Currently NixOS creates the swapfile (with the specified size) only if
it doesn't already exist. Changing the swapfile size afterwards will not
have any effect.
This commit changes that so the swapfile will be recreated whenever
swapDevices.*.size is changed (or more precisely, whenever the actual
file size differs from the configured one), allowing both growing and
shrinking the swapfile.
The service unit has "restartIfChanged = false", so we don't have to
worry about the swapfile being in use at the time this code is run (you
have to reboot for swapfile changes).
fallocate doesn't shrink files, use truncate for that. truncate can also
be used to grow files, but it creates "holes" in the file which doesn't
work with swapfiles.
Sadly, we can't instruct systemd to properly restart device-name.swap when this service restarts (or I haven't found the way to do so). As of now blindly restarting it would only get you a bunch of errors about device already used -- let's avoid it.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
