longer compares the current configuration to the previous
configuration, but instead compares the current Upstart state to the
intended state. Thus, if the switch script is interrupted, running
nixos-rebuild again will resume starting/stopping Upstart jobs where
the previous run left off.
We determine if an Upstart job has changed by having the pre-start
script of each Upstart job put a symlink to its .conf file in
/var/run/upstart-jobs. So if this symlink differs from the target
of /etc/init/<job>.conf, then the job has changed. This also
prevents multiple restarts of dependent jobs. E.g., if job B has
"start on started A" and "stop on stopping A", then restarting A
will cause B to be restarted, so B shouldn't B restarted a second
time.
We only start jobs that are not running if 1) they're tasks that
have been previously run (like mountall); or 2) they're jobs that
have a "start on" condition. This seems a reasonable heuristic.
svn path=/nixos/trunk/; revision=33222
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