Lennart suggested to use a more uniform algorithm for
the picking of the hostname words that is not biased
for long words by just (predictably) randomly going over
the offsets until we land on a word boundary. This is a
very nice suggestion so this commit implements it with
a fallback to the "old" behavior if we do not find a
word boundary within a reasonable amount of attempts.
A small python script shows that 64 iterations plus
fallback is a good number:
```
$ python3 simulate-hostname-pick.py 64
hostname-wordlist/adverbs
words=261 p_accept=0.1119 avg_bytes/word=1/p=8.94
max_iterations=64, n_trials=1000000
fallback rate : 0.051000% (510/1_000_000)
mean seeks per word : 8.93
hostname-wordlist/adjectives
words=449 p_accept=0.1380 avg_bytes/word=1/p=7.24
max_iterations=64, n_trials=1000000
fallback rate : 0.007500% (75/1_000_000)
mean seeks per word : 7.25
hostname-wordlist/nouns
words=449 p_accept=0.1472 avg_bytes/word=1/p=6.79
max_iterations=64, n_trials=1000000
fallback rate : 0.002700% (27/1_000_000)
mean seeks per word : 6.79
```
Combined with the fallback to the previous method if
we can't find anything within the 64 attemps this seems
to be the best tradeoff and give us very good uniformity.
This commit adds support to /etc/hostname for substitution
of $ wordlists from {/etc,/run,/usr/lib}/systemd/hostname-wordlist.
The first $ will lookup hostname-wordlist/1, the next
hostname-wordlist/2 and so on.
With that we can do a petname [1] style hostname in systemd, e.g.
below a possible expansion for a hostname template:
$-$-$-???? -> wildly-happy-octopus-92a9
The substitution of words is stable (based on machine-id) but
not persisted, it is picked on every boot via a stable file
offset so the operation is cheap. But this means that if the
wordlist changes the hostname would change. The next commit
will add the pattern to the firstboot.hostname credential which
is persistet with the resolved names to avoid this issue.
This also includes a wordlist from the "petname" project
that can be optionally installed.
Thanks to Dustin Kirkland for this wonderful project.
[1] https://github.com/dustinkirkland/petname
Michael Vogt