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1# The MB (Meta-Build wrapper) user guide
2
3[TOC]
4
5## Introduction
6
7`mb` is a simple python wrapper around the GYP and GN meta-build tools to
8be used as part of the GYP->GN migration.
9
10It is intended to be used by bots to make it easier to manage the configuration
11each bot builds (i.e., the configurations can be changed from chromium
12commits), and to consolidate the list of all of the various configurations
13that Chromium is built in.
14
15Ideally this tool will no longer be needed after the migration is complete.
16
17For more discussion of MB, see also [the design spec](design_spec.md).
18
19## MB subcommands
20
21### `mb analyze`
22
23`mb analyze` is reponsible for determining what targets are affected by
24a list of files (e.g., the list of files in a patch on a trybot):
25
26```
27mb analyze -c chromium_linux_rel //out/Release input.json output.json
28```
29
30Either the `-c/--config` flag or the `-m/--master` and `-b/--builder` flags
31must be specified so that `mb` can figure out which config to use.
32
33The first positional argument must be a GN-style "source-absolute" path
34to the build directory.
35
36The second positional argument is a (normal) path to a JSON file containing
37a single object with the following fields:
38
39  * `files`: an array of the modified filenames to check (as paths relative to
40    the checkout root).
41  * `test_targets`: an array of (ninja) build targets that needed to run the
42    tests we wish to run. An empty array will be treated as if there are
43    no tests that will be run.
44  * `additional_compile_targets`: an array of (ninja) build targets that
45    reflect the stuff we might want to build *in addition to* the list
46    passed in `test_targets`. Targets in this list will be treated
47    specially, in the following way: if a given target is a "meta"
48    (GN: group, GYP: none) target like 'blink_tests' or
49    'chromium_builder_tests', or even the ninja-specific 'all' target,
50    then only the *dependencies* of the target that are affected by
51    the modified files will be rebuilt (not the target itself, which
52    might also cause unaffected dependencies to be rebuilt). An empty
53    list will be treated as if there are no additional targets to build.
54    Empty lists for both `test_targets` and `additional_compile_targets`
55    would cause no work to be done, so will result in an error.
56  * `targets`: a legacy field that resembled a union of `compile_targets`
57    and `test_targets`. Support for this field will be removed once the
58    bots have been updated to use compile_targets and test_targets instead.
59
60The third positional argument is a (normal) path to where mb will write
61the result, also as a JSON object. This object may contain the following
62fields:
63
64  * `error`: this should only be present if something failed.
65  * `compile_targets`: the list of ninja targets that should be passed
66    directly to the corresponding ninja / compile.py invocation. This
67    list may contain entries that are *not* listed in the input (see
68    the description of `additional_compile_targets` above and
69    [design_spec.md](the design spec) for how this works).
70  * `invalid_targets`: a list of any targets that were passed in
71    either of the input lists that weren't actually found in the graph.
72  * `test_targets`: the subset of the input `test_targets` that are
73    potentially out of date, indicating that the matching test steps
74    should be re-run.
75  * `targets`: a legacy field that indicates the subset of the input `targets`
76    that depend on the input `files`.
77  * `build_targets`: a legacy field that indicates the minimal subset of
78    targets needed to build all of `targets` that were affected.
79  * `status`: a field containing one of three strings:
80
81    * `"Found dependency"` (build the `compile_targets`)
82    * `"No dependency"` (i.e., no build needed)
83    * `"Found dependency (all)"` (`test_targets` is returned as-is;
84       `compile_targets` should contain the union of `test_targets` and
85       `additional_compile_targets`. In this case the targets do not
86       need to be pruned).
87
88See [design_spec.md](the design spec) for more details and examples; the
89differences can be subtle.  We won't even go into how the `targets` and
90`build_targets` differ from each other or from `compile_targets` and
91`test_targets`.
92
93The `-b/--builder`, `-c/--config`, `-f/--config-file`, `-m/--master`,
94`-q/--quiet`, and `-v/--verbose` flags work as documented for `mb gen`.
