pantsbuild Sample Repo

This repo was created to provide a sample environment for testing out supporting Trunk Merge Queue Parallel Mode on Pants powered repos. In order to use Parallel Mode, the list of dependencies that a created PR affects needs to be sent to Merge Queue - this repo shows how to do that

Repo Dependency Layout

This repo contains the following Pants targets:

• is_number
  • dependent on nothing
• is_even
  • dependent on is_number
• is_odd
  • dependent on is_number
• is_even_or_odd
  • dependent on is_odd and is_even
  • transitively dependent on is_number
• main
  • dependent on is_even_or_odd
  • transitively dependent on is_odd, is_even, and is_number

In the pants world, you can run pants dependents --transitive (reference link) to see pants output the specific targets that each module has depending on it.

Calculating Impacted Targets

In order to determine what pants targets a PR you create specifically changes, pants must compare your current PR's branch against the branch that Trunk Merge Queue would be merging the PR into. Pants makes this easy with the --changed-since argument (reference link) as it understands git.

Knowing this, you can generate a list of impacted targets to send to Merge Queue with the following command assuming you currently have the PR's branch checked out:

pants --changed-since=origin/${MERGE_INSTANCE_BRANCH} --changed-dependents=transitive list

Broken down:

• --changed-since=origin/${MERGE_INSTANCE_BRANCH} will make Pants only consider targets that have changed directly between the current PR and the branch Merge Queue will merge the PR into
• --changed-dependents=transitive will also make Pants include the targets that are dependent on the directly changed targets above
• list will list out the changed targets and their dependents

This command will return a new-line delimited list than can then be sent directly to Trunk. If there are no targets changed, it'll instead output a warning log to stderr. To filter that out directly in bash, add 2>/dev/null to the end of the command above.

Testing Pants On This Repo

In order to test all of this out, all you need to do is run ./get-pants.sh - after that, the command pants should be available in your path. If you already have Pants installed, this step can be skipped

Running Tests

There are some sanity tests in this repo - to run them, you can either use python -m unittest or pants test :: to get Pants to do it