After certain upgrades (most notably upgrades between Ubuntu
releases), there are follow-up actions which should be taken by the
user. These are not logically part of the upgrade, but should be dealt
with at some point after the upgrade is complete.

A prime example is the UTF8MigrationTool. During an upgrade from
WartyWarthog to HoaryHedgehog, the system will be configured such that
UTF-8 encoding is available, and will be expected by applications per
default. The user should switch to a UTF-8 locale, and possibly rename
or re-encode files in order to achieve the best user experience. These
things cannot be done in automated fashion during the upgrade, but
must instead be dealt with interactively by the user.

Another example is the reboot notification. Packages like the kernel,
glibc, dbus and hal require a reboot. To simplify things
update-notifier installs a helper script in

/usr/share/update-notifier/notify-reboot-required

that can be called in the packages postinst script (make sure to check
for it first). It is i18n-able via langpacks and it will only be shown
once.  

Design:
 * Maintainer scripts create/copy a notification to
   /var/lib/update-notifier/user.d
 * update-notifier checks for those after each apt invocation (when
   /var/lib/update-notifier/dpkg-was-run changed) and notifies the user
   that they are pending
 * When the user selects to process pending actions, they are executed
   in sequence within the context of the user and the desktop (and so
   can display a GUI, interact with the user, etc.)

Details:
 * the directory /var/lib/update-notifier/user.d/ is used to store hooks
 * a hook file looks follows rfc822 rules:
   - a fieldname may have a "-$locale" in it. This is for i18n.
   - if "GettextDomain" is found, dgettext() will also be tried to get a
     i18n string (for langpacks)
   - each hook must have a unique "Name" key and a "Description" key
   - a optional "Priority" with {crititcal, high, medium, low} that
     gives the user a idea how important the hook is (currently not used) 
   - a optional "Command" field may be registered with a shell-command
     that the user can run (e.g. a script like /usr/share/package/do-something)
   - a optional "Terminal: True" field if the command should be
     execute in a terminal
   - update-notifier uses inotify to monitor
     /var/lib/update-notifier/dpkg-was-run and
     /var/lib/update-notifier/user.d/ and show a small information icon for
     hooks it has not seen yet
   - when the user clicks on the icon the name and description is
     given. If a shell script is registered a button is displayed that will
     run the script. the description in the hook should explain what the
     script will do
   - the encoding of the i18n fields is UTF-8
   - the optional key "DontShowAfterReboot" will make sure that the 
     notification is not shown when the system rebooted since it was 
     created/refreshed 
   - the optional key "DisplayIf" will run a shell command (system())
     to figure out if the note should be displayed or not. If the command
     returns 0 the note will be displayed, otherwise it will be ignored
   - the optional key "OnlyAdminUsers" controls if the note is shown to all
     users or only to the admin users (users in the admin group). the
     default is "OnlyAdminsUsers=true".

Example

Name: The great UTF-8 Migration
Name-de_DE: Die grosse UTF-8 Migration
Priority: Medium
Command: utf8-migration-assistant
Terminal: True
GettextDomain: utf8-migration-assistant
DisplayIf: shell-command
OnlyAdminUsers: False
Description: This command will convert your stuff into UTF-8.
 Use this command if you want a working gnome desktop and you
 feel the world should be a better place.

