Editing Files in Saltstack

As much as I ought to put up tech blogs as I work through projects, I can't keep up. So I tend to tech blog only when I discover something I couldn't find on the Internet beforehand. Today's entry is kind of like that and kind of not.

I have been working on configuring a server using the Saltstack configuration management system. A big part of this involves setting configuration files. Often these edits are done using the salt.states.file module. There are many methods provided. Which should you use?

In order of preference, here are the strategies I use when configuring a package:

If the package has a conf.d folder into which you can drop a configuration snippet, use file.managed and do that.
If it makes sense to replace the entire configuration file, use file.managed and do that with a Jinja template. I used to frown upon this option because if upgrades to the package provided new configuration options, you would miss them in your template. But when you upgrade these configuration files, Debian/Ubuntu bugs you about the changes, so maybe this is not so bad.
If it does not make sense to replace the entire configuration file, but you can insert a block at the beginning or end of the file, use file.blockreplace to do that. This leaves the original file intact, so you can look at available configuration options later. You may need to use file.comment to get rid of some default values. In my experience file.blockreplace is fairly robust to changes (although it does weird things with whitespace).
If you really need to edit a configuration file in place, use file.line and the trick I document below.

I do not think there are any other good options. file.append is terrible because it is not robust -- if you change one byte of the stuff you appended, Salt will re-append the snippet entirely. Maybe there are other good options, but I have not found them.

Editing Lines in a File

For some (probably stupid) reason, I decided I needed to edit the /etc/exim4/update-exim4.conf.conf file line by line. My strategy was as follows:

Define the changes I wanted to make using a Python dictionary
Loop through that dictionary and use file.line with the replace option to change the options.

Here is the dictionary I used in my edit. The first element of each tuple is the configuration option, and the second is the value:

{% set configlines = [('dc_eximconfig_configtype', 'satellite',),
                      ('dc_other_hostnames', '' ),
                      ('dc_use_split_config', 'true'),
                      ('dc_readhost', grains['fqdn']),
                      ('dc_smarthost', pillar['exim4']['smarthost']),
                      ('dc_hide_mailname', 'true'),
                     ] %}

Now you can loop through each of these elements and make a change. Note that each edit needs its own Salt ID, which is where conf[0] comes in:

{% for conf in configlines %}
/etc/exim4/update-conf-{{ conf[0] }}:
  file.line:
    - name: /etc/exim4/update-exim4.conf.conf
    - match: {{ conf[0] }}=
    - content: {{ conf[0] }}='{{ conf[1] }}'
    - mode: replace
    - backup: False
    - require:
      - pkg: exim4
{% endfor %}

The match directive looks for the line to edit. The content one sets the option. The replace actually does the change. Note that if the line is commented out then this will leave the line commented.

It is very important to note that you should only change ONE line at a time using file.line. Trying to change multiple lines will work, but is very fragile in the same way file.append is. In that situation you should use file.blockreplace instead.

Editing Files in Saltstack

Editing Lines in a File

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