Recently, I wrote about how I use multiple (10!) clones of the mozilla-inbound repository, with one Mercurial queue per clone, to work on multiple changes to the Mozilla codebase concurrently.

At times, I’ve felt almost guilty about using such a heavyweight branching mechanism, as opposed to a lightweight (i.e. intra-clone) branching mechanism such as git branches, or Mercurial bookmarks, or multiple Mercurial queues in a single clone (managed via hg qqueue). It seemed clumsy, like I was missing out on a compelling feature of modern version control systems.

But I now have come to understand that each approach is appropriate in a particular circumstances. In parcticular, lightweight branches are not appropriate when code modifications incur a non-trivial build-time cost.

Consider a Mozilla developer who works only on code that doesn’t need compilation, such as JavaScript, HTML, or Python. After modifying code, such a developer might incur a zero, or almost-zero build-time cost. For example, they might not have to do anything for their changes to propagate into the built product, or they might merely have to copy the modified file into the build directory.

For this developer, lightweight branches are entirely appropriate, because they can switch between branches with hardly a care in the world.

In contrast, consider a Mozilla developer (such as me!) who works mostly on C++ code within Gecko. After modifying code, this developer incurs a decidedly non-zero build cost — on my machine, just linking libxul takes around ten seconds. So any change to Gecko’s C++ code will require at least this much time, and it’s often substantially more, especially for anyone using a slow machine and/or OS.

For this developer, lightweight branches are not appropriate, because they will have to wait for rebuilding to occur every time they switch. ccache mitigates this problem, but it doesn’t solve it. In particular, the developer may well have switched away from one branch precisely because they are waiting for a long-running build to complete, and lightweight branches certainly don’t allow that.

These two distinct cases may be obvious to some people, but they weren’t to me. If nothing else, as someone who mostly works on C++ Mozilla code, I now can feel content with the heavyweight branching approach I use.

https://blog.mozilla.org/nnethercote/2014/01/21/lightweight-branches-arent-always-appropriate/