Fix the watch mode

CHANGES.md

@@ -1,3 +1,8 @@
+unreleased
+----------
+
+- Fix the watch mode (#1837, fix #1836, @diml)
+
 1.7.0 (12/02/2019)
 ------------------
 

src/build_system.ml

@@ -469,6 +469,7 @@ let get_build_system () =
   match !t with
   | Some t -> t
   | None -> Exn.code_error "build system not yet initialized" []
+let reset () = t := None
 let t = get_build_system
 
 let string_of_paths set =

src/build_system.mli

@@ -18,6 +18,8 @@ val init
   -> hook:(hook -> unit)
   -> unit
 
+val reset : unit -> unit
+
 type extra_sub_directories_to_keep =
   | All
   | These of String.Set.t

src/main.ml

@@ -96,6 +96,7 @@ let init_build_system ?only_packages ?external_lib_deps_mode w =
     | Rule_started   -> incr rule_total
     | Rule_completed -> incr rule_done
   in
+  Build_system.reset ();
   Build_system.init ~contexts:w.contexts ~file_tree:w.conf.file_tree ~hook;
   Scheduler.set_status_line_generator gen_status_line;
   Gen_rules.gen w.conf

src/memo/memo.ml

@@ -166,6 +166,16 @@ module Cached_value = struct
         t.calculated_at <- Run.current ();
         Some t.data
     end
+
+  (* We don't use this version of [get] yet as all the dependencies
+     are not yet properly specified. *)
+  let _ = get
+
+  let get t =
+    if Run.is_current t.calculated_at then
+      Fiber.return (Some t.data)
+    else
+      Fiber.return None
 end
 
 let ser_input (type a) (node : (a, _) Dep_node.t) =