[mb] Copy MB from Chromium repo

tools/mb/docs/user_guide.md

+# 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.
+
+