[Pkg-xfce-devel] Bug#829557: Bug#829557: Bug#829557: firefox: error box at start-up / D-BUS related issue

Simon McVittie smcv at debian.org
Mon Jul 11 09:38:11 UTC 2016

On Mon, 11 Jul 2016 at 11:07:07 +0200, Yves-Alexis Perez wrote:
> > Misc error when trying to watch fd 4: Invalid argument
> > Unable to add reload watch to main loopthis watch should have been
> > invalidatedMisc error when trying to watch fd 5: Invalid argument
> > Unable to add reload watch to main loop: (null)
> The message above seems to come from socket_set_epoll_add (https://dbus.freede
> sktop.org/doc/api/html/dbus-socket-set-epoll_8c_source.html#l00137) where
> epoll_ctl sets errno to EINVAL which means self->epfd is not an epoll file
> descriptor.

Something like that, yes. I'm really confused by this: it looks like it
might be a dbus-daemon bug, but so far I can't see why it would happen,
or how upgrading lightdm could trigger it.

The sequence of events here is:

* We create the epoll fd in _dbus_socket_set_epoll_new(). To have got
  a non-NULL result, we must have epfd != -1; and if we'd got NULL,
  we'd have failed already (_dbus_socket_set_new, _dbus_loop_new,
  bus_context_new and main all seem to catch errors correctly).

* dbus-daemon (bus/dir-watch-inotify.c) creates an inotify fd (fd 4)
  but then fails to add it to the main loop: _dbus_loop_add_watch()
  results in socket_set_epoll_add() which fails with EINVAL. Watching
  directories is considered non-critical so this is ignored.

* dbus-daemon (bus/main.c) creates what should probably have been a
  pipe-to-self to catch SIGHUP and SIGTERM from its own signal handler,
  but for historical reasons it's a socketpair() instead. The end we
  use as the read end is fd 5. socket_set_epoll_add() fails again.
  Catching our own SIGHUP and SIGTERM is considered to be critical
  (and really shouldn't fail!) so we exit.

I wonder whether there are other reasons why epoll_ctl can report EINVAL?

I also wonder whether the new lightdm is starting dbus-launch with a
different value for some arbitrary kernel limit, or whether your previous
session leaked some fds resulting in dbus-launch coming up with 90% of
an arbitrary limit already in use, or something like that?

The error reporting is definitely also bad here - unfortunately we're
constrained by 2005-era code that is partly designed for "autolaunching"
(spawning a dbus-daemon on-demand when you run a single GNOME/KDE app
in a twm session without proper distro D-Bus integration), which is why
stderr gets silenced. The autolaunch code path and the "proper distro
integration" code path both use dbus-launch, unless you're living in
the dbus-user-session future.


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