wip: set of subscribers

src/subscriber/trace_subscriber.ml

@@ -1,4 +1,5 @@
 open Trace_core
+module A = Trace_core.Internal_.Atomic_
 module Callbacks = Callbacks
 module Subscriber = Subscriber
 include Types
@@ -183,3 +184,40 @@ let collector (Sub { st; callbacks = (module CB) } : Subscriber.t) : collector =
       CB.on_init st ~time_ns
   end in
   (module M)
+
+module Handle = struct
+  type t = int
+
+  let compare : t -> t -> int = compare
+end
+
+(** how to allocate new handles *)
+let new_handle_ =
+  let a = A.make 0 in
+  fun () -> A.fetch_and_add a 1
+
+module Handle_map = Map.Make (Handle)
+
+(** Set of subscribers *)
+let top_subscribers : t Handle_map.t option A.t = A.make None
+
+let register_subscriber (s : t) : Handle.t =
+  let h = new_handle_ () in
+  let rec loop () =
+    let old = A.get top_subscribers in
+    let is_new, new_map =
+      match old with
+      | None -> true, Handle_map.singleton h s
+      | Some m -> false, Handle_map.add h s m
+    in
+    if A.compare_and_set top_subscribers old (Some new_map) then
+      is_new, new_map
+    else
+      loop ()
+  in
+
+  let is_new, map = loop () in
+
+  h
+
+let unregister_subscriber (h : Handle.t) : unit = ()

src/subscriber/trace_subscriber.mli

@@ -24,6 +24,28 @@ val collector : t -> Trace_core.collector
 
   It uses [mtime] (if available) to obtain timestamps. *)
 
+(** {2 Global set of subscribers}
+
+    There is a global set of subscribers (it is not mandatory to use it).
+    When the first subscriber is added to it, a {!Trace_core.collector}
+    is registered (see {!Trace_core.setup_collector}).
+
+    When the last subscriber is unregistered, the collector is shut down. *)
+
+module Handle : sig
+  type t = private int
+  (** A unique handle for a subscriber *)
+end
+
+val register_subscriber : t -> Handle.t
+(** Register a subscriber. This calls {!Trace_core.setup_collector} if
+  no other subscriber is registered.
+  @raise Invalid_argument if {!Trace_core.setup_collector} fails. *)
+
+val unregister_subscriber : Handle.t -> unit
+(** Remove a subscriber using its handle. If no subscriber remains,
+  {!Trace_core.shutdown} is called to remove the collector. *)
+
 (**/**)
 
 module Private_ : sig