Mutex the batches

src/client-cohttp-eio/opentelemetry_client_cohttp_eio.ml

@@ -179,9 +179,7 @@ end
 
 (** Batch of resources to be pushed later.
 
-    This type is thread-safe
-
-    TODO: This is NOT safe accross domains. Need to wrap in mutex or re-architect. *)
+    This type is safe accross threads and domains. *)
 module Batch : sig
   type 'a t
 
@@ -197,6 +195,24 @@ module Batch : sig
   val make : ?batch:int -> ?timeout:Mtime.span -> unit -> 'a t
   (** Create a new batch *)
 end = struct
+
+  module Q : sig
+    type _ t
+    val make : ?batch:int -> ?timeout:Mtime.span -> unit -> _ t
+    val timeout : _ t -> Mtime.span option
+    val size : _ t -> int
+    val start : _ t -> Mtime.t
+    val is_full : _ t -> bool
+
+    (* Take all items queued for the batch, emptying the queue in the process *)
+    val drain_q : 'a t -> 'a list
+
+    val push : 'a t -> 'a -> bool
+  end = struct
+    let mutex = Eio.Mutex.create ()
+    let w_mutex f = Eio.Mutex.use_rw ~protect:true mutex f
+    let r_mutex f = Eio.Mutex.use_ro mutex f
+
     type 'a t = {
       mutable size: int;
       mutable q: 'a list;
@@ -218,47 +234,65 @@ end = struct
         high_watermark;
       }
 
+    let drain_q t = w_mutex (fun () ->
+        let l = t.q in
+        t.q <- [];
+        t.size <- 0;
+        l)
+
+    (* Immutable fields that don't need a mutex for safe access *)
+    let size t = t.size
+
+    let is_full_ t : bool =
+        match t.batch with
+        | None -> t.size > 0
+        | Some b -> t.size >= b
+
+    let is_full t : bool = r_mutex (fun () -> is_full_ t)
+
+    let push (t : _ t) x : bool = w_mutex (fun () ->
+      if t.size >= t.high_watermark then (
+        (* drop this to prevent queue from growing too fast *)
+        Atomic.incr n_dropped;
+        true
+      ) else (
+        if t.size = 0 && Option.is_some (t.timeout) then
+          (* current batch starts now *)
+          t.start <- Mtime_clock.now ();
+
+        (* add to queue *)
+        t.size <- 1 + t.size;
+        t.q <- x :: t.q;
+        let ready = is_full_ t in
+        ready
+      ))
+
+    (* Access to mutable fields, requiring a mutex for safe access *)
+    let timeout t = r_mutex (fun () -> t.timeout)
+    let start t = r_mutex (fun () -> t.start)
+  end
+
+  type 'a t = 'a Q.t
+
+  let make = Q.make
+
   let timeout_expired_ ~now self : bool =
-    match self.timeout with
+    match Q.timeout self with
     | Some t ->
-      let elapsed = Mtime.span now self.start in
+      let elapsed = Mtime.span now (Q.start self) in
       Mtime.Span.compare elapsed t >= 0
     | None -> false
 
-  let is_full_ self : bool =
-    match self.batch with
-    | None -> self.size > 0
-    | Some b -> self.size >= b
-
   let pop_if_ready ?(force = false) ~now (self : _ t) : _ list option =
-    if self.size > 0 && (force || is_full_ self || timeout_expired_ ~now self)
+    if Q.size self > 0 && (force || Q.is_full self || timeout_expired_ ~now self)
     then (
-      let l = self.q in
-      self.q <- [];
-      self.size <- 0;
+      let l = Q.drain_q self in
       assert (l <> []);
       Some l
     ) else
       None
 
-  let push (self : _ t) x : bool =
-    if self.size >= self.high_watermark then (
-      (* drop this to prevent queue from growing too fast *)
-      Atomic.incr n_dropped;
-      true
-    ) else (
-      if self.size = 0 && Option.is_some self.timeout then
-        (* current batch starts now *)
-        self.start <- Mtime_clock.now ();
-
-      (* add to queue *)
-      self.size <- 1 + self.size;
-      self.q <- x :: self.q;
-      let ready = is_full_ self in
-      ready
-    )
-
-  let push' self x = ignore (push self x : bool)
+  let push' self x = ignore (Q.push self x : bool)
 end
 
 (** An emitter. This is used by {!Backend} below to forward traces/metrics/…