Stephen's Work Ramblings

I’ve recently been working on splitting the LCFG client code base from the LCFG component which is used to configure and manage the daemon. This allows the client Perl modules to be built in the style of a standard Perl module. The immediate benefit of this is the enhanced portability, it makes it much easier to build the code on platforms other than DICE Linux if you can use standard Perl module building tools. We could also upload the code to CPAN which would make it even easier to download and install.

There are also benefits for maintainability, the standard Perl build tools make it easy to run unit tests and do other tasks such as checking test and API documentation coverage for your code. It is not impossible to do these things without some tool like Module::Build but it is a lot more awkward. Also, without the standard tools you have to know, or be able to discover, where certain files should be installed, we have some of this built into the LCFG build tools CMake framework but it only handles fairly simple scenarios.

The new project which contains all the Perl modules for the client is named LCFG-Client-Perl in subversion and the component continues to be named lcfg-client in the standard LCFG component naming style. This completes stage 9 of the project plan.

One thing that we need to do very frequently in the LCFG client, and also in many LCFG components, is comparing files. Typically we want to see if the new file we have just generated is any different from the previous version, in which case we will need to replace old with new and possibly carry out some action afterwards.

There are clearly many ways to solve this problem. We could read in the two files and do a simple string comparison (conceptually simple but tends to be messy, particularly if you want to minimise the memory requirements). It is also possible to calculate checksums for a file (MD5, SHA1, etc). I quite like this approach and it is nice and fast for small files. Up until now I’ve been using a mix of methods based on Text::Diff (wastes time since I don’t actually what the differences are) or calculating check sums, neither of which is an ideal approach in most cases.

What I really want though is a standard API which can simply answer the question of “are these two files the same?”. Some of the older LCFG code shows its shell heritage by using the cmp command. This command does exactly what we want and does it in a fairly efficient manner. The downside is that we have to execute another process every time we want to compare two files.

Step forwards, File::Compare. I’m not sure why I hadn’t spotted this module in the past. It works in a very similar way to the good old cmp command. It is also part of the set of core Perl modules which means it is available everywhere and it has a nice simple interface. I think I shall be converting various modules over to this approach in the future.