gotcha: using ccache in Debian package builds

Before I upload packages to Debian, I always do a full build from source under sbuild. This ensures that the package can build from source on a clean environment, implying that the set of build dependencies is complete.

But when iterating on a non-trivial package locally, I will usually build the package directly on my Debian testing system, and I want to take advantage of ccache to cache native (C/C++) code compilation to speed things up. In Debian, the easiest way to enable ccache is to add /usr/lib/ccache to your $PATH. I do this by doing something similar to the following in my ~/.bashrc:

export PATH=/usr/lib/ccache:$PATH

I noticed, however, that my Debian package builds were not using the cache. When building the same small package manually using make, the cache was used, but not when the build was wrapped with dpkg-buildpackage.

I tracked it down to the fact that in compatibility level 13+, debhelper will set $HOME to a temporary directory. For what's it worth, I think that's a good thing: you don't want package builds reaching for your home directory as that makes it harder to make builds reproducible, among other things.

This behavior, however, breaks ccache. The default cache directory is $HOME/.ccache, but that only gets resolved when ccache is actually used. So we end up starting with an empty cache on each build, get a 100% cache miss rate, and still pay for the overhead of populating the cache.

The fix is to explicitly set $CCACHE_DIR upfront, so that by the time $HOME gets overriden, it doesn't matter anymore for ccache. I did this in my ~/.bashrc:

export CCACHE_DIR=$HOME/.ccache

This way, $HOME will be expanded right there when the shell starts, and by the time ccache is called, it will use the persistent cache in my home directory even though $HOME will, at that point, refer to a temporary directory.