Implement mb - a meta-build wrapper for bots to use in the GYP->GN migration.

tools/mb/docs/design_spec.md

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+# The MB (Meta-Build wrapper) design spec
+
+[TOC]
+
+## Intro
+
+MB is intended to address two major aspects of the GYP -> GN transition
+for Chromium:
+
+1. "bot toggling" - make it so that we can easily flip a given bot
+   back and forth between GN and GYP.
+
+2. "bot configuration" - provide a single source of truth for all of
+   the different configurations (os/arch/`gyp_define` combinations) of
+   Chromium that are supported.
+
+MB must handle at least the `gen` and `analyze` steps on the bots, i.e.,
+we need to wrap both the `gyp_chromium` invocation to generate the
+Ninja files, and the `analyze` step that takes a list of modified files
+and a list of targets to build and returns which targets are affected by
+the files.
+
+## Design
+
+MB is intended to be as simple as possible, and to defer as much work as
+possible to GN or GYP. It should live as a very simple Python wrapper
+that offers little in the way of surprises.
+
+### Command line
+
+It is structured as a single binary that supports a list of subcommands:
+
+* `mb gen -c linux_rel_bot //out/Release`
+* `mb analyze -m tryserver.chromium.linux -b linux_rel /tmp/input.json /tmp/output.json`
+
+### Configurations
+
+`mb` looks in the `//tools/mb/mb_config.pyl` config file to determine whether
+to use GYP or GN for a particular build directory, and what set of flags
+(`GYP_DEFINES` or `gn args`) to use.
+
+A config can either be specified directly (useful for testing) or by specifying
+the master name and builder name (useful on the bots so that they do not need
+to specify a config directly and can be hidden from the details).
+
+See the [user guide](user_guide.md#mb_config.pyl) for details.
+
+### Handling the analyze step
+
+The interface to `mb analyze` is described in the
+[user\_guide](user_guide.md#mb_analyze).
+
+Since the interface basically mirrors the way the "analyze" step on the bots
+invokes gyp\_chromium today, when the config is found to be a gyp config,
+the arguments are passed straight through.
+
+It implements the equivalent functionality in GN by calling `'gn refs
+[list of files] --type=executable --all --as=output` and filtering the
+output to match the list of targets.
+
+## Detailed Design Requirements and Rationale
+
+This section is collection of semi-organized notes on why MB is the way
+it is ...
+
+### in-tree or out-of-tree
+
+The first issue is whether or not this should exist as a script in
+Chromium at all; an alternative would be to simply change the bot
+configurations to know whether to use GYP or GN, and which flags to
+pass.
+
+That would certainly work, but experience over the past two years
+suggests a few things:
+
+  * we should push as much logic as we can into the source repositories
+    so that they can be versioned and changed atomically with changes to
+    the product code; having to coordinate changes between src/ and
+    build/ is at best annoying and can lead to weird errors.
+  * the infra team would really like to move to providing
+    product-independent services (i.e., not have to do one thing for
+    Chromium, another for NaCl, a third for V8, etc.).
+  * we found that during the SVN->GIT migration the ability to flip bot
+    configurations between the two via changes to a file in chromium
+    was very useful.
+
+All of this suggests that the interface between bots and Chromium should
+be a simple one, hiding as much of the chromium logic as possible.
+
+### Why not have MB be smarter about de-duping flags?
+
+This just adds complexity to the MB implementation, and duplicates logic
+that GYP and GN already have to support anyway; in particular, it might
+require MB to know how to parse GYP and GN values. The belief is that
+if MB does *not* do this, it will lead to fewer surprises.
+
+It will not be hard to change this if need be.
+
+### Integration w/ gclient runhooks
+
+On the bots, we will disable `gyp_chromium` as part of runhooks (using
+`GYP_CHROMIUM_NO_ACTION=1`), so that mb shows up as a separate step.
+
+At the moment, we expect most developers to either continue to use
+`gyp_chromium` in runhooks or to disable at as above if they have no
+use for GYP at all. We may revisit how this works once we encourage more
+people to use GN full-time (i.e., we might take `gyp_chromium` out of
+runhooks altogether).
+
+### Config per flag set or config per (os/arch/flag set)?
+
+Currently, mb_config.pyl does not specify the host_os, target_os, host_cpu, or
+target_cpu values for every config that Chromium runs on, it only specifies
+them for when the values need to be explicitly set on the command line.
+
+Instead, we have one config per unique combination of flags only.
+
+In other words, rather than having `linux_rel_bot`, `win_rel_bot`, and
+`mac_rel_bot`, we just have `rel_bot`.
+
+This design allows us to determine easily all of the different sets
+of flags that we need to support, but *not* which flags are used on which
+host/target combinations.
+
+It may be that we should really track the latter. Doing so is just a 
+config file change, however.
+
+### Non-goals
+
+* MB is not intended to replace direct invocation of GN or GYP for
+  complicated build scenarios (aka ChromeOS), where multiple flags need
+  to be set to user-defined paths for specific toolchains (e.g., where
+  ChromeOS needs to specify specific board types and compilers).
+
+* MB is not intended at this time to be something developers use frequently,
+  or to add a lot of features to. We hope to be able to get rid of it once
+  the GYP->GN migration is done, and so we should not add things for
+  developers that can't easily be added to GN itself.
+
+* MB is not intended to replace the 
+  [CR tool](https://code.google.com/p/chromium/wiki/CRUserManual). Not
+  only is it only intended to replace the gyp\_chromium part of `'gclient
+  runhooks'`, it is not really meant as a developer-facing tool.
+
+### Open issues
+
+* Some common flags (goma\_dir being the obvious one) may need to be
+  specified via the user, and it's unclear how to integrate this with
+  the concept of build\_configs.
+  
+  Right now, MB has hard-coded support for a few flags (i.e., you can
+  pass the --goma-dir flag, and it will know to expand "${goma\_dir}" in
+  the string before calling out to the tool. We may want to generalize
+  this to a common key/value approach (perhaps then meeting the
+  ChromeOS non-goal, above), or we may want to keep this very strictly
+  limited for simplicity.