Managing apt_preferences with CFEngine

Linux Mint Debian EditionSince when I started using Linux Mint Debian Edition (LMDE), it has become increasingly important to have my apt_preferences set up correctly, so that I could pull packages from Debian testing (currently codenamed “jessie”) if I wanted, but use LMDE packages by default. It took me some time and research to find the right settings.

Once found the right settings, and discovered that I needed to “hardcode” the hostname of the LMDE mirror I was using, there came the problem to have the apt sources and apt preferences in sync. It seemed a nice task for CFEngine, and I started to work on managing my apt preferences. The first solution I had in mind didn’t work properly due to a bug in CFEngine that wasn’t yet fixed in version 3.4.4, the one I’m using. The solution below works. It’s a bit more cumbersome than the one that I had in mind, but it should be useful nonetheless 🙂 It’s an edit_line bundle:

# This bundle is used to compile an apt-preferences file (usually
# /etc/apt/preferences, or another file in /etc/apt/preferences.d.
# It receives in input the name of an array with the following
# structure:
#
# pin[id][Package] string => "package_identifier"
# pin[id][Pin]     string => "pin for this package"
# pin[id][Prio]    string => "priority for this package"
#
# We use preserve_block in insert_lines, so all the three items must
# be present (Package, Prio, and Pin). Besides, it is advisable that
# the whole file is managed from a blank state.

bundle edit_line apt_preferences(pins) {
  vars:
      "n" string => "$(const.n)" ;
      "ids" slist => getindices("$(pins)") ;

  insert_lines:
      "Package: $($(pins)[$(ids)][Package])$(n)Pin: $($(pins)[$(ids)][Pin])$(n)Pin-Priority: $($(pins)[$(ids)][Prio])$(n)",
	  insert_type => "preserve_block" ;

}

Put the bundle in one of your libraries, import it, and then you can use it like this:

bundle agent test {
  vars:
      "ids" slist => {"lmde","mint","jessie","wheezy"} ;

      "pin[lmde][Prio]"   int => "700" ;
      "pin[mint][Prio]"   int => "700" ;
      "pin[jessie][Prio]" int => "-1"  ;
      "pin[wheezy][Prio]" int => "1"   ;

      "pin[lmde][Pin]"   string => "origin debian.lth.se" ;
      "pin[mint][Pin]"   string => "origin packages.linuxmint.com" ;
      "pin[jessie][Pin]" string => "release n=jessie"  ;
      "pin[wheezy][Pin]" string => "release n=wheezy"   ;

      "pin[$(ids)][Package]" string => "*" ;

  files:
      "/tmp/prefs"
	  create        => "yes",
	  edit_defaults => empty,
	  perms         => owner("bronto"),
	  edit_line     => apt_preferences("test.pin") ;

}

The test bundle above will create a file like this:

Package: *
Pin: origin debian.lth.se
Pin-Priority: 700

Package: *
Pin: origin packages.linuxmint.com
Pin-Priority: 700

Package: *
Pin: release n=jessie
Pin-Priority: -1

Package: *
Pin: release n=wheezy
Pin-Priority: 1

Enjoy!

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