aptitude::Delete-Unused behaves incorrectly

[Ren Xiaolei]

Binary package hint: aptitude

when I execute "aptitude install trac", libapr1 is installed, too ,for dependencies.
then, I execute "aptitude install apache2", apache2-mpm-worker is installed for dependencies.

later, I "aptitude purge apache2", it prompted me that only apache2 package will be purged.
"aptitude purge trac", apache2-mpm-worker be uninstalled too.

I guess:
trac depend on subverion, subversion depend on libapr1.
apache2 depend on apache2-mpm-worker, apache2-mpm-worker depend on libapr1.

When I purge apache2, libapr1 is still depended on by trac/subverion.
Aptitude thinks that apache2-mpm-worker (the package associated to libapr1) shouldn't be uninstall.
This is wrong.

[Yunfeng Wang (uhuruh)]

another example related with aptitude shown below:

$ LANG=C sudo aptitude install mailx
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
The following NEW packages will be installed:
  bsd-mailx{a} exim4{a} exim4-base{a} exim4-config{a} exim4-daemon-light{a} mailx
0 packages upgraded, 6 newly installed, 0 to remove and 262 not upgraded.
Need to get 0B/1871kB of archives. After unpacking 3817kB will be used.
Do you want to continue? [Y/n/?]
[snippped]

$ LANG=C sudo aptitude purge mailx
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information... 0%
Reading extended state information
Initializing package states... Done
The following packages will be REMOVED:
  mailx{p}
0 packages upgraded, 0 newly installed, 1 to remove and 262 not upgraded.
Need to get 0B of archives. After unpacking 36.9kB will be freed.
[snipped]

In the above purge command, bsd-mailx and exim4* was supposed to be removed along with mailx,
but isn't.

[Daniel Hartwig (wigs)]

apache2-mpm-worker is recommended by trac (through 'provides'):

$ apt-cache show trac | grep Recommends
Recommends: apache2 | httpd, ...
$ apt-cache show apache2-mpm-worker | grep Provides
Provides: apache2, apache2-mpm, httpd, httpd-cgi

so it is expected that having trac installed prevents apache2-mpm-worker from being removed. Same situation with bsd-mailx, etc.:

$ apt-cache show bsd-mailx | grep Provides
Provides: mail-reader, mailx