Frequently Asked Questions¶
How do I report bugs?¶
Bugs against the
mach core can be filed in Bugzilla in the Firefox
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 profile a slow command?¶
To diagnose bottlenecks, you can collect a performance profile:
./mach --profile-command SLOW-COMMAND ARGS ...
Then, you can visualize
# 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
--profile-command only profiles commands, you’ll need to invoke
directly to profile
python3 -m cProfile -o mach.cProfile ./mach ... python3 -m snakeviz mach.cProfile
mach a build system?¶
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
mach grew to fit those needs.
How do I add features to
If you would like to add a new feature to
mach that cannot be implemented as
mach command, the first step is to file a bug in the
Firefox Build System :: Mach Core component.
Should I implement X as a
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
maketarget. The build team generally does not like one-off
maketargets that aren’t part of building (read: compiling) the tree. This includes things related to testing and packaging. These weigh down
Makefilesand 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
machis often trivial.
mach fit into the modules system?¶
Mozilla operates with a modules governance
there are different components with different owners. There is not
mach module. There may or may never be one; currently
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 owne by the
team/module that owns the system they interact with. In other words,
is not a power play to consolidate authority for tooling. Instead, it aims to
expose that tooling through a common, shared interface.