Hunting the Shmoo

Mach is an amazing tool which facilitates a large number of common user stories in the mozilla source tree. You can perform initial setup, execute a build, run tests, examine diagnostics, even search Google. Many of these things require an object directory. This can potentially lead to some confusion if you typically have more than one object directory at any given time. How does mach know which object directory to operate on?

It turns out that mach is pretty smart. It takes a very good guess at which object directory you want. Here is a simplification of the steps in order:

1. If cwd is an objdir or a subdirectory of an objdir, use that
2. If a mozconfig is detected and MOZ_OBJDIR is in it, use that
3. Attempt to guess the objdir with build/autoconf/config.guess

The cool thing about this is that there are tons of different workflows that fit nicely into this model. For example, many people put the mach binary on their $PATH and then always make sure to ‘cd’ into their objdirs before invoking related mach commands.

It turns out that mach works really well with a tool I had written quite awhile back called mozconfigwrapper. I won’t go into details about mozconfigwrapper here. For more info, see my previous post on it. Now for the sake of example, let’s say we have a regular and debug build called ‘regular’ and ‘debug’ respectively. Now let’s say I wanted to run the ‘mochitest-plain’ test suite on each build, one after the other. My workflow would be (from any directory other than an objdir):

$ buildwith regular
$ mach mochitest-plain
$ buildwith debug
$ mach mochitest-plain

How does this work? Very simply, mozconfigwrapper is exporting the $MOZCONFIG environment variable under the hood anytime you call ‘buildwith’. Mach will then pick up on this due to the second step listed above.

Your second question might be why bother installing mozconfigwrapper when you can just export MOZCONFIG directly? This is a matter of personal preference, but one big reason for me is the buildwith command has full tab completion, so it is easy to see which mozconfigs you have available to choose from. Also, since they are hidden away in your home directory, you don’t need to memorize any paths. There are other advantages as well which you can see in the mozconfigwrapper readme.

I’ve specially found this workflow useful when building several platforms at once (e.g firefox and b2g desktop) and switching back and forth between them with a high frequency. In the end, to each their own and this is just one possible workflow out of many. If you have a different workflow please feel free to share it in the comments.