Check that rules do produce the expected targets

src/build_system.ml

@@ -238,6 +238,22 @@ let create_file_specs t targets rule ~allow_override =
 
 module Pre_rule = Build_interpret.Rule
 
+let refresh_targets_timestamps_after_rule_execution t targets =
+  let missing =
+    List.fold_left targets ~init:Pset.empty ~f:(fun acc fn ->
+      match Unix.lstat (Path.to_string fn) with
+      | exception _ -> Pset.add fn acc
+      | stat ->
+        let ts = stat.st_mtime in
+        Hashtbl.add t.timestamps ~key:fn ~data:ts;
+        acc)
+  in
+  if not (Pset.is_empty missing) then
+    die "@{<error>Error@}: Rule failed to generate the following targets:\n%s"
+      (Pset.elements missing
+       |> List.map ~f:(fun fn -> sprintf "- %s" (Path.to_string fn))
+       |> String.concat ~sep:"\n")
+
 let compile_rule t ~all_targets_by_dir ?(allow_override=false) pre_rule =
   let { Pre_rule. build; targets = target_specs } = pre_rule in
   let deps = Build_interpret.deps build ~all_targets_by_dir in
@@ -296,8 +312,9 @@ let compile_rule t ~all_targets_by_dir ?(allow_override=false) pre_rule =
           acc || prev_hash <> hash)
     in
     if rule_changed || min_timestamp t targets < max_timestamp t all_deps then begin
-      List.iter targets ~f:(Hashtbl.remove t.timestamps);
-      Action.exec action
+      List.iter targets ~f:Path.unlink_no_err;
+      Action.exec action >>| fun () ->
+      refresh_targets_timestamps_after_rule_execution t targets
     end else
       return ()
   ) in

src/path.ml

@@ -335,6 +335,7 @@ let readdir t = Sys.readdir (to_string t) |> Array.to_list
 let is_directory t = Sys.is_directory (to_string t)
 let rmdir t = Unix.rmdir (to_string t)
 let unlink t = Unix.unlink (to_string t)
+let unlink_no_err t = try Unix.unlink (to_string t) with _ -> ()
 
 let extend_basename t ~suffix = t ^ suffix
 

src/path.mli

@@ -71,3 +71,4 @@ val readdir : t -> string list
 val is_directory : t -> bool
 val rmdir : t -> unit
 val unlink : t -> unit
+val unlink_no_err : t -> unit