95
96### `mb audit`
97
98`mb audit` is used to track the progress of the GYP->GN migration. You can
99use it to check a single master, or all the masters we care about. See
100`mb help audit` for more details (most people are not expected to care about
101this).
102
103### `mb gen`
104
105`mb gen` is responsible for generating the Ninja files by invoking either GYP
106or GN as appropriate. It takes arguments to specify a build config and
107a directory, then runs GYP or GN as appropriate:
108
109```
110% mb gen -m tryserver.chromium.linux -b linux_rel //out/Release
111% mb gen -c linux_rel_trybot //out/Release
112```
113
114Either the `-c/--config` flag or the `-m/--master` and `-b/--builder` flags
115must be specified so that `mb` can figure out which config to use. The
116`--phase` flag must also be used with builders that have multiple
117build/compile steps (and only with those builders).
118
119By default, MB will look for a bot config file under `//ios/build/bots` (see
120[design_spec.md](the design spec) for details of how the bot config files
121work). If no matching one is found, will then look in
122`//tools/mb/mb_config.pyl` to look up the config information, but you can
123specify a custom config file using the `-f/--config-file` flag.
124
125The path must be a GN-style "source-absolute" path (as above).
126
127You can pass the `-n/--dryrun` flag to mb gen to see what will happen without
128actually writing anything.
129
130You can pass the `-q/--quiet` flag to get mb to be silent unless there is an
131error, and pass the `-v/--verbose` flag to get mb to log all of the files
132that are read and written, and all the commands that are run.
133
134If the build config will use the Goma distributed-build system, you can pass
135the path to your Goma client in the `-g/--goma-dir` flag, and it will be
136incorporated into the appropriate flags for GYP or GN as needed.
137
138If gen ends up using GYP, the path must have a valid GYP configuration as the
139last component of the path (i.e., specify `//out/Release_x64`, not `//out`).
140The gyp script defaults to `//build/gyp_chromium`, but can be overridden with
141the `--gyp-script` flag, e.g. `--gyp-script=gypfiles/gyp_v8`.
142
143### `mb help`
144
145Produces help output on the other subcommands
146
147### `mb lookup`
148
149Prints what command will be run by `mb gen` (like `mb gen -n` but does
150not require you to specify a path).
151
152The `-b/--builder`, `-c/--config`, `-f/--config-file`, `-m/--master`,
153`--phase`, `-q/--quiet`, and `-v/--verbose` flags work as documented for
154`mb gen`.
155
156### `mb validate`
157
158Does internal checking to make sure the config file is syntactically
159valid and that all of the entries are used properly. It does not validate
160that the flags make sense, or that the builder names are legal or
161comprehensive, but it does complain about configs and mixins that aren't
162used.
163
164The `-f/--config-file` and `-q/--quiet` flags work as documented for
165`mb gen`.
166
167This is mostly useful as a presubmit check and for verifying changes to
168the config file.
169
170## Isolates and Swarming
171
172`mb gen` is also responsible for generating the `.isolate` and
173`.isolated.gen.json` files needed to run test executables through swarming
174in a GN build (in a GYP build, this is done as part of the compile step).
175
176If you wish to generate the isolate files, pass `mb gen` the
177`--swarming-targets-file` command line argument; that arg should be a path
178to a file containing a list of ninja build targets to compute the runtime
179dependencies for (on Windows, use the ninja target name, not the file, so
180`base_unittests`, not `base_unittests.exe`).
181
182MB will take this file, translate each build target to the matching GN
183label (e.g., `base_unittests` -> `//base:base_unittests`, write that list
184to a file called `runtime_deps` in the build directory, and pass that to
185`gn gen $BUILD ... --runtime-deps-list-file=$BUILD/runtime_deps`.
186
187Once GN has computed the lists of runtime dependencies, MB will then
188look up the command line for each target (currently this is hard-coded
189in [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
190matching `.isolate` and `.isolated.gen.json` files.
