redshift: Fix default value of $DISPLAY

[8573]

Before commit 54fa0cf, the redshift
service was run with the environment variable DISPLAY set to :0.

Commit 54fa0cf changed this to
instead use the value of the services.xserver.display configuration
option in the value of the DISPLAY variable. In so doing, no default
value was provided for the case where services.xserver.display is
null.

While the default value of services.xserver.display is 0, use of
which by the redshift module would result in DISPLAY again being
set to :0, services.xserver.display may also be null, to which
value it is set by, e.g., the lightdm module.

In the case that services.xserver.display is null, with the change
made in commit 54fa0cf, the DISPLAY
variable in the environment of the redshift service would be set to
: (a single colon), which, according to my personal experience,
would result in —

• the redshift service failing to start; and
• systemd repeatedly attempting to restart the redshift service,
  looping indefinitely, while the hapless redshift spews error
  messages into the journal.

It can be observed that the malformed value of DISPLAY is likely at
fault for this issue by executing the following commands in an
ordinary shell, with a suitable redshift executable, and the X11
display not already tinted:

• redshift -O 2500 — This command should reduce the color
  temperature of the display (making it more reddish).
• DISPLAY=':' redshift -O 6500 — This command should raise the
  color temperature back up, were it not for the DISPLAY
  environment variable being set to : for it, which should cause
  it to, instead, fail with several error messages.

This commit attempts to fix this issue by having the DISPLAY
environment variable for the redshift service default to its old
value of :0 in the case that services.xserver.display is null.

I have tested this solution on NixOS, albeit without the benefit of a
system with multiple displays.

[mention-bot]

@8573, thanks for your PR! By analyzing the annotation information on this pull request, we identified @anderspapitto, @nckx and @ocharles to be potential reviewers

[anderspapitto]

hmm. any idea why lightdm sets DISPLAY to null? there's also a number of other services that want to know what the display is - it would be good to have a fix that addresses all of them (e.g. by making lightdm not mess with DISPLAY)

[8573]

@anderspapitto I don't know, alas. DISPLAY itself seems fine though* —

$ echo $DISPLAY
:0

— it's only NixOS's configuration option services.xserver.display that's set to null.

(* Edit: In normal user environments, not in the environment of the Redshift service, that is.)

[8573]

Thanks for merging!