apt-listchanges
===============

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Due to the demand, here is a quick overview of how apt-listchanges works.
******************************************************************************

While upgrading a package _package_, apt-listchanges does the following:

1. finds the name of its _source package_ ;
2. see if this _source package_ has already showed up in its status database
   or not; if it has take the last seen version from the database
   and jump to 4 ;
3. in case the status database does not contain _source package_ entry, check
   with dpkg which version of the _package_ is installed (if any) and use
   that version as the last seen version of _source package_;
   otherwise skip this _package_;
4. if the last seen version is not lesser than the version being upgraded,
   then skip this _package_;
5. read _package_ NEWS and/or changelog files from its .deb and append the data
   to the data to be shown ;
6. update this _source package_ as seen and save its version in the status
   database.

This is done for every package that is going to be installed or upgraded.

The reasons for a NEWS and/or changelog not to show up are:
* you never installed any binary package from that source package before,
  meaning that `apt-listchanges` breaks each time a source package is renamed,
  I can't fix that ;
* there is no new NEWS/changelog entry:
  - `apt-get install --reinstall`) ;
  - or you already installed a binary package from the same source and the
    same version.
* `apt-listchanges` was unable to guess the package source name (should not
  happen);
* `apt-listchanges` was unable to find the Debian changelog in the binary
  package (should not happen either).


Workaround in case of libdb downgrade or similar issues
-------------------------------------------------------

During the early months of 2008, there was an unfortunate situation
where python bumped libdb to 4.6 to then downgrade it to 4.5 later
on. That generates a situation where the apt-listchanges database was
not readable anymore because updated to libdb2.6 format but then
accessed with libdb2.5.

You can read the full discussion at https://bugs.debian.org/469221 .

In case situations like that one will happen again, we want to mention
a way to recover the database:

 # install utilities to handle both db4.6 and db4.5 libdb files
 aptitude install db4.6-util db4.5-util
 # define a temporary database location
 TMPDB=$(mktemp -d)/listchanges.db
 # convert the current apt-listchanges database from 4.6 to 4.5
 db4.6_dump /var/lib/apt/listchanges.db | db4.5_load "$TMPDB"
 # keep a copy of the old db and move the converted db in place
 mv /var/lib/apt/listchanges.db /var/lib/apt/listchanges.db.old
 mv "$TMPDB" /var/lib/apt/listchanges.db

Adapt the solution to the situation you're facing.

Alternatively you can try to replace the database with its previous
version stored in /var/lib/apt/listchanges-old.db (since version 3.1
of apt-listchanges) or as a last resort you can remove the database,
in which case you lose the history, but apt-listchanges should still
work, as it will switch to comparing the currently installed version
of the package with the version being installed (see point 3 of the
algorithm given at the top of this file).


Sharing the status database between machines
--------------------------------------------

The status database can be shared between machines,
see https://bugs.debian.org/658444 for a use case,
using NFS-mounted storage, as long as you do not run more than one
instance of apt-listchanges at time (otherwise the database might
get corrupted due to lack of locking).  The '--dump-seen' option
can be used to examine the contents of the database.