Operating systems

During my ventures in the Linux-land, I constantly package and re-package RPMs. Sometimes to introduce new functionality to existing package or to simply get a newer version that distro vendor is prepared to offer. Number of times I've created packaging to software that is not in the distro at all.

Another thing I love using are symlinks. I can have newer and older package of a software and can simply switch with updating the symlink into correct version. When I combined those two, it bit me in the ass.

I had quite simple script-blocks to handle the symlink:

%post
cd /my/package/directory/
%{__rm} -f my.cool.symlink-name
ln -s package/library/my.cool my.cool.symlink-name

%preun
%{__rm} -f /my/package/directory/my.cool.symlink-name

On install, that worked, but on update/freshen there was no symlink left. I was puzzled, why is that? Little bit of googling revealed two pieces of information: RPM spec-file documentation about scripts, especially the Install/Erase-time Scripts -part and 2nd the Fedora Project's packaging information, especially the scriptlet ordering. I'll abbreviate the Fedora's ordering here omitting the non-interesting parts:

1. %pre of new package
   This is the part where my script confirms that the symlink exists.
   
2. (package install)
3. %post of new package
4. %preun of old package
   During update/freshen, this is the part where my script removes the symlink created in 1.)
   Crap!
   
5. (removal of old package)
6. %postun of old package

Further reading of RPM spec-docs said "the argument passed to version 1.0's scripts is 1". Ok, nice to how, but now what? How can I utilize the information? What is the exact syntax for the script? The only usable information I found was in the Fedora packaging instructions, there was an example:

%preun
if [ $1 = 0 ] ; then
  /sbin/install-info --delete %{_infodir}/%{name}.info %{_infodir}/dir || :
fi

So this was the thing I had to try. My solution is to change the %preun-block:

%preun
if [ $1 -lt 1 ] ; then
  # This is really an un-install, not deleting previous version on update
  %{__rm} -f /my/package/directory/my.cool.symlink-name
fi

I did that and upgraded the package. Poooof! The symlink was gone like there was no change at all. WHY? I upgraded the revision number of the package and upgraded again. NOW it worked! Nice.

There is a simple explanation what happened. It says in the Fedora project's order-list that "%preun of old package". OLD package! It works starting from the next update, but not on the first one.

Anyway I was delighted to get that one sorted.