RFC: Add --plan flag

[jaredpalmer]

Describe the feature you'd like to request

Add support for --plan flag that will output JSON that includes information about what is about to happen for a given command. This can be used with scripts such as Vercel's Ignore Build Command and also in Circle and GitLab for dynamic provisioning of CI/CD workflows.

Nx's custom Ignore Command something like this as a reference: https://github.com/nrwl/nx/blob/master/scripts/vercel/ignore-build.sh

Describe the solution you'd like

Collect up info through a traversal and print out to JSON

• Should this hit caches / wonder if we need an endpoint specifically to check without downloading which we don't yet support?

Describe alternatives you've considered

• Possibly need to do another full pass through the graph / topological sort in order to hash
• Split up scheduler so that the graph generation and execution occur in separate steps (so that it can be interrupted if --plan is passed)
• Split this into multiple commands like plz query does

[wvanrensselaer]

Wonder if it would make sense to record/cache runtime metrics for plan tasks to optimize future plans. Our CI runners usually have ~20s setup/cleanup time on top of the task being run. So suppose we have 4 uncached tasks to run:

Could initially mirror the Turborepo plan in CI using 4 machines as follows:

But in the future we can optimize the CI plan with prior runtime knowledge. In this case it would be better to group B, C, and D on the same machine:

[felixmosh]

This task will allow implement a custom github action based on actions/cache 🙏🏼

[cipriancaba]

Is there any other way atm to retrieve changed packages since a specific commit?

[bobaaaaa]

This would be a solution for #800

[jaredpalmer]

Now available in [email protected] as --dry and --dry=json