:sig
pid_t task(String key, std::function<void()> exec_func)

:params
key : string uniquely identifying the task
exec_func : function to execute
return value : the process ID of the started (or still running) task

:see
>task

:content
Starts `exec_func` in a new process and returns that process ID.

If a process with the same `key` is already running, `task()` does not start a second copy. Instead it returns the PID of the already-running task.

Use the `key` to make a background job idempotent across repeated calls.

Related:

- PHP: background-process patterns using `proc_open()`, queues, cron, or worker supervisors
- JavaScript / Node.js: Node `child_process`, worker queues, timers, schedulers, and supervised background jobs