191
192## The `mb_config.pyl` config file
193
194The `mb_config.pyl` config file is intended to enumerate all of the
195supported build configurations for Chromium. Generally speaking, you
196should never need to (or want to) build a configuration that isn't
197listed here, and so by using the configs in this file you can avoid
198having to juggle long lists of GYP_DEFINES and gn args by hand.
199
200`mb_config.pyl` is structured as a file containing a single PYthon Literal
201expression: a dictionary with three main keys, `masters`, `configs` and
202`mixins`.
203
204The `masters` key contains a nested series of dicts containing mappings
205of master -> builder -> config . This allows us to isolate the buildbot
206recipes from the actual details of the configs. The config should either
207be a single string value representing a key in the `configs` dictionary,
208or a list of strings, each of which is a key in the `configs` dictionary;
209the latter case is for builders that do multiple compiles with different
210arguments in a single build, and must *only* be used for such builders
211(where a --phase argument must be supplied in each lookup or gen call).
212
213The `configs` key points to a dictionary of named build configurations.
214
215There should be an key in this dict for every supported configuration
216of Chromium, meaning every configuration we have a bot for, and every
217configuration commonly used by develpers but that we may not have a bot
218for.
219
220The value of each key is a list of "mixins" that will define what that
221build_config does. Each item in the list must be an entry in the dictionary
222value of the `mixins` key.
223
224Each mixin value is itself a dictionary that contains one or more of the
225following keys:
226
227  * `gyp_crosscompile`: a boolean; if true, GYP_CROSSCOMPILE=1 is set in
228    the environment and passed to GYP.
229  * `gyp_defines`: a string containing a list of GYP_DEFINES.
230  * `gn_args`: a string containing a list of values passed to gn --args.
231  * `mixins`: a list of other mixins that should be included.
232  * `type`: a string with either the value `gyp` or `gn`;
233    setting this indicates which meta-build tool to use.
234
235When `mb gen` or `mb analyze` executes, it takes a config name, looks it
236up in the 'configs' dict, and then does a left-to-right expansion of the
237mixins; gyp_defines and gn_args values are concatenated, and the type values
238override each other.
239
240For example, if you had:
241
242```
243{
244  'configs`: {
245    'linux_release_trybot': ['gyp_release', 'trybot'],
246    'gn_shared_debug': None,
247  }
248  'mixins': {
249    'bot': {
250      'gyp_defines': 'use_goma=1 dcheck_always_on=0',
251      'gn_args': 'use_goma=true dcheck_always_on=false',
252    },
253    'debug': {
254      'gn_args': 'is_debug=true',
255    },
256    'gn': {'type': 'gn'},
257    'gyp_release': {
258      'mixins': ['release'],
259      'type': 'gyp',
260    },
261    'release': {
262      'gn_args': 'is_debug=false',
263    }
264    'shared': {
265      'gn_args': 'is_component_build=true',
266      'gyp_defines': 'component=shared_library',
267    },
268    'trybot': {
269      'gyp_defines': 'dcheck_always_on=1',
270      'gn_args': 'dcheck_always_on=true',
271    }
272  }
273}
274```
275
276and you ran `mb gen -c linux_release_trybot //out/Release`, it would
277translate into a call to `gyp_chromium -G Release` with `GYP_DEFINES` set to
278`"use_goma=true dcheck_always_on=false dcheck_always_on=true"`.
279
280(From that you can see that mb is intentionally dumb and does not
281attempt to de-dup the flags, it lets gyp do that).
282
283## Debugging MB
284
285By design, MB should be simple enough that very little can go wrong.
286
287The most obvious issue is that you might see different commands being
288run than you expect; running `'mb -v'` will print what it's doing and
289run the commands; `'mb -n'` will print what it will do but *not* run
290the commands.
291
292If you hit weirder things than that, add some print statements to the
293python script, send a question to gn-dev@chromium.org, or
294[file a bug](https://crbug.com/new) with the label
295'mb' and cc: dpranke@chromium.org.
296
297
298