The MB (Meta-Build wrapper) user guide


mb is a simple python wrapper GN meta-build tool; it was originally written as part of the GYP->GN migration.

It is intended to be used by bots to make it easier to manage the configuration each bot builds (i.e., the configurations can be changed from chromium commits), and to consolidate the list of all of the various configurations that Chromium is built in.

Ideally this tool will no longer be needed after the migration is complete.

For more discussion of MB, see also the design spec.

MB subcommands

mb analyze

mb analyze is reponsible for determining what targets are affected by a list of files (e.g., the list of files in a patch on a trybot):

mb analyze -c chromium_linux_rel //out/Release input.json output.json

Either the -c/--config flag or the --builder-group and -b/--builder flags must be specified so that mb can figure out which config to use.

The first positional argument must be a GN-style “source-absolute” path to the build directory.

The second positional argument is a (normal) path to a JSON file containing a single object with the following fields:

  • files: an array of the modified filenames to check (as paths relative to the checkout root).
  • test_targets: an array of (ninja) build targets that needed to run the tests we wish to run. An empty array will be treated as if there are no tests that will be run.
  • additional_compile_targets: an array of (ninja) build targets that reflect the stuff we might want to build in addition to the list passed in test_targets. Targets in this list will be treated specially, in the following way: if a given target is a “meta” (GN: group) target like ‘blink_tests’ or or even the ninja-specific ‘all’ target, then only the dependencies of the target that are affected by the modified files will be rebuilt (not the target itself, which might also cause unaffected dependencies to be rebuilt). An empty list will be treated as if there are no additional targets to build. Empty lists for both test_targets and additional_compile_targets would cause no work to be done, so will result in an error.
  • targets: a legacy field that resembled a union of compile_targets and test_targets. Support for this field will be removed once the bots have been updated to use compile_targets and test_targets instead.

The third positional argument is a (normal) path to where mb will write the result, also as a JSON object. This object may contain the following fields:

  • error: this should only be present if something failed.

  • compile_targets: the list of ninja targets that should be passed directly to the corresponding ninja / invocation. This list may contain entries that are not listed in the input (see the description of additional_compile_targets above and [](the design spec) for how this works).

  • invalid_targets: a list of any targets that were passed in either of the input lists that weren't actually found in the graph.

  • test_targets: the subset of the input test_targets that are potentially out of date, indicating that the matching test steps should be re-run.

  • targets: a legacy field that indicates the subset of the input targets that depend on the input files.

  • build_targets: a legacy field that indicates the minimal subset of targets needed to build all of targets that were affected.

  • status: a field containing one of three strings:

    • "Found dependency" (build the compile_targets)
    • "No dependency" (i.e., no build needed)
    • "Found dependency (all)" (test_targets is returned as-is; compile_targets should contain the union of test_targets and additional_compile_targets. In this case the targets do not need to be pruned).

See [](the design spec) for more details and examples; the differences can be subtle. We won't even go into how the targets and build_targets differ from each other or from compile_targets and test_targets.

The -b/--builder, -c/--config, -f/--config-file, --builder-group, -q/--quiet, and -v/--verbose flags work as documented for mb gen.

mb gen

mb gen is responsible for generating the Ninja files by invoking GN with the right sets of build args for the given bot. It takes arguments to specify a build config and a directory, then runs GN as appropriate:

% mb gen -m tryserver.chromium.linux -b linux_rel //out/Release
% mb gen -c linux_rel_trybot //out/Release

Either the -c/--config flag or the --builder-group and -b/--builder flags must be specified so that mb can figure out which config to use. The --phase flag must also be used with builders that have multiple build/compile steps (and only with those builders).

By default, MB will look for a bot config file under //ios/build/bots (see [](the design spec) for details of how the bot config files work). If no matching one is found, will then look in //tools/mb/mb_config.pyl to look up the config information, but you can specify a custom config file using the -f/--config-file flag.

The path must be a GN-style “source-absolute” path (as above).

You can pass the -n/--dryrun flag to mb gen to see what will happen without actually writing anything.

You can pass the -q/--quiet flag to get mb to be silent unless there is an error, and pass the -v/--verbose flag to get mb to log all of the files that are read and written, and all the commands that are run.

If the build config will use the Goma distributed-build system, you can pass the path to your Goma client in the -g/--goma-dir flag, and it will be incorporated into the appropriate flags for GN as needed.

mb help

Produces help output on the other subcommands

mb isolate

Builds a given (ninja) target and produces an .isolated file suitable for then running the command either locally in an isolated environment, or remotely by uploading it to an isolate server and running it under swarming. See below for more information on isolates and swarming.

mb lookup

Prints what command will be run by mb gen (like mb gen -n but does not require you to specify a path).

The -b/--builder, -c/--config, -f/--config-file, --builder-group, --phase, -q/--quiet, and -v/--verbose flags work as documented for mb gen.

mb run

Builds and runs a given (ninja) target. By default the target will be run locally but isolated (i.e., outside of the source tree, just as it would be run under swarming). If the -s/--swarming flag is passed, the target will be built, run, uploaded to the isolate server, and run under swarming.

