perf(trace-tef): improve behavior of collector under contention

writers still do one lock+unlock per event, but the collector can read
multiple events at once using Queue.transfer.

src/tef/b_queue.ml

@@ -51,3 +51,22 @@ let pop (self : 'a t) : 'a =
     )
   in
   loop ()
+
+let transfer (self : 'a t) q2 : unit =
+  Mutex.lock self.mutex;
+  while
+    if Queue.is_empty self.q then (
+      if self.closed then (
+        Mutex.unlock self.mutex;
+        raise Closed
+      );
+      Condition.wait self.cond self.mutex;
+      true
+    ) else (
+      Queue.transfer self.q q2;
+      Mutex.unlock self.mutex;
+      false
+    )
+  do
+    ()
+  done

src/tef/b_queue.mli

@@ -14,5 +14,10 @@ val pop : 'a t -> 'a
 (** [pop q] pops the next element in [q]. It might block until an element comes.
    @raise Closed if the queue was closed before a new element was available. *)
 
+val transfer : 'a t -> 'a Queue.t -> unit
+(** [transfer bq q2] transfers all items presently
+    in [bq] into [q2], and clears [bq].
+    It blocks if no element is in [bq]. *)
+
 val close : _ t -> unit
 (** Close the queue, meaning there won't be any more [push] allowed. *)

src/tef/trace_tef.ml

@@ -201,6 +201,7 @@ let bg_thread ~out (events : event B_queue.t) : unit =
   let writer = Writer.create ~out () in
   protect ~finally:(fun () -> Writer.close writer) @@ fun () ->
   let spans : span_info Span_tbl.t = Span_tbl.create 32 in
+  let local_q = Queue.create () in
 
   (* how to deal with an event *)
   let handle_ev (ev : event) : unit =
@@ -225,8 +226,15 @@ let bg_thread ~out (events : event B_queue.t) : unit =
 
   try
     while true do
-      let ev = B_queue.pop events in
+      (* work on local events, already on this thread *)
+      while not (Queue.is_empty local_q) do
+        let ev = Queue.pop local_q in
         handle_ev ev
+      done;
+
+      (* get all the events in the incoming blocking queue, in
+         one single critical section. *)
+      B_queue.transfer events local_q
     done
   with B_queue.Closed ->
     (* warn if app didn't close all spans *)