Tools for migrating to starlark

This directory contains tools for migrating configuration to starlark from other locations.

Migrating tests to starlark

Make sure to install buildozer and that it is on your path.

Commands assume the working directory is //infra/config.

Execute //infra/config/migration/migrate-targets.py.
sh
```
./migration/migrate-targets.py $BUILDER_GROUP
```
This will modify the starlark files so that it will generate per-builder targets spec files for all builders in the builder group that have their tests set in //testing/buildbot/waterfalls.pyl.

To migrate a subset of builders from a builder group, instead run
sh
```
./migration/migrate-targets.py $BUILDER_GROUP $BUILDER1 $BUILDER2 ... $BUILDER_N
```
When migrating an entire builder group, it might be necessary to do this to migrate some builders in another builder group since all builders that are related via parent-child triggering or try-builder mirroring must be migrated in one CL.

This script will assume the starlark file for the builder group is located in either //infra/config/subprojects/ci/{builder_group}.star or //infra/config/subprojects/try/{builder_group}.star (groups prefixed with tryserver will fall in the latter category). If a builder group's starlark file location does not match these patterns, instead run
sh
```
./migration/migrate-targets.py $BUILDER_GROUP --star-file $STARLARK_FILE_PATH
```
Executing this script may fail at the ./main.star command because there is a test modification or removal for a test not present in any of the bundles within a builder, for example: Error: attempting to remove test 'perfetto_unittests' that is not contained in the bundle.

This is most likely due to outdated configurations in //testing/buildbot, where an update in the test suites in //testing/buildbot/waterfalls.pyl was not reflected in //testing/buildbot/test_suite_exceptions.pyl. Remove any offending modifications or removals from //testing/buildbot/test_suite_exceptions.pyl, and rerun the script and those errors should go away.

After updating starlark, the script will put the working copy into the state where git index contains the updated starlark and the new json files with the specs for each builder set to the same content that the testing/buildbot json files have for the builder. The unindexed new json files will have the content generated from the starlark, so git diff will show what is different between what generate_buildbot_json.py and what starlark generate.
Make any necessary manual changes to resolve the output of git diff.

Differences could be due to:
- Some manual changes are required that can't be handled by the migration script
  - The builder sets the isolated_scripts suite type with a suite containing gtests: the "expand-as-isolated-script" mixin needs to be applied to the suite; see examples. Note that if the builder was using both the gtest_tests and isolated_scripts suite types, then an anonymous bundle may need to be created to limit the scope that the mixin is applied to; see examples.
- Some feature is not supported by starlark and/or the migration script
- The difference doesn't actually have an effect (the targets-config-verifier try builder will pass in this case)
If there are no differences, it's still possible that the migration will be rejected by the targets-config-verifier builder if the change is missing migrating some related builders.
Remove the entries for all migrated builders from //testing/buildbot/waterfalls.pyl and //testing/buildbot/test_suite_exceptions.pyl. In waterfalls.pyl, make sure to add breadcrumb comments to indicate the builders that are migrated at the top of the builder group. If the entire builder group is migrated, add the breadcrumb comment for the entire builder group near the top of the file.
If any builder group was fully migrated, manually remove the corresponding .json file from //testing/buildbot.
Execute //infra/config/migration/post-migrate-targets.py.
sh
```
./migration/post-migrate-targets.py
```
This will do the following:
- Regenerate the .json files in //testing/buildbot.
- Replace test suite definitions with bundle definitions for any test suites no longer referenced in //testing/buildbot.
- Mark mixins and variants that are no longer referenced in //testing/buildbot so that they no longer generate pyl entries.
- Mark isolates that are no longer referenced in //testing/buildbot so that they no longer trigger an error from //testing/buildbot/check.py.
- Regenerate all //infra/config config files and sync necessary .pyl files to //testing/buildbot.
Some manual modifications may be necessary to the starlark files:
- buildozer reformats the entire file, some of this is undesirable.
- Top-level function calls added by the script should be reformatted so that each argument is on its own line (it's not possible to force this with buildozer).
- Comments associated with the entries in test_suite_exceptions.pyl and waterfalls.pyl should be transferred to the newly added starlark declarations
- Tests that are removed require a reason specified. The script puts in a reason string that will prevent the change from being submitted. This allows for confirming the generated files, without risking an undocumented removal. This may overlap with the previous bullet point since comments on the removal may serve as an adequate value for the reason field.
- Comments associated with removed test suite declarations should be transferred to the corresponding bundle declarations.

Out-of-tree migration script

In order to decouple the migration script from the src revision, which makes it easier to manage changing the migration script separately from the config files, you can keep an extra spare checkout of chromium/src.

Run the following commands in some directory not in your checkout.

git clone https://chromium.googlesource.com/chromium/src/ --no-checkout starlark-migration --depth 1
cd starlark-migration
git sparse-checkout init --cone
git sparse-checkout set infra/config/migration
git checkout

This will checkout a subset of the chromium/src repo containing the infra/config/migration directory. The migrate-targets.py and post-migrate-targets.py scripts in these directory can be run in the infra/config directory of your main checkout to modify the files there.

git rebase-update won't work with the sparse checkout since it errors trying to get the submodules. If you want to pull in submitted changes, either do git pull on your branch or do git fetch origin && git checkout origin/main to just get the current head version.

When uploading changes, you'll need to use git cl upload --bypass-hooks because due to the nature of spare checkouts, PRESUBMIT.py scripts in ancestor directories will be included but those presubmit checks assume a full checkout.

buildozer

The migration scripts make use of buildozer, which is a tool for making edits to starlark files. It's geared towards modifying BUILD files but can be used for other types of starlark files with some care. See installation instructions to get buildozer.

Ideally all edits of a script could be done in a single invocation of buildozer by using the -f flag to pass commands in a file, but command files don't currently support values with escaped newlines which are necessary in order to force values onto multiple lines when buildozer would normally put them on a single line: calls with one argument, dicts with one item or lists with one element. There is a pull request to enable this functionality waiting for review.