Frequently Asked Questions

How do I report bugs?

Bugs against the mach core can be filed in Bugzilla in the Firefox Build System::Mach Core component.


Most mach bugs are bugs in individual commands, not bugs in the core mach code. Bugs for individual commands should be filed against the component that command is related to. For example, bugs in the build command should be filed against Firefox Build System :: General. Bugs against testing commands should be filed somewhere in the Testing product.

How do I debug a command failing with a Python exception?

You can run a command and break into pdb, the Python debugger, when the command is invoked with:

./mach --debug-command FAILING-COMMAND ARGS ...

How do I debug mach itself?

If you are editing the mach code, or other Python modules you can open the terminal and start debugging with pdb with the following:

python3 -m pdb ./mach <command>

How do I debug pytest tests?

First, before debugging, run ./mach python-test once to ensure that the testing virtualenv is up-to-date:

./mach python-test path/to/

Then, using the testing virtualenv, debug the test file:

<objdir>/_virtualenvs/python-test/bin/python -m pdb path/to/

How do I profile a slow command?

To diagnose bottlenecks, you can collect a performance profile:

./mach --profile-command SLOW-COMMAND ARGS ...

Then, you can visualize mach_profile_SLOW-COMMAND.cProfile using snakeviz:

# If you don't have snakeviz installed yet:
python3 -m pip install snakeviz
python3 -m snakeviz mach_profile_SLOW-COMMAND.cProfile

How do I profile mach itself?

Since --profile-command only profiles commands, you’ll need to invoke cProfile directly to profile mach itself:

python3 -m cProfile -o mach.cProfile ./mach ...
python3 -m snakeviz mach.cProfile

Is mach a build system?

No. mach is just a generic command dispatching tool that happens to have a few commands that interact with the real build system. Historically, mach was born to become a better interface to the build system. However, its potential beyond just build system interaction was quickly realized and mach grew to fit those needs.

How do I add features to mach?

If you would like to add a new feature to mach that cannot be implemented as a mach command, the first step is to file a bug in the Firefox Build System :: Mach Core component.

Should I implement X as a mach command?

There are no hard or fast rules. Generally speaking, if you have some piece of functionality or action that is useful to multiple people (especially if it results in productivity wins), then you should consider implementing a mach command for it.

Some other cases where you should consider implementing something as a mach command:

  • When your tool is a random script in the tree. Random scripts are hard to find and may not conform to coding conventions or best practices. Mach provides a framework in which your tool can live that will put it in a better position to succeed than if it were on its own.

  • When the alternative is a make target. The build team generally does not like one-off make targets that aren’t part of building (read: compiling) the tree. This includes things related to testing and packaging. These weigh down Makefiles and add to the burden of maintaining the build system. Instead, you are encouraged to implement ancillary functionality in Python. If you do implement something in Python, hooking it up to mach is often trivial.

How do I use 3rd-party Python packages in my mach command?

See Using third-party Python packages.

How does mach fit into the modules system?

Mozilla operates with a modules governance system where there are different components with different owners. There is not currently a mach module. There may or may never be one; currently mach is owned by the build team.

Even if a mach module were established, mach command modules would likely never belong to it. Instead, mach command modules are owned by the team/module that owns the system they interact with. In other words, mach is not a power play to consolidate authority for tooling. Instead, it aims to expose that tooling through a common, shared interface.

Who do I contact for help or to report issues?

You can ask questions in #build.