fix: logger app blocks until ready

This change makes the Logger application startup process _block_ until
the logger is ready to take in requests. This means applications relying
on the logger will take a tiny bit longer to boot, but we can guarantee
that the log requests will not be dropped because of process spawn
order.

Close #82

riot/lib/logger/logger.ml

@@ -3,6 +3,7 @@ module Log = Riot_runtime.Log
 open Global
 
 type opts = { print_source : bool; print_time : bool; color_output : bool }
+type config = { opts : opts; started_by : Riot_runtime.Core.Pid.t }
 
 type ('a, 'b) logger_format =
   (('a, Format.formatter, unit, 'b) format4 -> 'a) -> 'b

riot/lib/logger_app.ml

@@ -5,6 +5,8 @@ open Logger.Make (struct
   let namespace = [ "riot"; "logger" ]
 end)
 
+type Message.t += Logger_ready
+
 module Formatter = struct
   type Message.t += Log of log
 
@@ -47,7 +49,8 @@ module Formatter = struct
     let pid =
       spawn_link (fun () ->
           Process.flag (Priority High);
-          formatter_loop config)
+          send config.started_by Logger_ready;
+          formatter_loop config.opts)
     in
     set_on_log (fun log -> send pid (Log log));
     Ok pid
@@ -59,5 +62,12 @@ let default_opts =
   { print_time = true; print_source = false; color_output = true }
 
 let start () =
-  let child_specs = [ Formatter.child_spec default_opts ] in
-  Supervisor.start_link ~child_specs ()
+  let this = self () in
+  let config = { opts = default_opts; started_by = this } in
+  let child_specs = [ Formatter.child_spec config ] in
+  let result = Supervisor.start_link ~child_specs () in
+  let `ready =
+    let selector msg = if msg = Logger_ready then `select `ready else `skip in
+    receive ~selector ()
+  in
+  result