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Declarative Diversions - Report 1

The following is the design spec so far for this project. It has to be fleshed out a bit more but most of the basic structure is there. Anyone who cares to give input is more than welcome.
A further update will be posted on Monday to my blog, planet debian, the wiki page and to the dpkg mailing list.

A diversion is when it is possible to have dpkg not overwrite a file when it 
reinstalls the package it belongs to, and to have it put the file from the 
package somewhere else instead.

Declarative diversions involves a new control file with a declarative 
syntax which dpkg will parse and process directly as part of the package unpack 
and removal phases, eliminating the problems resulting from non-atomic 
handling of diversions.

There are a number of topics involved in implementing this kind of project

 * What syntax do we use for the new control file?
 * What dpkg does with the control file
 * How do we handle diversions to a non-existant directory?
 * How do we order unpacking a new package which adds a diversion?
 * How do we order removing a package which had a diversion?
 * How do we handle errors?
 * What happens to dpkg-divert?

Details - Control File Syntax
It will conform to RFC2822 style with the following format:
 * Divert-From:
 * Divert-To:
 * Blank lines and lines beginning with '#' will be comments
'Divert-To' will be optional and if it is ommitted then files being diverted 
will have their filename changed to 'file.distrib'

The above style has it's advantages, one of the main ones being that there is
no need to escape whitespace within filenames.

Details - Control File Handling
Within control.tar.gz the file should be named 'diversions'
This file is then copied to /var/lib/info/$package.diversions
This is ensuring we have a copy of the control file just in case it is needed.
Any input on this subject would be appreciated.

Details - Handling Diversions to non-existant directories
Diverting files to directories that don't exist can cause a number of problems. 
If the package does not 'own' the directory it may be left orphaned on
removal of the package
The package is responsible for ensuring the availability of the target directory in the unpack phase.

Details - Ordering Requirements
 =>Unpacking a new package that adds a diversion
   *1. Unpack folders
   *2. Add diversions
   *3. Unpack files
   Performing diversions this way avoids trouble with creating folders and 
   accidentally overwrtiting the wrong version of files. 
   If we don't use this method the problems that can occur include creating 
   folders before the package itself would create them which may cause 
   permission errors. 
   Another major problem would be extracting a new version of the file and  
   overwriting the old one. 
   This might cause the wrong version of the file to be diverted and the 
   package to break.
 =>Removing a package which had a diversion
   *1. Remove files
   *2. Remove diversions
   *3. Remove folders
   This ensures that all files, diversions and folders are removed correctly

Details - Error handling
Errors in diversions will have to handled with a great deal of care due to
the fact that if they are not the package could be broken.
This means that a great deal of checks must be done to ensure that all the files
can be diverted properly before any actual diverting takes place. If they can't 
the package installation/update must be stopped and rolled back to avoid the 
package being installed incorrectly or broken.

Details - 'dpkg-divert'
When we impliment the new diversion method we should keep the current 
dpkg-divert. This allows maintainers to catch up with the new method without 
breaking their packages. It also allows maintainers to perform some operations 
that aren't support by the new method.

Example Usage #1
The file to be diverted is '/usr/share/foo'
It needs to be moved to '/usr/share/bar'

The syntax of the control file would be:

#Start File
Divert-From: /usr/share/foo
Divert-To: /usr/share/bar
#End File

Example Usage #2
In this example the maintainer doesn't want to move the file to any specific

The syntax of the control file would be:

#Start File
Divert-From: /usr/share/foo
#End File

This would divert the file to '/usr/share/foo.distrib'

RFC2822 Guide: 
 * http://www.faqs.org/rfcs/rfc2822.html

First Email Thread on Declarative Diversions (First Message in Thread):
 * http://lists.debian.org/debian-dpkg/2011/05/msg00102.html
Declarative Diversions Wiki:
 * http://wiki.debian.org/SummerOfCode2011/DeclarativeDiversions
My Blog for this Project: 
 * http://blog.sam-dunne.com

Sam Dunne

BSc Computer Science, UCD Dublin.

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