Comment # 19 on bug 1181018 from Michal Suchanek

The reason for O_NONBLOCK is a regression since SLE11 causing the CD tray to
close without user action by random software (typically udev) trying to probe
the CD state.

In the case of CD the O_NONBLOCK means something like: give me a fd, and if
there is no disk or if it's not probed read will fail. Without O_NONBLOCK the
drive tries to close tray, spin up the disk, and detect what kind of disk is
inserted. The drive normally does the detection when the user closes the tray
anyway so if the disk was interted in advance O_NONBLOCK gives usable fd.