# The MB (Meta-Build wrapper) design spec

[TOC]

## Intro

MB was originally 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/`gn_args` combinations) of
   Chromium that are supported.

Now that everything is using GN, only the second purpose is really relevant,
but it's still important. MB must handle at least the `gen` and `analyze`
steps on the bots, i.e., we need to wrap both the `gn gen` 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.

For more information on how to actually use MB, see
[the user guide](user_guide.md).

## Design

MB is intended to be as simple as possible, and to defer as much work as
possible to GN. 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` will first look for a bot config file in a set of different locations
(initially just in //ios/build/bots). Bot config files are JSON files that
contain keys for 'gn_args' (a list of strings that will be joined together).
Bot config files require the full list of settings to be given explicitly.

If no matching bot config file is found, `mb` looks in the
`//tools/mb/mb_config.pyl` config file to determine what set of flags
(`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).

The way analyze works can be subtle and complicated (see below).

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.

## Analyze

The goal of the `analyze` step is to speed up the cycle time of the try servers
by only building and running the tests affected by the files in a patch, rather
than everything that might be out of date. Doing this ends up being tricky.

We start with the following requirements and observations:

* In an ideal (un-resource-constrained) world, we would build and test
  everything that a patch affected on every patch. This does not
  necessarily mean that we would build 'all' on every patch (see below).

* In the real world, however, we do not have an infinite number of machines,
  and try jobs are not infinitely fast, so we need to balance the desire
  to get maximum test coverage against the desire to have reasonable cycle
  times, given the number of machines we have.

* Also, since we run most try jobs against tip-of-tree Chromium, by
  the time one job completes on the bot, new patches have probably landed,
  rendering the build out of date.

* This means that the next try job may have to do a build that is out of
  date due to a combination of files affected by a given patch, and files
  affected for unrelated reasons. We want to rebuild and test only the
  targets affected by the patch, so that we don't blame or punish the
  patch author for unrelated changes.

So:

1. We need a way to indicate which changed files we care about and which
   we don't (the affected files of a patch).

2. We need to know which tests we might potentially want to run, and how
   those are mapped onto build targets. For some kinds of tests (like
   GTest-based tests), the mapping is 1:1 - if you want to run base_unittests,
   you need to build base_unittests. For others (like the telemetry and
   layout tests), you might need to build several executables in order to
   run the tests, and that mapping might best be captured by a *meta*
   target (a GN group like `webkit_tests`) that depends on the right list
   of files. Because the BUILD.gn files know nothing about test steps, we
   have to have some way of mapping back and forth between test steps and
   build targets. That mapping is *not* currently available to MB (or GN),
   and so we have to provide enough information to make it possible for
   the caller to do the mapping.

3. We might also want to know when test targets are affected by data files
   that aren't compiled (python scripts, or the layout tests themselves).

4. We also want to ensure that particular targets still compile even if they
   are not actually tested; consider testing the installers themselves, or
   targets that don't yet have good test coverage. We might want to use meta
   targets for this purpose as well.

5. However, for some meta targets, we don't necessarily want to rebuild the
   meta target itself, perhaps just the dependencies of the meta target that
   are affected by the patch. For example, if you have a meta target like
   `blink_tests` that might depend on ten different test binaries. If a patch
   only affects one of them (say `wtf_unittests`), you don't want to
   build `blink_tests`, because that might actually also build the other nine
   targets.  In other words, some meta targets are *prunable*.

6. As noted above, in the ideal case we actually have enough resources and
   things are fast enough that we can afford to build everything affected by a
   patch, but listing every possible target explicitly would be painful. The
   GN Ninja generator provides an 'all' target that captures (nearly,
   see [crbug.com/503241](https://crbug.com/503241)) everything, but
   unfortunately GN doesn't actually represent 'all' as a meta target in the
   build graph, so we will need to write code to handle that specially.

7. In some cases, we will not be able to correctly analyze the build graph to
   determine the impact of a patch, and need to bail out (e.g,. if you change a
   build file itself, it may not be easy to tell how that affects the graph).
   In that case we should simply build and run everything.

The interaction between 2) and 5) means that we need to treat meta targets
two different ways, and so we need to know which targets should be
pruned in the sense of 5) and which targets should be returned unchanged
so that we can map them back to the appropriate tests.

So, we need three things as input:

* `files`: the list of files in the patch
* `test_targets`: the list of ninja targets which, if affected by a patch,
  should be reported back so that we can map them back to the appropriate
  tests to run. Any meta targets in this list should *not* be pruned.
* `additional_compile_targets`: the list of ninja targets we wish to compile
  *in addition to* the list in `test_targets`. Any meta targets
  present in this list should be pruned (we don't need to return the
  meta targets because they aren't mapped back to tests, and we don't want
  to build them because we might build too much).

We can then return two lists as output:

* `compile_targets`, which is a list of pruned targets to be
  passed to Ninja to build. It is acceptable to replace a list of
  pruned targets by a meta target if it turns out that all of the
  dependendencies of the target are affected by the patch (i.e.,
  all ten binaries that blink_tests depends on), but doing so is
  not required.
* `test_targets`, which is a list of unpruned targets to be mapped
  back to determine which tests to run.

There may be substantial overlap between the two lists, but there is
no guarantee that one is a subset of the other and the two cannot be
used interchangeably or merged together without losing information and
causing the wrong thing to happen.

