The MB (Meta-Build wrapper) user guide


mb is a simple python wrapper around the GYP and GN meta-build tools to be used 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 -m/--master 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 two fields:

  • files: an array of the modified filenames to check (as paths relative to the checkout root).
  • targets: an array of the unqualified target names to check.

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.
  • targets: the subset of the input targets that depend on the input files.
  • build_targets: the minimal subset of targets needed to build all of targets that were affected.
  • status: one of three strings:
    • "Found dependency" (build the build_targets)
    • "No dependency" (i.e., no build needed)
    • "Found dependency (all)" (build everything, in which case targets and build_targets are not returned).

The -b/--builder, -c/--config, -f/--config-file, -m/--master, -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 either GYP or GN as appropriate. It takes arguments to specify a build config and a directory, then runs GYP or 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 -m/--master and -b/--builder flags must be specified so that mb can figure out which config to use.

By default, MB will 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 GYP or GN as needed.

If gen ends up using GYP, the path must have a valid GYP configuration as the last component of the path (i.e., specify //out/Release_x64, not //out).

mb help

Produces help output on the other subcommands

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, -m/--master, -q/--quiet, and -v/--verbose flags work as documented for mb gen.

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.

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 (in a GYP build, this is done as part of the compile step).

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 GYP_DEFINES and gn args by hand.

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

The masters key contains a nested series of dicts containing mappings of master -> builder -> config . This allows us to isolate the buildbot recipes from the actual details of the configs.

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:

  • gyp_crosscompile: a boolean; if true, GYP_CROSSCOMPILE=1 is set in the environment and passed to GYP.
  • gyp_defines: a string containing a list of GYP_DEFINES.
  • gn_args: a string containing a list of values passed to gn --args.
  • mixins: a list of other mixins that should be included.
  • type: a string with either the value gyp or gn; setting this indicates which meta-build tool to use.

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; gyp_defines and gn_args values are concatenated, and the type values override each other.

For example, if you had:

  'configs`: {
    'linux_release_trybot': ['gyp_release', 'trybot'],
    'gn_shared_debug': None,
  'mixins': {
    'bot': {
      'gyp_defines': 'use_goma=1 dcheck_always_on=0',
      'gn_args': 'use_goma=true dcheck_always_on=false',
    'debug': {
      'gn_args': 'is_debug=true',
    'gn': {'type': 'gn'},
    'gyp_release': {
      'mixins': ['release'],
      'type': 'gyp',
    'release': {
      'gn_args': 'is_debug=false',
    'shared': {
      'gn_args': 'is_component_build=true',
      'gyp_defines': 'component=shared_library',
    'trybot': {
      'gyp_defines': 'dcheck_always_on=1',
      'gn_args': 'dcheck_always_on=true',

and you ran `mb gen -c linux_release_trybot //out/Release`, it would
translate into a call to `gyp_chromium -G Release` with `GYP_DEFINES` set to
`"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 gyp 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: