Relocatable Wrinkles: Things to Consider

While it's certainly no problem to add a prefix tag line to a spec file, it's necessary to consider a few other issues:

Let's cover each of these issues, starting with the %files list.

%files List Restrictions

As mentioned above, each file in the %files list must start with the relocation prefix. If this isn't done, the build will fail:
# rpm -ba cdplayer-1.0.spec
* Package: cdplayer
+ umask 022
+ echo Executing: %prep
Binary Packaging: cdplayer-1.0-1
Package Prefix = usr/local
File doesn't match prefix (usr/local): /usr/doc/cdplayer-1.0-1
File not found: /usr/doc/cdplayer-1.0-1
Build failed.

In our example, the build proceeded normally until the time came to create the binary package file. At that point RPM detected the problem. The error message says it all: The prefix line in the spec file (/usr/local) was not present in the first part of the file's (/usr/doc/cdplayer-1.0-1) path. This stopped the build in its tracks.

The fact that every file in a relocatable package must be installed under the directory specified in the prefix line, raises some issues. For example, what about a program that reads a configuration file normally kept in /etc?

This question leads right into our next section.

Relocatable Packages Must Contain Relocatable Software

While this section's title seems pretty obvious, it's not always easy to tell if a particular piece of software can be relocated. Let's take a look at the question raised at the end of the previous section. If a program has been written to read its configuration file from /etc, there are three possible approaches to making that program relocatable:

  1. Set the prefix to /etc and package everything under /etc.

  2. Package everything somewhere other than /etc and leave out the config file entirely.

  3. Modify the program.

The first approach would certainly work from a purely technical standpoint, but not many people would be happy with a program that installed itself in /etc. So this approach isn't viable.

The second approach might be more appropriate, but it forces users to complete the install by having them create the config file themselves. If RPM's goal is to make software easier to install and remove, this is not a viable approach, either!

The final approach might be the best. Once the program is installed, when the rewritten software is first run, it could see that no configuration file existed in /etc, and create one.

However, even though this would work, when the time came to erase the package, the config file would be left behind. RPM had never installed it, so RPM couldn't get rid of it. There's also the fact that this approach is probably more labor intensive than most package builders would like.

None of these approaches are very appealing, are they? Some software just doesn't relocate very well. In general, any of the following things are warning signs that relocation is going to be a problem:

If these kinds of issues crop up, then making the software relocatable is going to be tough. And there's still one issue left to consider.

The Relocatable Software Is Referenced By Other Software

Even assuming the software is written so that it can be put in a relocatable package, there still might be a problem. And that problem centers not on the relocatable software itself, but on other programs that reference the relocatable software.

For example, there are times when a package needs to execute other programs. This might include backup software that needs to send mail, or a communications program that needs to compress files. If these underlying programs were relocatable, and not installed where other packages expect them, then they would be of little use.

Granted, this isn't a common problem, but it can happen. And for the package builder interested in building relocatable packages, it's an issue that needs to be explored. Unfortunately, this type of problem can be the hardest to find.

If, however, a software product has been found to be relocatable, the mechanics of actually building a relocatable package are pretty straightforward. Let's give it a try.