src/tools/mb/docs/user_guide.md

# The MB (Meta-Build wrapper) user guide

[TOC]

## Introduction

`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](design_spec.md).

## 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 / compile.py invocation. This
    list may contain entries that are *not* listed in the input (see
    the description of `additional_compile_targets` above and
    [design_spec.md](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 [design_spec.md](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
[design_spec.md](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.

### 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 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 [mb.py](https://source.chromium.org/chromium/chromium/src/+/main:tools/mb/mb.py;l=1370)), 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_remoteexec=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_remoteexec=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 gn-dev@chromium.org, or
[file a bug](https://crbug.com/new) with the label
'mb' and cc: dpranke@chromium.org.