feat trace-tef: add a ticker thread to ensure we flush the file regularly

src/tef/trace_tef.ml

@@ -33,6 +33,7 @@ let protect ~finally f =
     Printexc.raise_with_backtrace exn bt
 
 type event =
+  | E_tick
   | E_message of {
       tid: int;
       msg: string;
@@ -104,6 +105,8 @@ module Writer = struct
     flush self.oc;
     if self.must_close then close_out self.oc
 
+  let[@inline] flush (self : t) : unit = flush self.oc
+
   let emit_sep_ (self : t) =
     if self.first then
       self.first <- false
@@ -206,6 +209,7 @@ let bg_thread ~out (events : event B_queue.t) : unit =
   (* how to deal with an event *)
   let handle_ev (ev : event) : unit =
     match ev with
+    | E_tick -> Writer.flush writer
     | E_message { tid; msg; time_us; data } ->
       Writer.emit_instant_event ~tid ~name:msg ~ts:time_us ~args:data writer
     | E_define_span { tid; name; id; time_us; data } ->
@@ -243,6 +247,14 @@ let bg_thread ~out (events : event B_queue.t) : unit =
         (Span_tbl.length spans);
     ()
 
+let tick_thread events : unit =
+  try
+    while true do
+      Thread.delay 0.5;
+      B_queue.push events E_tick
+    done
+  with B_queue.Closed -> ()
+
 type output =
   [ `Stdout
   | `Stderr
@@ -262,6 +274,9 @@ let collector ~out () : collector =
     (* writer thread. It receives events and writes them to [oc]. *)
     let t_write : Thread.t = Thread.create (fun () -> bg_thread ~out events) ()
 
+    (* ticker thread, regularly sends a message to the writer thread *)
+    let t_tick : Thread.t = Thread.create (fun () -> tick_thread events) ()
+
     let shutdown () =
       if A.exchange active false then (
         B_queue.close events;