Optional task dependencies; Add a way to not explicitly depend on a task, but prevent running until afterwards if it's going to be run

[me4502]

Goals

1. There is no way to specify "conflicts" in terms of two tasks that cannot be run at the same time
2. There is no way to specify inherent order without a hard dependency; a task that must run after X, but only if the task X is going to run in the first place

Non-goals

1. I feel this makes sense to be uni-directional. It's not a goal to also support the inverse of tasks declaring they can only be run before another.

Background

The concept of optional dependencies isn't new, it just doesn't currently seem to be possible currently with Turborepo.

The main usecase is for situations where there is some sort of "lock" that prevents multiple tasks from interacting with one system at the same time.

Another alternative would be a "required resource" system, where resources can be checked out by a task and put back when complete, and only one task can have the allocation at a time. That would solve the conflict issue that I'm proposing this for, however feels more complex than optional dependencies, and would have less flexibility due to not allowing declaration of inherent order.

Proposal

My proposal for how this would work would be a new configuration option for tasks called "dependsOnOptional" (or a similar better name) that is used for ordering the dependency graph but does not add to it. I'm not familiar with Rust so I don't believe I'd be able to contribute this sorry.

[brokenmass]

I believe I've covered this requirement in this discussion: #9535

[me4502]

thanks for that link, it's a super interesting idea that definitely feels useful. i'm not sure if i missed anything in there so please correct me if i'm wrong, but i'm not sure if it technically does fill every requirement of mine.

the two goals for a task under my proposal would be:

• cannot run at the same time (similar to a dependency)
• does not trigger the other task to run

so it's for cases where you don't want to add to the task graph, but if the other task is in there you want to make sure this one runs afterwards. eg, a task in a workspace that can't be run alongside that workspace's build task, but doesn't need the build to have happened. the closest current setup would require adding the build task as a dependency to prevent them running at the same time, which is non-ideal as it's a lot of unnecessary overhead.

this probably would fit well into that linked proposal as an "optional" field on the dependency block though, to state that it shouldn't inject the task into the task graph