Workaround: dpkg goes Segmentation Fault

So, your apt-get [dist-]upgrade stops when a post/pre-installation script goes in Segmentation Fault. The strange thing is that the same script, extracted from your favourite deb package, works if launched standalone. That's probably a debconf or dpkg bug but, frankly, I didn't even google to find if it's really a known bug: I get this problem quite often on my Debian testing (especially while upgrading libc6, tzdatam console_common), so I chose to spend my time looking for a workaround. And, finally, I found a very simple one.

That's, more or less, the error I get (note: my system is localized in italian):

(Lettura del database ... 184524 file e directory attualmente installati.)
Mi preparo a sostituire libc6 2.7-3 (con libc6 2.7-5) ...
Spacchetto il sostituto di libc6 ...
Configuro libc6 (2.7-5) ...
dpkg: errore processando libc6 (--install):
il sottoprocesso post-installation script è stato terminato dal segnale (Segmentation fault)
Sono occorsi degli errori processando:
libc6

In a nutshell, my solution is:

• Unpack the .deb package causing the problem and grab the meta-files
• Clear the pre/post-installation script (depending on which one causes the segmentation fault)
• Pack a new .deb package with these fake scripts and install it
• Run manually the scripts you modified (do this before the installation if it was a preinst or pre/post-rm)
  
• Done: you can go on with your dist-upgrade

That's easy to do within a bash shell if you know the right commands. And they are:

# Make a copy of crashing package
cp /var/cache/apt/archives/crashing_pkg_x.y.z.deb ./crashing_package.deb

# Extracts package data
dpkg-deb -x crashing_package.deb ./temp_dir

# Extracts package meta-files
dpkg-deb -e crashing_package.deb ./temp_dir/DEBIAN

# Make fake post/pre installation scripts
echo "echo Fake" > ./temp_dir/DEBIAN/postinst

# Pack a new modified package
dpkg-deb -b ./temp_dir/ mod_package.deb

# Install the hacked package (should not go in segfault)
dpkg -i mod_package.deb

# Manually run postinst script
./temp_dir/DEBIAN/postinst

# Clean
rm -rf ./temp_dir
rm crashing_package.deb mod_package.deb

Dirty and functional, as usual. But pay attention to the manual execution of scripts: they may crash, or refuse to go on, or they may contain debconf commands that bash can't execute; so my advice is: take a look inside and try to manually do, step by step, what the script was supposed to do (uninstalling packages, stopping processes, restarting daemons, upgrading configurations, and so on).

I guess you're experienced enough to change them to suit your needs (and, above all, I'm in a hurry now!). So, good luck and may the GNU-force be with you =)