Goals

1. Some tasks, like headed browser tests, don't play well with each other and should be run sequentially (--concurrency=1)
2. The dependency tasks, builds, etc can all be run at full concurrency

Non-goals

Background

Currently we solve this by running a build task across all workspaces before running our tests with --concurrency=1. This is imprecise, and leads to us running more build/readiness tasks than we actually need, as it's not based on the actual dependency tree. While it does work, it's slower than it could be.

Proposal

I have two possible proposed ways to achieve this, with a preference for the second,

1. Separate --concurrency option for dependencies and explicit tasks. This would allow dependencies to be run with full concurrency, and the written task to run with a concurrency of one. The existing --concurrency option would set both.

2. An --only-dependencies option, that allows you to run only the dependency tasks in the dependency graph. This would still be a two command process, but would be significantly more flexible and reduce any oddities with the concurrency options like the other solution.

This means you could run turbo run browser-tests --only-dependencies, and then run turbo run browser-tests --concurrency=1, where the dependencies are completed & cached, making the next task significantly faster, while only completing the required dependencies.