Function prepare_launch 

pub fn prepare_launch(
    root: &Path,
    selection: &Selection,
    runtime: &Path,
    platform: RunnerPlatform,
    holds_runner: &dyn Fn(&Path) -> bool,
) -> Result<LaunchCaches, CacheError>

Leases a slot in selection’s namespace for the attempt at runtime, creates every directory its variables name, and marks the namespace used.

holds_runner decides whether an existing lease is still held; production passes runtime_holds_runner.

Errors

The lock could not be taken, or a directory or lease could not be written.