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-# The MB (Meta-Build wrapper) user guide
-
-[TOC]
-
-## Introduction
-
-`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](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 `-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 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, GYP: none) target like 'blink_tests' or
-    'chromium_builder_tests', 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`, `-m/--master`,
-`-q/--quiet`, and `-v/--verbose` flags work as documented for `mb gen`.
-
-### `mb audit`
-
-`mb audit` is used to track the progress of the GYP->GN migration. You can
-use it to check a single master, or all the masters we care about. See
-`mb help audit` for more details (most people are not expected to care about
-this).
-
-### `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. 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.
-
-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`).
-The gyp script defaults to `//build/gyp_chromium`, but can be overridden with
-the `--gyp-script` flag, e.g. `--gyp-script=gypfiles/gyp_v8`.
-
-### `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`,
-`--phase`, `-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 [mb.py](https://code.google.com/p/chromium/codesearch?q=mb.py#chromium/src/tools/mb/mb.py&q=mb.py%20GetIsolateCommand&sq=package:chromium&type=cs)), 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 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:
-
-  * `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 gn-dev@chromium.org, or
-[file a bug](https://crbug.com/new) with the label
-'mb' and cc: dpranke@chromium.org.
-
-