From a0b421dcdc5484788338ca457c5e28b5ce327128 Mon Sep 17 00:00:00 2001
From: Simon Cruanes <simon.cruanes.2007@m4x.org>
Date: Thu, 4 Dec 2025 14:33:39 -0500
Subject: [PATCH] feat lib/tracer: restore `with_` and `with_thunk_and_finally`
 !!

---
 src/lib/tracer.ml | 88 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)

diff --git a/src/lib/tracer.ml b/src/lib/tracer.ml
index 4b6009c3..4e507943 100644
--- a/src/lib/tracer.ml
+++ b/src/lib/tracer.ml
@@ -22,3 +22,91 @@ let dummy () : t = Emitter.dummy ()
 let dynamic_forward_to_main_exporter : t =
   Main_exporter.Util.dynamic_forward_to_main_exporter () ~get_emitter:(fun e ->
       e.emit_spans)
+
+let (add_event [@deprecated "use Span.add_event"]) = Span.add_event
+
+let (add_attrs [@deprecated "use Span.add_attrs"]) = Span.add_attrs
+
+let with_thunk_and_finally (self : t) ?(force_new_trace_id = false) ?trace_state
+    ?(attrs : (string * [< Value.t ]) list = []) ?kind ?trace_id ?parent ?links
+    name cb =
+  let parent =
+    match parent with
+    | Some _ -> parent
+    | None -> Ambient_span.get ()
+  in
+  let trace_id =
+    match trace_id, parent with
+    | _ when force_new_trace_id -> Trace_id.create ()
+    | Some trace_id, _ -> trace_id
+    | None, Some p -> Span.trace_id p
+    | None, None -> Trace_id.create ()
+  in
+  let start_time = Timestamp_ns.now_unix_ns () in
+  let span_id = Span_id.create () in
+
+  let parent_id = Option.map Span.id parent in
+
+  let span : Span.t =
+    Span.make ?trace_state ?kind ?parent:parent_id ~trace_id ~id:span_id ~attrs
+      ?links ~start_time ~end_time:start_time name
+  in
+  (* called once we're done, to emit a span *)
+  let finally res =
+    let end_time = Timestamp_ns.now_unix_ns () in
+    Proto.Trace.span_set_end_time_unix_nano span end_time;
+
+    (match Span.status span with
+    | Some _ -> ()
+    | None ->
+      (match res with
+      | Ok () ->
+        (* By default, all spans are Unset, which means a span completed without error.
+             The Ok status is reserved for when you need to explicitly mark a span as successful
+             rather than stick with the default of Unset (i.e., “without error”).
+
+              https://opentelemetry.io/docs/languages/go/instrumentation/#set-span-status *)
+        ()
+      | Error (e, bt) ->
+        Span.record_exception span e bt;
+        let status =
+          make_status ~code:Status_code_error ~message:(Printexc.to_string e) ()
+        in
+        Span.set_status span status));
+
+    Emitter.emit self [ span ]
+  in
+  let thunk () = Ambient_span.with_ambient span (fun () -> cb span) in
+  thunk, finally
+
+(** Sync span guard.
+
+    Notably, this includes {e implicit} scope-tracking: if called without a
+    [~scope] argument (or [~parent]/[~trace_id]), it will check in the
+    {!Ambient_context} for a surrounding environment, and use that as the scope.
+    Similarly, it uses {!Scope.with_ambient_scope} to {e set} a new scope in the
+    ambient context, so that any logically-nested calls to {!with_} will use
+    this span as their parent.
+
+    {b NOTE} be careful not to call this inside a Gc alarm, as it can cause
+    deadlocks.
+
+    @param force_new_trace_id
+      if true (default false), the span will not use a ambient scope, the
+      [~scope] argument, nor [~trace_id], but will instead always create fresh
+      identifiers for this span *)
+let with_ (self : t) ?force_new_trace_id ?trace_state ?attrs ?kind ?trace_id
+    ?parent ?links name (cb : Span.t -> 'a) : 'a =
+  let thunk, finally =
+    with_thunk_and_finally self ?force_new_trace_id ?trace_state ?attrs ?kind
+      ?trace_id ?parent ?links name cb
+  in
+
+  try
+    let rv = thunk () in
+    finally (Ok ());
+    rv
+  with e ->
+    let bt = Printexc.get_raw_backtrace () in
+    finally (Error (e, bt));
+    raise e