refactor: avoid some deadlocks in trace-tef

src/tef/b_queue.ml

@@ -38,12 +38,26 @@ let push (self : _ t) x : unit =
 
 let rec pop_all (self : 'a t) : 'a list =
   match Mpsc_bag.pop_all self.q with
-  | l -> List.rev l
-  | exception Mpsc_bag.Empty ->
+  | Some l -> l
+  | None ->
     if self.closed then raise Closed;
     Mutex.lock self.mutex;
     Atomic.set self.consumer_waiting true;
-    Condition.wait self.cond self.mutex;
-    Atomic.set self.consumer_waiting false;
-    Mutex.unlock self.mutex;
-    pop_all self
+    (* check again, a producer might have pushed an element since we
+       last checked. However if we still find
+       nothing, because this comes after [consumer_waiting:=true],
+       any producer arriving after that will know to wake us up. *)
+    (match Mpsc_bag.pop_all self.q with
+    | Some l ->
+      Atomic.set self.consumer_waiting false;
+      Mutex.unlock self.mutex;
+      l
+    | None ->
+      if self.closed then (
+        Mutex.unlock self.mutex;
+        raise Closed
+      );
+      Condition.wait self.cond self.mutex;
+      Atomic.set self.consumer_waiting false;
+      Mutex.unlock self.mutex;
+      pop_all self)

src/tef/mpsc_bag.ml

@@ -24,9 +24,7 @@ let rec add backoff t x =
 
 let[@inline] add t x = add Backoff.default t x
 
-exception Empty
-
-let[@inline] pop_all t : _ list =
+let[@inline] pop_all t : _ list option =
   match Atomic.exchange t.bag [] with
-  | [] -> raise_notrace Empty
-  | l -> l
+  | [] -> None
+  | l -> Some (List.rev l)

src/tef/mpsc_bag.mli

@@ -7,8 +7,5 @@ val create : unit -> 'a t
 val add : 'a t -> 'a -> unit
 (** [add q x] adds [x] in the bag. *)
 
-exception Empty
-
-val pop_all : 'a t -> 'a list
-(** Return all current items in an unspecified order.
-    @raise Empty if empty *)
+val pop_all : 'a t -> 'a list option
+(** Return all current items in the insertion order. *)