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The main functionality
======================
`pristine-etc-keeper` is a program that hooks into `etckeeper` to
maintain a git branch that is a "pristine" version of `/etc`--just the
raw files provided by installed packages.
It assumes that the real directory being tracked by etckeeper is
`/etc`, that the VCS using used is `git`, and that the system package
manager is `pacman`.
It works by asking `pacman` which packages are installed, and
extracting `/etc` files from the cached copies of the package
tarballs. A litteral pristine version of `/etc` lives at
`/var/lib/pristine-etc/etc`.
The spool (AKA, invoking pristine-etc-keeper)
=============================================
It would be really simple to just provide a program that you run when
you want to update the pristine-etc. Unfortunately,
pristine-etc-keeper is very slow, and this would make many tasks
painful. So we run it asyncronously.
But that opens a whole other slew of problems. What happens if we try
to run it again while it is already running? The second instance
should wait until the first instance is finished. But only one
instance should be queued at a time; if a 3rd instance tries to start,
it should just be discarded/merged with the one already waiting.
A simple thing to do would have been to write a daemon that is always
running, waiting to receive a signal that it should run. I don't like
that because I don't really want the process to be long lived.
So, I created a sort of spool. You can think of the spool as a queue
of jobs to run, and that when you run `pristine-etc-keeper.service`,
it runs repeatedly until the spool is empty. What makes this
different than the job queue that I just described it as is that it
drains the entire spool each run; if it has to run again, it's because
the spool filled back up while it was running.
Anyway, to put a job in the spool, you do 4 things:
1. `open(2)` the file `/var/lib/pristine-etc/lock`
1. `flock(2)` the lockfile file descriptor with an exclusive lock
2. write to /var/lib/pristine-etc/spool with your message and
`O_CREAT` | `O_APPEND`.
3. close the `flock`ed file descriptor
4. run `systemctl start pristin-etc-keeper.service` to start (if
nescessary) it draining the spool.
You may wish to run
`systemctl reset-failed pristin-etc-keeper.service` before trying to
start it.
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