Properly handle empty tag list

[kempniu]

This PR attempts to fix a bug which makes it impossible to remove the last existing tag for a device.

Consider the power-switch tag. elogind ships with a file called 70-power-switch.rules which applies that tag to certain devices. Try removing it e.g. for the ACPI Power Button device by creating an appropriately numbered rule file (e.g. /etc/udev/rules.d/75-remove-power-switch.rules) with the following contents:

SUBSYSTEM=="input", KERNEL=="event*", ENV{ID_INPUT_KEY}=="1", TAG-="power-switch"

This will not remove the power-switch tag for the device:

$ udevadm test "/sys/$(udevadm info -q path /dev/input/event1)" 2>&1 | grep TAGS
TAGS=:power-switch

If you add any other tag to the tag list for the same device, e.g. by making the aforementioned rule file look like this:

SUBSYSTEM=="input", KERNEL=="event*", ENV{ID_INPUT_KEY}=="1", TAG-="power-switch"
SUBSYSTEM=="input", KERNEL=="event*", ENV{ID_INPUT_KEY}=="1", TAG+="foo"

the power-switch tag will be removed correctly:

$ udevadm test "/sys/$(udevadm info -q path /dev/input/event1)" 2>&1 | grep TAGS
TAGS=:foo:

The reason for this behavior is that udev_device_get_properties_list_entry() only updates the TAGS property if the tag list is non-empty.

I believe this is an upstream bug which was fixed in v220 as a side effect of commits 57fa1d094cd2c5ac68970526ad0a0754c548e75d and f4ac4d1a82e2c468761fffa333323841ad886221. Since eudev did not adopt these changes, the bug remained unfixed.

I was not sure how to sum up all of the above in the commit log message, so please let me know if it should be adjusted.

Hope this helps!

[kempniu]

(AFAICT, Travis GCC build failed due a network error during apt-get update, not a code issue.)

[blueness]

Thanks, this is a good catch. I'll test later and commit if okay.

[blueness]

pushed. thanks for the contribution.