Switch to using nonblocking fds with lwt

Summary:
This is the result of a week long investigation into Flow
occasionally hanging. Here's what you should know:

* A read/write of a blocking fd will block until the fd is readable/writable
* A read/write of a nonblocking fd will return -1 immediately if the fd is not readable/writable
* Lwt super extra hates running blocking system calls. It will create system threads and run the blocking system call on them
* There's a bug in Lwt in scheduling the system threads: ocsigen/lwt#569

From our point of view, there's very little difference in using blocking
or non-blocking fds. We had just been using blocking fds since I
couldn't really tell if one was better than the other,

However, using non-blocking keeps us from needing system threads and
works around the bug, so let's use them!

Reviewed By: samwgoldman

Differential Revision: D7464034

fbshipit-source-id: e0ba602381a8bef7dd374ee1cd5fb0fdef9ad7d9

hack/dfind/dfindLibLwt.ml

@@ -17,8 +17,8 @@ struct
   let return = Lwt.return
   let (>>=) = Lwt.(>>=)
 
-  let descr_of_in_channel ic = Lwt_unix.of_unix_file_descr (Daemon.descr_of_in_channel ic)
-  let descr_of_out_channel oc = Lwt_unix.of_unix_file_descr (Daemon.descr_of_out_channel oc)
+  let descr_of_in_channel ic = Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true (Daemon.descr_of_in_channel ic)
+  let descr_of_out_channel oc = Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true (Daemon.descr_of_out_channel oc)
 
   let to_fd_with_preamble ?timeout ?flags fd v =
     if timeout <> None

hack/procs/workerControllerLwt.ml

@@ -40,8 +40,8 @@ let call w (type a) (type b) (f : a -> b) (x : a) : b Lwt.t =
 
   let infd = Daemon.descr_of_in_channel inc in
   let outfd = Daemon.descr_of_out_channel outc in
-  let infd_lwt = Lwt_unix.of_unix_file_descr ~blocking:true infd in
-  let outfd_lwt = Lwt_unix.of_unix_file_descr ~blocking:true outfd in
+  let infd_lwt = Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true infd in
+  let outfd_lwt = Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true outfd in
 
   let request = wrap_request w f x in
 

src/monitor/flowServerMonitor.ml

@@ -126,7 +126,7 @@ let internal_start ~is_daemon ?waiting_fd monitor_options =
     let handle_waiting_start_command = match waiting_fd with
     | None -> Lwt.return_unit
     | Some fd ->
-      let fd = Lwt_unix.of_unix_file_descr ~blocking:true fd in
+      let fd = Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true fd in
       handle_waiting_start_command fd
     in
 
@@ -139,11 +139,13 @@ let internal_start ~is_daemon ?waiting_fd monitor_options =
     (* We can start up the socket acceptor even before the server starts *)
     Lwt.async (fun () ->
       SocketAcceptor.run
-        (Lwt_unix.of_unix_file_descr ~blocking:true monitor_socket_fd)
+        (Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true monitor_socket_fd)
         monitor_options.FlowServerMonitorOptions.autostop
     );
     Lwt.async (fun () ->
-      SocketAcceptor.run_legacy (Lwt_unix.of_unix_file_descr ~blocking:true legacy_socket_fd)
+      SocketAcceptor.run_legacy (
+        Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true legacy_socket_fd
+      )
     );
 
     (* Wait forever! Mwhahahahahaha *)

src/monitor/flowServerMonitorServer.ml

@@ -165,11 +165,11 @@ end = struct
     let in_fd =
       ic
       |> Daemon.descr_of_in_channel
-      |> Lwt_unix.of_unix_file_descr ~blocking:true in
+      |> Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true in
     let out_fd =
       oc
       |> Daemon.descr_of_out_channel
-      |> Lwt_unix.of_unix_file_descr ~blocking:true in
+      |> Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true in
 
     let close_if_open fd =
       try Lwt_unix.close fd

src/monitor/logger/flowServerMonitorLogger.ml

@@ -75,7 +75,7 @@ let init_logger log_fd =
 
   let template = "$(date).$(milliseconds) [$(level)] $(message)" in
 
-  let log_fd = Option.map log_fd ~f:Lwt_unix.of_unix_file_descr in
+  let log_fd = Option.map log_fd ~f:(Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true) in
 
   let fds = Lwt_unix.stderr :: (Option.value_map log_fd ~default:[] ~f:(fun fd -> [fd])) in
   Lwt.async (fun () -> WriteLoop.run fds);

src/monitor/monitorRPC.ml

@@ -31,7 +31,8 @@ let with_outfd ~on_disabled ~f = with_channel snd ~on_disabled ~f
 
 (* The main server process will initialize this with the channels to the monitor process *)
 let init ~channels:(ic,oc) =
-  let infd = Lwt_unix.of_unix_file_descr (Daemon.descr_of_in_channel ic) in
+  let infd =
+    Lwt_unix.of_unix_file_descr ~blocking:false ~set_flags:true (Daemon.descr_of_in_channel ic) in
   let outfd = Daemon.descr_of_out_channel oc in
   state := Initialized {infd; outfd}