Move !is_android to build_engine_artifacts declaration (#54006)

The `//flutter/build/archives:artifacts` target is used to build a zip
archive (artifacts.zip) of host tools such as flutter_tester, the Dart
kernel compiler, the impellerc shader compiler, and other tooling that
is bundled in debug-mode host builds.

This moves the `!is_android` to the definition of
`build_engine_artifacts`. This is required because of the way that we
produce 32-bit arm gen_snapshot for Android on Windows hosts, which
relies on the regular x64 host toolchain due to us having no 32-bit arm
toolchain for Windows. As such, `current_toolchain == host_toolchain` on
that platform.

```
build_engine_artifacts =
    flutter_build_engine_artifacts &&
    (current_toolchain == host_toolchain ||
     (is_linux && !is_chromeos && current_cpu != "arm") || is_mac || is_win)
```

On iOS builds, we don't have this issue since `current_toolchain` will
be one of:
* `//build/toolchain/mac:ios_clang_arm`
* `//build/toolchain/mac:ios_clang_arm_sim`
* `//build/toolchain/mac:ios_clang_x64_sim`

Whereas `host_toolchain` will be one of:
* `//build/toolchain/mac:clang_arm64`
* `//build/toolchain/mac:clang_x64`

This patch also adds documentation to clarify the purpose of this target
and where related artifacts are produced so that future readers don't
need to do a deep dive into our build plumbing to figure this out.

While the target itself is primarily intended for producing host
binaries, one target binary (gen_snapshot) is bundled into the same
archive bundle as the host tools. This should be refactored such that
just like iOS and Android, they are bundled into their own
target-platform-specific archive, and the tool code accordingly updated
to pull these down into the appropriate cache directory.

As a side-note, on macOS we do rely on this archive target for the host
tools, but the bundled gen_snapshot is unused -- instead, one produced
by the //flutter/sky/tools/create_macos_gen_snapshots.py script used.
This should be fixe in a followup patch.

Related: https://github.com/flutter/flutter/issues/38935

Identified while trying to resolve:
Issue: https://github.com/flutter/flutter/issues/101138
Issue: https://github.com/flutter/flutter/issues/69157

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I signed the [CLA].
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[C++, Objective-C, Java style guides]:
https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
[testing the engine]:
https://github.com/flutter/flutter/wiki/Testing-the-engine
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat
https://dart.googlesource.com/external/github.com/flutter/engine/+/5810b3fc791f4bb82b9a454014310990eddc1181
2 files changed
tree: 77711ecb57ad8214418846cb9c67428d98a8bfac
  1. ci/
  2. tools/
  3. .gitignore
  4. commits.json
  5. DEPS
  6. OWNERS
  7. README.md
README.md

Monorepo

A gclient solution for checking out Dart and Flutter source trees

Monorepo is:

  • Optimized for Tip-of-Tree testing: The Monorepo DEPS used to check out Dart and Flutter dependencies comes from the Flutter engine DEPS with updated dependencies from Dart.

Checking out Monorepo

With depot_tools installed and on your path, create a directory for your monorepo checkout and run these commands to create a gclient solution in that directory:

mkdir monorepo
cd monorepo
gclient config --unmanaged https://dart.googlesource.com/monorepo
gclient sync -D

This gives you a checkout in the monorepo directory that contains:

monorepo/
  DEPS - the DEPS used for this gclient checkout
  commits.json - the pinned commits for Dart, flutter/engine,
                 and flutter/flutter
  tools/ - scripts used to create monorepo DEPS
engine/src/ - the flutter/buildroot repo
    flutter/ - the flutter/engine repo
    out/ - the build directory, where Flutter engine builds are created
    third_party/ - Flutter dependencies checked out by DEPS
      dart/ - the Dart SDK checkout.
        third_party - Dart dependencies, also used by Flutter
flutter/ - the flutter/flutter repo

Building Flutter engine

Flutter's instructions for building the engine are at Compiling the engine

They can be followed closely, with a few changes:

  • Googlers working on Dart do not need to switch to Fuchsia's Goma RBE, except for Windows. The GOMA_DIR enviroment variable can just point to the .cipd_bin directory in a depot_tools installation, and just goma_ctl ensure_start is sufficient.
  • The --no-prebuilt-dart-sdk option has to be added to every gn command, so that the build is set up to build and use a local Dart SDK.
  • The --full-dart-sdk option must be added to gn for the host build target if you will be building web or desktop apps.

Example build commands that work on linux:

MONOREPO_PATH=$PWD
if [[ ! $PATH =~ (^|:)$MONOREPO_PATH/flutter/bin(:|$) ]]; then
  PATH=$MONOREPO_PATH/flutter/bin:$PATH
fi

export GOMA_DIR=$(dirname $(command -v gclient))/.cipd_bin
goma_ctl ensure_start

pushd engine/src
flutter/tools/gn --goma --no-prebuilt-dart-sdk --unoptimized --full-dart-sdk
autoninja -C out/host_debug_unopt
popd

Building Flutter apps

The Flutter commands used to build and run apps will use the locally built Flutter engine and Dart SDK, instead of the one downloaded by the Flutter tool, if the --local-engine option is provided.

For example, to build and run the Flutter spinning square sample on the web platform,

MONOREPO_PATH=$PWD
cd flutter/examples/layers
flutter --local-engine=host_debug_unopt \
  -d chrome run widgets/spinning_square.dart
cd $MONOREPO_PATH

To build for desktop, specify the desktop platform device in flutter run as -d macos or -d linux or -d windows. You may also need to run the command

flutter create --platforms=windows,macos,linux

on existing apps, such as sample apps. New apps created with flutter create already include these support files. Details of desktop support are at Desktop Support for Flutter

Testing

Tests in the Flutter source tree can be run with the flutter test command, run in the directory of a package containing tests. For example:

MONOREPO_PATH=$PWD
cd flutter/packages/flutter
flutter test --local-engine=host_debug_unopt
cd $MONOREPO_PATH

Troubleshooting

Please file an issue or email the dart-engprod team with any problems with or questions about using monorepo.

We will update this documentation to address them.

  • flutter commands may download the engine and Dart SDK files for the configured channel, even though they will be using the local engine and its SDK.

Windows

  • On Windows, gclient sync needs to be run in an administrator session, because some installed dependencies create symlinks.