TechnicalTrivia

V0.1, 2020-01-21

This code depends on udev, the Linux subsystem for managing device events. 
(https://wiki.debian.org/udev and https://wiki.archlinux.org/index.php/Udev 
are useful references.)

When a USB CD drive is plugged in, the Raspbian kernel and its baseline 
udev rules identify it as a CD drive. Normally, it is enumerated in the 
device tree as /dev/sr0 but occasionally it shows up as /dev/sr1. By default, 
the device's permissions are brw-rw----. For convenience, a symbolic link 
to it, /dev/cdrom (and also /dev/cdrw if it's capable of writing), is created 
by udev. A variety of attributes and environment variables pertaining to the 
device are maintained in the udev database. The udevadm tool is convenient 
for exploring these attributes and variable as they change with device events.

Many online articles on writing udev rules show introductory examples using 
the actions "add" and "remove". These actions apply to the drive itself and 
not to the removable media. Only the "change" action is appropriate for 
detecting the insertion and ejection of a compact disc.

The ruleset written for this code takes advantage of two udev environment 
variables. 

When, specifically an audio disc is inserted, the udev environment variable 
ID_CDROM_MEDIA_TRACK_COUNT_AUDIO is created with a value equal to the number 
of tracks found on the disc. The ruleset triggers on the existence of this 
variable (its value is not used) to signal systemd to invoke the 
addaudiocd.sh script.

When the disc is ejected using the drive's manual eject button, the udev 
environment variable DISK_EJECT_REQUEST is created with a value equal to 1. 
The ruleset triggers on the existence of this variable and its value to 
signal systemd to invoke the remaudiocd.sh script.

The addaudiocd.sh script marks the CD drive as readable by all (may be 
redundant if the drive has already been used), gets the audio track count
from the udev database, and creates the track list in the moOde queue.

The remaudiocd.sh script clears the moOde queue and ejects the disc. Note
that using the drive's manual eject button triggers ejection anyway, but
it's included here so remaudiocd.sh can be used as a general utility script.
Think "eject CD" on a remote control or in the moOdeUI.