This guide explains how to use ucf's new --three-way-merge-command functionality in conjunction with Elektra in order to ultilize Elektra's powerful tools in order to allow automatic three-way merges of your package's configuration during upgrades in a way that is more reliable than a diff3 merge. This guide assumes that you are familiar with ucf already and are just trying to implement the --three-way-merge-command option using Elektra.

The New Option

The addition of the --three-way-merge-command option was a part of my Google Summer of Code Project. This option takes the form: –three-way-merge-command command <New file>=""> <Destination>

Where command is the command you would like to use for the merge. New File and Destination are the same as always.

elektra-merge

We added a new script to Elektra called elektra-merge for use with this new option in ucf. This script acts as a liaison between ucf and Elektra, allowing a regular ucf command to run a kdb merge even though ucf commands only pass New File and Destination whereas kdb merge requires ourpath, theirpath, basepath, and resultpath. Since ucf already performs a three-way merge, it keeps track of all the necessary files to do so, even though it only takes in New File and Destination.

In order to use elektra-merge, the current configuration file must be mounted to KDB to serve as ours in the merge. The script automatically mounts theirs, base, and result using the kdb remount command in order to use the same backend as ours (since all versions of the same file should use the same backend anyway) and this way users don't need to worry about specifying the backend for each version of the file. Then the script attempts a merge on the newly mounted KeySets. Once this is finished, either on success or not, the script finishes by unmouting all but our copy of the file to cleanup KDB. Then, if the merge was successful ucf will replace ours with the result providing the package with an automatically merged configuration which will also be updated in KDB itself.

Additionally, we added two other scripts, elektra-mount and elektra-umount which act as simple wrappers for kdb mount and kdb umount. They work identically but are more script friendly.

The Full Command

The full command to use elektra-merge to perform a three-way merge on a file managed by ucf is: ucf –three-way –threeway-merge-command elektra-merge <New file>=""> <Destination>

That's it! As described above, elektra-merge is smart enough to run the whole merge off of the information from that command and utilizes the new kdb remount command to do so.

How-To Integrate

Integrating elektra-merge into a package that already uses ucf is very easy! In postinst you should have a line similar to: ucf <New file>=""> <Destination>

or perhaps: ucf –three-way <New file>=""> <Destination>

All you must do is in postinst, when run with the configure option you must mount the config file to Elektra: kdb elektra-mount <New file>=""> <Mouting destination>=""> <Backend>

Next, you must update the line containing ucf with the options --three-way and --threeway-merge-command like so: ucf –three-way –threeway-merge-command elektra-merge <New file>=""> <Destination>

Then, in your postrm script, during a purge, you must umount the config file before deleting it: kdb elektra-umount <name>

That's it! With those small changes you can use Elektra to perform automatic three-way merges on any files that your package uses ucf to handle!

Example

Below is a diff representing the changes we made to the samba-common package in order to allow automatic configuration merging for smb.conf using Elektra. We chose this package because it already uses ucf to handle smb.conf but it frequently requires users to manually merge changes across versions. Here is the patch showing what we changed:

    diff samba_orig/samba-3.6.6/debian/samba-common.postinst samba/samba-3.6.6/debian/samba-common.postinst
    92c92,93
    < ucf --three-way --debconf-ok "$NEWFILE" "$CONFIG"
    ---
    > kdb elektra-mount "$CONFIG" system/samba/smb ini
    > ucf --three-way --threeway-merge-command elektra-merge --debconf-ok "$NEWFILE" "$CONFIG"
    Only in samba/samba-3.6.6/debian/: samba-common.postinst~
    diff samba_orig/samba-3.6.6/debian/samba-common.postrm samba/samba-3.6.6/debian/samba-common.postrm
    4a5
    >       kdb elektra-umount system/samba/smb

As you can see, all we had to do was add the line to mount smb.conf during install, update the ucf command to include the new --threeway-merge-command option, and unmount system/samba/smb during a purge. It really is that easy!