By default, a set of dimensions appropriate for running the target in the default pool for the build will be provided. You can specify additional dimensions with the -d/--dimension flags, and you can skip the default dimensions with the --no-default-dimensions flag (which can be useful if you need to run on devices or in a different pool). See below for more information on isolates and swarming.

In either case, any flags past -- will be passed on to the command to be run inside the isolate.

mb try

Tries your change on the trybots. Right now this is essentially a fancy tryjob, like one you could trigger via git cl try or via CQ dry runs. Basic usage is try -m tryserver.chromium.linux -b linux-rel base_unittests

Your change must be uploaded to Gerrit. Local changes will not be uploaded for you. It uses the gerrit CL associated with your given git branch.

You still have to specify the builder group(--builder-group) and buildername (-b) arguments. See for a mapping of which bots are on which tryservers, and what those bots mirror. Any trybot in is supported; you can test your code on windows, for example. The tryjob will compile and run your code on windows.

The target (base_unittests) in the example is a ninja build target. Most ninja unittest targets can be put here which currently runs on the bots.

mb validate

Does internal checking to make sure the config file is syntactically valid and that all of the entries are used properly. It does not validate that the flags make sense, or that the builder names are legal or comprehensive, but it does complain about configs and mixins that aren't used.

The -f/--config-file and -q/--quiet flags work as documented for mb gen.

This is mostly useful as a presubmit check and for verifying changes to the config file.

mb zip

Builds and isolates a given (ninja) target like the isolate command does, and then takes all of the files in the isolate and writes them into a single zip file that can then easily be redistributed.

Isolates and Swarming

mb gen is also responsible for generating the .isolate and .isolated.gen.json files needed to run test executables through swarming in a GN build.

If you wish to generate the isolate files, pass mb gen the --swarming-targets-file command line argument; that arg should be a path to a file containing a list of ninja build targets to compute the runtime dependencies for (on Windows, use the ninja target name, not the file, so base_unittests, not base_unittests.exe).

MB will take this file, translate each build target to the matching GN label (e.g., base_unittests -> //base:base_unittests, write that list to a file called runtime_deps in the build directory, and pass that to gn gen $BUILD ... --runtime-deps-list-file=$BUILD/runtime_deps.

Once GN has computed the lists of runtime dependencies, MB will then look up the command line for each target (currently this is hard-coded in, and write out the matching .isolate and .isolated.gen.json files.

The mb_config.pyl config file

The mb_config.pyl config file is intended to enumerate all of the supported build configurations for Chromium. Generally speaking, you should never need to (or want to) build a configuration that isn't listed here, and so by using the configs in this file you can avoid having to juggle long lists of gn args by hand.

mb_config.pyl is structured as a file containing a single PYthon Literal expression: a dictionary with three main keys, builder_groups, configs and mixins.

The builder_groups key contains a nested series of dicts containing mappings of builder_group -> builder -> config . This allows us to isolate the builder recipes from the actual details of the configs. The config should either be a single string value representing a key in the configs dictionary, or a list of strings, each of which is a key in the configs dictionary; the latter case is for builders that do multiple compiles with different arguments in a single build, and must only be used for such builders (where a --phase argument must be supplied in each lookup or gen call).

The configs key points to a dictionary of named build configurations.

There should be an key in this dict for every supported configuration of Chromium, meaning every configuration we have a bot for, and every configuration commonly used by develpers but that we may not have a bot for.

The value of each key is a list of “mixins” that will define what that build_config does. Each item in the list must be an entry in the dictionary value of the mixins key.

Each mixin value is itself a dictionary that contains one or more of the following keys:

  • gn_args: a string containing a list of values passed to gn --args.
  • mixins: a list of other mixins that should be included.

When mb gen or mb analyze executes, it takes a config name, looks it up in the ‘configs’ dict, and then does a left-to-right expansion of the mixins; gn_args values are concatenated.

For example, if you had:

  'configs`: {
    'linux_release_trybot': ['gn_release', 'trybot'],
    'gn_shared_debug': None,
  'mixins': {
    'bot': {
      'gn_args': 'use_goma=true dcheck_always_on=false',
    'debug': {
      'gn_args': 'is_debug=true',
    'gn_release': {
      'mixins': ['release'],
    'release': {
      'gn_args': 'is_debug=false',
    'shared': {
      'gn_args': 'is_component_build=true',
    'trybot': {
      'gn_args': 'dcheck_always_on=true',

and you ran mb gen -c linux_release_trybot //out/Release, it would translate into a call to gn --args="use_goma=true dcheck_always_on=false dcheck_always_on=true".

(From that you can see that mb is intentionally dumb and does not attempt to de-dup the flags, it lets GN do that).

Debugging MB

By design, MB should be simple enough that very little can go wrong.

The most obvious issue is that you might see different commands being run than you expect; running 'mb -v' will print what it's doing and run the commands; 'mb -n' will print what it will do but not run the commands.

If you hit weirder things than that, add some print statements to the python script, send a question to, or file a bug with the label ‘mb’ and cc: