From 5fd5ef9403ef539702297e4ff0fb6f4894fd2ebd Mon Sep 17 00:00:00 2001
From: Jon Ludlam <jon@recoil.org>
Date: Mon, 11 Aug 2025 13:37:44 +0100
Subject: [PATCH] Improve file magic handling

Before this change, the magic string was read by reading the number
of bytes we're expecting to read based on our magic string. If ours
happens to be a prefix of the real one (e.g. ours is
'odoc-3.1.0' and the real one is 'odoc-3.1.0-2-g12345789') then
the check was succeeding and then the subsequent unmarshalling was
failing.

This fixes that by writing the length too, so we always unmarshal
the right length.
---
 src/odoc/odoc_file.ml | 51 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 42 insertions(+), 9 deletions(-)

diff --git a/src/odoc/odoc_file.ml b/src/odoc/odoc_file.ml
index 67f9b07098..cc96b68ef2 100644
--- a/src/odoc/odoc_file.ml
+++ b/src/odoc/odoc_file.ml
@@ -29,13 +29,24 @@ type content =
 type t = { content : content; warnings : Odoc_model.Error.t list }
 
 (** Written at the top of the files. Checked when loading. *)
-let magic = "odoc-%%VERSION%%"
+let magic = "ODOC"
+
+let magic_version = "%%VERSION%%"
 
 (** Exceptions while saving are allowed to leak. *)
 let save_ file f =
+  let len = String.length magic_version in
+  (* Sanity check, see similar check in load_ *)
+  if len > 255 then
+    failwith
+      (Printf.sprintf
+         "Magic version string %S is too long, must be <= 255 characters" magic);
+
   Fs.Directory.mkdir_p (Fs.File.dirname file);
   Io_utils.with_open_out_bin (Fs.File.to_string file) (fun oc ->
       output_string oc magic;
+      output_binary_int oc len;
+      output_string oc magic_version;
       f oc)
 
 let save_unit file (root : Root.t) (t : t) =
@@ -78,19 +89,41 @@ let save_unit file ~warnings m =
 
 let load_ file f =
   let file = Fs.File.to_string file in
-  (if Sys.file_exists file then Ok file
-   else Error (`Msg (Printf.sprintf "File does not exist")))
-  >>= fun file ->
-  Io_utils.with_open_in_bin file @@ fun ic ->
-  try
+
+  let check_exists () =
+    if Sys.file_exists file then Ok ()
+    else Error (`Msg (Printf.sprintf "File %s does not exist" file))
+  in
+
+  let check_magic ic =
     let actual_magic = really_input_string ic (String.length magic) in
-    if actual_magic = magic then f ic
+    if actual_magic = magic then Ok ()
+    else
+      Error
+        (`Msg
+           (Printf.sprintf "%s has invalid magic %S, expected %S\n%!" file
+              actual_magic magic))
+  in
+  let version_length ic () =
+    let len = input_binary_int ic in
+    if len > 0 && len <= 255 then Ok len
+    else Error (`Msg (Printf.sprintf "%s has invalid version length" file))
+  in
+  let check_version ic len =
+    let actual_magic = really_input_string ic len in
+    if actual_magic = magic_version then Ok ()
     else
       let msg =
-        Printf.sprintf "%s: invalid magic number %S, expected %S\n%!" file
-          actual_magic magic
+        Printf.sprintf "%s has invalid version %S, expected %S\n%!" file
+          actual_magic magic_version
       in
       Error (`Msg msg)
+  in
+
+  check_exists () >>= fun () ->
+  Io_utils.with_open_in_bin file @@ fun ic ->
+  try
+    check_magic ic >>= version_length ic >>= check_version ic >>= fun () -> f ic
   with exn ->
     let msg =
       Printf.sprintf "Error while unmarshalling %S: %s\n%!" file