test: add test to check Span.dummy never gets modified

tests/core/dune

@@ -1,4 +1,4 @@
 (tests
- (names test_trace_context t_size t_histogram)
+ (names test_trace_context t_size t_histogram test_span_dummy)
  (package opentelemetry)
  (libraries pbrt opentelemetry opentelemetry-client))

tests/core/test_span_dummy.expected

@@ -0,0 +1,27 @@
+ok: add_attrs
+ok: add_attrs'
+ok: add_event
+ok: add_event'
+ok: add_links
+ok: add_links'
+ok: set_status
+ok: set_kind
+ok: record_exception
+span dummy at the end: { trace_id = <bytes len=16>;
+                         span_id = <bytes len=8>;
+                         trace_state = "" (* absent *);
+                         parent_span_id = <bytes len=0> (* absent *);
+                         flags = 0 (* absent *);
+                         name = "" (* absent *);
+                         kind = Span_kind_unspecified (* absent *);
+                         start_time_unix_nano = 0 (* absent *);
+                         end_time_unix_nano = 0 (* absent *);
+                         attributes = [];
+                         dropped_attributes_count = 0 (* absent *);
+                         events = [];
+                         dropped_events_count = 0 (* absent *);
+                         links = [];
+                         dropped_links_count = 0 (* absent *);
+                         status = None;
+                       }
+all ok

tests/core/test_span_dummy.ml

@@ -0,0 +1,47 @@
+open Opentelemetry
+
+(** Check that Span.dummy is never modified by mutation functions *)
+
+let check_pristine () =
+  let d = Span.dummy in
+  assert (Span.attrs d = []);
+  assert (Span.events d = []);
+  assert (Span.links d = []);
+  assert (Span.status d = None);
+  assert (Span.kind d = None);
+  assert (not (Span.is_not_dummy d))
+
+let check name f =
+  f ();
+  check_pristine ();
+  Printf.printf "ok: %s\n" name
+
+let trace_id = Trace_id.create ()
+
+let span_id = Span_id.create ()
+
+let () =
+  check_pristine ();
+  check "add_attrs" (fun () -> Span.add_attrs Span.dummy [ "k", `String "v" ]);
+  check "add_attrs'" (fun () ->
+      Span.add_attrs' Span.dummy (fun () -> [ "k", `Int 42 ]));
+  check "add_event" (fun () -> Span.add_event Span.dummy (Event.make "ev"));
+  check "add_event'" (fun () ->
+      Span.add_event' Span.dummy (fun () -> Event.make "ev"));
+  check "add_links" (fun () ->
+      Span.add_links Span.dummy [ Span_link.make ~trace_id ~span_id () ]);
+  check "add_links'" (fun () ->
+      Span.add_links' Span.dummy (fun () ->
+          [ Span_link.make ~trace_id ~span_id () ]));
+  check "set_status" (fun () ->
+      Span.set_status Span.dummy
+        (Span_status.make ~message:"err" ~code:Span_status.Status_code_error));
+  check "set_kind" (fun () -> Span.set_kind Span.dummy Span_kind_server);
+  check "record_exception" (fun () ->
+      try raise Exit
+      with exn ->
+        let bt = Printexc.get_raw_backtrace () in
+        Span.record_exception Span.dummy exn bt);
+  Format.printf "span dummy at the end: %a@." Opentelemetry_proto.Trace.pp_span
+    Span.dummy;
+  print_endline "all ok"