The implementation is responsible for recognizing 'all' as a magic string
and mapping it onto the list of all root nodes in the build graph.

There may be files listed in the input that don't actually exist in the build
graph: this could be either the result of an error (the file should be in the
build graph, but isn't), or perfectly fine (the file doesn't affect the build
graph at all). We can't tell these two apart, so we should ignore missing
files.

There may be targets listed in the input that don't exist in the build
graph; unlike missing files, this can only indicate a configuration error,
and so we should return which targets are missing so the caller can
treat this as an error, if so desired.

Any of the three inputs may be an empty list:

* It normally doesn't make sense to call analyze at all if no files
  were modified, but in rare cases we can hit a race where we try to
  test a patch after it has already been committed, in which case
  the list of modified files is empty. We should return 'no dependency'
  in that case.

* Passing an empty list for one or the other of test_targets and
  additional_compile_targets is perfectly sensible: in the former case,
  it can indicate that you don't want to run any tests, and in the latter,
  it can indicate that you don't want to do build anything else in
  addition to the test targets.

* It doesn't make sense to call analyze if you don't want to compile
  anything at all, so passing [] for both test_targets and 
  additional_compile_targets should probably return an error.

In the output case, an empty list indicates that there was nothing to
build, or that there were no affected test targets as appropriate.

Note that passing no arguments to Ninja is equivalent to passing
`all` to Ninja (at least given how GN works); however, we
don't want to take advantage of this in most cases because we don't
actually want to build every out of date target, only the targets
potentially affected by the files. One could try to indicate
to analyze that we wanted to use no arguments instead of an empty
list, but using the existing fields for this seems fragile and/or
confusing, and adding a new field for this seems unwarranted at this time.

There is an "error" field in case something goes wrong (like the
empty file list case, above, or an internal error in MB/GN). The
analyze code should also return an error code to the shell if appropriate
to indicate that the command failed.

In the case where build files themselves are modified and analyze may
not be able to determine a correct answer (point 7 above, where we return
"Found dependency (all)"), we should also return the `test_targets` unmodified
and return the union of `test_targets` and `additional_compile_targets` for
`compile_targets`, to avoid confusion.

### Examples

Continuing the example given above, suppose we have the following build
graph:

* `blink_tests` is a meta target that depends on `webkit_unit_tests`,
  `wtf_unittests`, and `webkit_tests` and represents all of the targets
  needed to fully test Blink. Each of those is a separate test step.
* `webkit_tests` is also a meta target; it depends on `content_shell`
  and `image_diff`.
* `base_unittests` is a separate test binary.
* `wtf_unittests` depends on `Assertions.cpp` and `AssertionsTest.cpp`.
* `webkit_unit_tests` depends on `WebNode.cpp` and `WebNodeTest.cpp`.
* `content_shell` depends on `WebNode.cpp` and `Assertions.cpp`.
* `base_unittests` depends on `logging.cc` and `logging_unittest.cc`.

#### Example 1

We wish to run 'wtf_unittests' and 'webkit_tests' on a bot, but not
compile any additional targets.

If a patch touches WebNode.cpp, then analyze gets as input:

    {
      "files": ["WebNode.cpp"],
      "test_targets": ["wtf_unittests", "webkit_tests"],
      "additional_compile_targets": []
    }

and should return as output:

    {
      "status": "Found dependency",
      "compile_targets": ["webkit_unit_tests"],
      "test_targets": ["webkit_tests"]
    }

Note how `webkit_tests` was pruned in compile_targets but not in test_targets.

#### Example 2

Using the same patch as Example 1, assume we wish to run only `wtf_unittests`,
but additionally build everything needed to test Blink (`blink_tests`):

We pass as input:

    {
      "files": ["WebNode.cpp"],
      "test_targets": ["wtf_unittests"],
      "additional_compile_targets": ["blink_tests"]
    }

And should get as output:

    {
      "status": "Found dependency",
      "compile_targets": ["webkit_unit_tests"],
      "test_targets": []
    }

Here `blink_tests` was pruned in the output compile_targets, and
test_targets was empty, since blink_tests was not listed in the input
test_targets.

#### Example 3

Build everything, but do not run any tests.

Input:

    {
      "files": ["WebNode.cpp"],
      "test_targets": [],
      "additional_compile_targets": ["all"]
    }

Output:

    {
      "status": "Found dependency",
      "compile_targets": ["webkit_unit_tests", "content_shell"],
      "test_targets": []
    }

#### Example 4

Same as Example 2, but a build file was modified instead of a source file.

Input:

    {
      "files": ["BUILD.gn"],
      "test_targets": ["wtf_unittests"],
      "additional_compile_targets": ["blink_tests"]
    }

Output:

    {
      "status": "Found dependency (all)",
      "compile_targets": ["webkit_unit_tests", "wtf_unittests"],
      "test_targets": ["wtf_unittests"]
    }

test_targets was returned unchanged, compile_targets was pruned.

## Random 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 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 GN already has to support anyway; in particular, it might
require MB to know how to parse 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.

### 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 for
  complicated build scenarios (a.k.a. Chrome OS), where multiple flags need
  to be set to user-defined paths for specific toolchains (e.g., where
  Chrome OS 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
  eventually.

* MB is not intended to replace the
  [CR tool](https://code.google.com/p/chromium/wiki/CRUserManual), and
  it is not really meant as a developer-facing tool.
