Enabling tracing on a phase to debug?

[Maxim Cournoyer]

Hello Guix,

I was toying with the idea to programatically enable tracing the
execution of a single build phase to aid with debugging, and came up
with this with the help of robin on #guix (thanks!)

--8<---------------cut here---------------start------------->8---
@@ -4349,7 +4349,19 @@ (define-public vala
              (substitute* "valadoc/tests/libvaladoc\
 /tests-extra-environment.sh"
                (("export PKG_CONFIG_PATH=" m)
-                (string-append m "$PKG_CONFIG_PATH:"))))))))
+                (string-append m "$PKG_CONFIG_PATH:")))))
+
+         (replace 'glib-or-gtk-wrap
+           (lambda args
+             (use-modules (system vm trace)
+                          (system vm vm))
+             (call-with-vm
+              (lambda ()
+                (set-vm-engine! 'debug)
+                (call-with-trace
+                  (lambda _
+                    (apply (assoc-ref %standard-phases 'glib-or-gtk-wrap)
+                           args))))))))))
     (native-inputs
      `(("bison" ,bison)
        ("dbus" ,dbus)                   ; for dbus tests
--8<---------------cut here---------------end--------------->8---

Unfortunately it doesn't cause any trace to be output.  Is there
something missing to the above incantation, perhaps?

Thanks,

Maxim