fix(fsevents): better sync file handling

We move the sync file to _build/.sync. This is done to be able to setup
a private watch for it and ignore the _build directory. This is good for
fsevents as it allows us to drop watching the possibly massive build
directory.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

Author: rgrinberg

src/dune_file_watcher/dune_file_watcher.ml

@@ -25,7 +25,7 @@ type kind =
       { pid : Pid.t
       ; wait_for_watches_established : unit -> unit
       }
-  | Fsevents of Fsevents.t
+  | Fsevents of Fsevents.t Nonempty_list.t
   | Inotify of Inotify_lib.t
 
 type t =
@@ -171,8 +171,11 @@ let shutdown t =
   | Fsevents fsevents ->
     `Thunk
       (fun () ->
-        let runloop = Option.value_exn (Fsevents.runloop fsevents) in
-        Fsevents.stop fsevents;
+        let fsevents = Nonempty_list.to_list fsevents in
+        let runloop =
+          List.hd fsevents |> Fsevents.runloop |> Option.value_exn
+        in
+        List.iter fsevents ~f:Fsevents.stop;
         Fsevents.RunLoop.stop runloop)
 
 let buffer_capacity = 65536
@@ -218,7 +221,11 @@ module Buffer = struct
 end
 
 let special_file_for_inotify_sync =
-  let path = lazy (Path.Build.relative Path.Build.root "dune-inotify-sync") in
+  let path =
+    lazy
+      (let dir = Path.Build.relative Path.Build.root ".sync" in
+       Path.Build.relative dir "dune-inotify-sync")
+  in
   fun () -> Lazy.force path
 
 let special_file_for_inotify_sync_absolute =
@@ -299,10 +306,12 @@ let select_watcher_backend () =
     fswatch_backend ()
 
 let emit_sync () =
-  Io.write_file (Path.build (special_file_for_inotify_sync ())) "z"
+  let path = Path.build (special_file_for_inotify_sync ()) in
+  Io.write_file path "z"
 
 let prepare_sync () =
-  Path.mkdir_p (Path.parent_exn (Path.build (special_file_for_inotify_sync ())));
+  let dir = Path.parent_exn (Path.build (special_file_for_inotify_sync ())) in
+  Path.mkdir_p dir;
   emit_sync ()
 
 let spawn_external_watcher ~root ~backend =
@@ -425,16 +434,39 @@ let create_inotifylib ~scheduler =
 let create_fsevents ~(scheduler : Scheduler.t) =
   prepare_sync ();
   let ignored_files = Table.create (module String) 64 in
+  let latency = 0.2 in
+  let path event =
+    Fsevents.Event.path event |> Path.of_string
+    |> Path.Expert.try_localize_external
+  in
+  let fsevents_special =
+    Fsevents.create
+      ~paths:
+        [ special_file_for_inotify_sync ()
+          |> Path.Build.parent_exn |> Path.build |> Path.to_absolute_filename
+        ]
+      ~latency
+      ~f:(fun events ->
+        scheduler.thread_safe_send_emit_events_job (fun () ->
+            List.filter_map events ~f:(fun event ->
+                let action = Fsevents.Event.action event in
+                if is_special_file_for_inotify_sync (path event) then
+                  match action with
+                  | Unknown
+                  | Create
+                  | Modify ->
+                    Some Event.Sync
+                  | Remove -> None
+                else
+                  None)))
+  in
   let fsevents =
     let paths = [ Path.to_string Path.root ] in
-    Fsevents.create ~paths ~latency:0.2 ~f:(fun events ->
+    Fsevents.create ~paths ~latency ~f:(fun events ->
         scheduler.thread_safe_send_emit_events_job (fun () ->
             List.filter_map events ~f:(fun event ->
-                let path =
-                  Fsevents.Event.path event |> Path.of_string
-                  |> Path.Expert.try_localize_external
-                in
                 let action = Fsevents.Event.action event in
+                let path = path event in
                 if is_special_file_for_inotify_sync path then
                   match action with
                   | Unknown
@@ -463,19 +495,20 @@ let create_fsevents ~(scheduler : Scheduler.t) =
   scheduler.spawn_thread (fun () ->
       let runloop = Fsevents.RunLoop.in_current_thread () in
       Fsevents.start fsevents runloop;
+      Fsevents.start fsevents_special runloop;
       match Fsevents.RunLoop.run_current_thread runloop with
       | Ok () -> ()
       | Error exn ->
         Code_error.raise "fsevents callback raised" [ ("exn", Exn.to_dyn exn) ]);
   Fsevents.set_exclusion_paths fsevents
     ~paths:
-      ((* For now, we don't ignore the build directroy because we want to
-          receive events from the special event sync event *)
-       [ "_esy"; "_opam"; ".git"; ".hg" ]
-      |> List.rev_map ~f:(fun base ->
-             let path = Path.relative (Path.source Path.Source.root) base in
-             Path.to_absolute_filename path));
-  { kind = Fsevents fsevents; ignored_files }
+      (Path.(build Build.root)
+       :: ([ "_esy"; "_opam"; ".git"; ".hg" ]
+          |> List.rev_map ~f:(fun base ->
+                 let path = Path.relative (Path.source Path.Source.root) base in
+                 path))
+      |> List.rev_map ~f:Path.to_absolute_filename);
+  { kind = Fsevents [ fsevents; fsevents_special ]; ignored_files }
 
 let create_external ~root ~debounce_interval ~scheduler ~backend =
   match debounce_interval with