process cmd in
+    let rec matches_version ic =
+      try
+        let line = input_line ic in
+        let prefix = "Wrong magic number" in
+        let str_len = String.length line in
+        if str_len >= 18 && String.sub line 0 18 = prefix then (
+          Printf.eprintf "ERROR: Ocamlobjinfo on %s: %s\n"
+            (Fpath.to_string path) line;
+          false)
+        else if str_len > 0 then matches_version ic
+        else true
+      with End_of_file -> true
+    in
+    let matches_version = matches_version ic in
+    let _ = close_process (ic, oc) in
+    matches_version
+  in
+
   (* Some packages produce ocaml artefacts that can't be processed with the switch's
       ocaml compiler - most notably the secondary compiler! This switch is intended to
       be used to ignore those files *)
-  let process_ocaml_artefacts =
+  let is_not_blacklisted =
     let package_blacklist = [ "ocaml-secondary-compiler" ] in
     not (List.mem package.name package_blacklist)
   in
@@ -33,16 +55,17 @@ let process_package : Fpath.t -> Package.t -> Fpath.t list -> unit =
     let no_ext = Fpath.rem_ext filename in
     let has_hyphen = String.contains (Fpath.to_string filename) '-' in
     let is_module =
-      process_ocaml_artefacts
+      is_not_blacklisted
       && List.mem ext [ ".cmt"; ".cmti"; ".cmi" ]
-      && not has_hyphen
+      && (not has_hyphen)
+      && format_matches_compiler_version Fpath.(root // fpath)
     in
     let do_copy =
       (not in_build_dir)
       && (is_in_doc_dir || is_module
          || List.mem no_ext (List.map Fpath.v [ "META"; "dune-package" ]))
     in
-    let is_cma = process_ocaml_artefacts && List.mem ext [ ".cma"; ".cmxa" ] in
+    let is_cma = is_not_blacklisted && List.mem ext [ ".cma"; ".cmxa" ] in
     let copy =
       if do_copy then Fpath.(root // fpath, dest // fpath) :: acc.copy
       else acc.copy