Marshal alternative GPT header

lib/gpt.ml

@@ -373,9 +373,19 @@ let unmarshal buf ~sector_size =
             partitions_crc32;
           })
 
-let marshal_header ~sector_size (buf : Cstruct.t) t =
+let marshal_header ~sector_size ~primary (buf : Cstruct.t) t =
   if Cstruct.length buf < sector_size || Cstruct.length buf < sizeof then
     invalid_arg "Gpt.marshal_header";
+  let t =
+    if primary then
+      t
+    else
+      (* The backup header has the current and backup LBAs swapped *)
+      let t =
+        { t with current_lba = t.backup_lba; backup_lba = t.current_lba }
+      in
+      { t with header_crc32 = Optint.to_int32 (calculate_header_crc32 t) }
+  in
   Cstruct.blit_from_string signature 0 buf signature_offset revision_offset;
   Cstruct.LE.set_uint32 buf revision_offset t.revision;
   Cstruct.LE.set_uint32 buf header_size_offset t.header_size;

lib/gpt.mli

@@ -89,7 +89,7 @@ val unmarshal : Cstruct.t -> sector_size:int ->
    @raise Invalid_argument when [buf] is too small to contain a GPT header
 *)
 
-val marshal_header : sector_size:int -> Cstruct.t -> t -> unit
+val marshal_header : sector_size:int -> primary:bool -> Cstruct.t -> t -> unit
 (** [marshal_header ~sector_size buf t] serializes the GPT header to [buf].
     The caller is expected to write the contents of [buf] to [t.current_lba]
     and [t.backup_lba]. *)

test/test_gpt.ml

@@ -167,7 +167,7 @@ let test_marshal_unmarshal () =
     512 * ((len + 511) / 512)
   in
   let buf_partition_table = Cstruct.create partition_table_len in
-  Gpt.marshal_header ~sector_size:512 buf_header morig;
+  Gpt.marshal_header ~sector_size:512 ~primary:true buf_header morig;
   Gpt.marshal_partition_table ~sector_size:512 buf_partition_table morig;
   match Gpt.unmarshal ~sector_size:512 buf_header with
   | Error e -> Alcotest.failf "Failed to parse marshalled gpt header: %s" e