Add regression test emitting custom event from signal handler

testsuite/tests/lib-runtime-events/test_user_event_signal.ml

@@ -0,0 +1,31 @@
+(* TEST
+ include runtime_events;
+ include unix;
+*)
+
+(* Check that emitting a custom event from a signal handler works (see #12900).
+ *)
+
+let unit =
+  let encode _buf () =
+    print_endline "Start encoding event";
+    Unix.sleepf 1.5;
+    print_endline "Finished encoding event";
+    0
+  in
+  let decode _buf _len = () in
+  Runtime_events.Type.register ~encode ~decode
+
+type Runtime_events.User.tag += My_event
+let my_event = Runtime_events.User.register "event" My_event unit
+
+let handle_signal _ =
+  print_endline "Signal handler called; writing trace event...";
+  Runtime_events.User.write my_event ()
+
+let () =
+  Runtime_events.start ();
+  Sys.set_signal Sys.sigalrm (Signal_handle handle_signal);
+  ignore (Unix.alarm 1 : int);
+  Runtime_events.User.write my_event ();
+  print_endline "Done"

testsuite/tests/lib-runtime-events/test_user_event_signal.reference

@@ -0,0 +1,6 @@
+Start encoding event
+Signal handler called; writing trace event...
+Start encoding event
+Finished encoding event
+Finished encoding event
+Done