Add support for hooks

README.org

@@ -1,5 +1,4 @@
 * JBUILDER - A composable build system for OCaml and Reason
-%%VERSION%%
 
 Jbuilder is a build system designed for OCaml/Reason projects only. It
 focuses on providing the user with a consistent experience and takes

bin/main.ml

@@ -647,6 +647,18 @@ let subst =
     ; `P {|If you use topkg to handle the releases of your project, then you
            should add this line to the $(b,build:) instructions in your opam file:|}
     ; `Pre {|  ["jbuilder" "subst" name] {pinned}|}
+    ; `S "HOOKS"
+    ; `I ("pre_subst",
+          {|If a a file $(b,pkg/pre_subst_hook.ml) exists, it is evaluated before
+            performing the expansion.|})
+    ; `I ("post_subst",
+          {|If a a file $(b,pkg/post_subst_hook.ml) exists, it is evaluated after
+            performing the expansion.|})
+    ; `P ({|A typical example of $(b,pre_subst) hook is a script that adds %%|} ^
+          {|VERSION%%
+            to the README, so that the version is present in releases and you
+            don't get the variable unexpanded on github.
+            Jbuilder itself does this.|})
     ; `Blocks help_secs
     ]
   in

pkg/pre_subst_hook.ml

@@ -0,0 +1,23 @@
+let split_at_first_empty_line s =
+  let len = String.length s in
+  let rec loop i =
+    if i + 1 >= len then
+      (s, "")
+    else if s.[i] = '\n' && s.[i + 1] = '\n' then
+      let i = i + 1 in
+      (String.sub s 0 i,
+       String.sub s i (len - i))
+    else
+      loop (i + 1)
+  in
+  loop 0
+
+let () =
+  let ic = open_in_bin "README.org" in
+  let len = in_channel_length ic in
+  let s = really_input_string ic len in
+  close_in ic;
+  let oc = open_out_bin "README.org" in
+  let before, after = split_at_first_empty_line s in
+  Printf.fprintf oc "%s%%%%%,VERSION%%%%\n%s" before after;
+  close_out oc

src/watermarks.ml

@@ -204,6 +204,19 @@ let get_name ~files ?name () =
       die "@{<error>Error@}: cannot determine name automatically.\n\
            You must pass a [--name] command line argument."
 
+let ocaml = lazy (
+  match Bin.which "ocaml" with
+  | None -> Utils.program_not_found "ocaml"
+  | Some p -> Path.to_string p
+)
+
+let run_hook ~files hook =
+  let fn = sprintf "pkg/%s_hook.ml" hook in
+  if List.mem fn ~set:files then
+    Future.run Strict (Lazy.force ocaml) [fn]
+  else
+    Future.return ()
+
 let subst_git ?name () =
   let rev = "HEAD" in
   let git =
@@ -217,6 +230,8 @@ let subst_git ?name () =
        (Future.run_capture Strict git ["rev-parse"; rev]))
     (Future.run_capture_lines Strict git ["ls-tree"; "-r"; "--name-only"; rev])
   >>= fun ((version, commit), files) ->
+  run_hook "pre_subst" ~files
+  >>= fun () ->
   let version = String.trim version in
   let commit  = String.trim commit  in
   let name = get_name ~files ?name () in
@@ -224,7 +239,7 @@ let subst_git ?name () =
   List.iter files ~f:(fun fn ->
     if is_a_source_file fn then
       subst_file fn ~map:watermarks);
-  Future.return ()
+  run_hook "post_subst" ~files
 
 let subst ?name () =
   if Sys.file_exists ".git" then