Support concurrent tasks

[cassaundra]

Concurrent tasks are useful for both performance and usecases like running a frontend and backend simultaneously.

Implementation

Usage

Add a --concurrent flag to tasks which runs the task without blocking.

Implementation

EngineState would need to be cloned and used for each concurrent task, meaning changes to the global state (e.g. mutating variables) would be isolated to the task. If this is done, it should be done for all tasks for consistency. For that matter, it might be best to either modify the engine state to make all variables immutable, or create a clean engine state altogether.

Output handling

For now, tasks output directly to stdout/stderr. Introducing concurrent tasks would mingle the outputs of concurrent tasks with all others, resulting in potentially confusing output. Future work might include redirecting these streams elsewhere (e.g. logs) to resolve this problem.

Services

14 proposes using EngineState::ctrlc to interrupt tasks, which would cause all tasks (including concurrent tasks) to terminate. For the purposes of running multiple concurrent services, this would cause all services to be terminated (and not all to necessarily be restarted), which is not ideal.

To solve this, a --service flag could be added to tasks which implies that it is concurrent and should maintain a separate ctrlc state.

[fdncred]

Not sure if you're aware but nushell has a par-each command that runs things in parallel. You might find some insight from there if you don't already know about it.

[cassaundra]

Implemented as of now. Will refactor more when adding hot reloading and runtime configuration options.