:sig
pid_t task_repeat(String key, f64 interval, std::function<void()> exec_func, u64 timeout = 60*10)

:params
key : string uniquely identifying the task
interval : repeat interval in seconds
exec_func : function to execute repeatedly
timeout : optional task timeout value
return value : the process ID of the started (or still running) task

:desc
Starts a repeating background worker process. The function `exec_func` is executed in a loop and the worker sleeps for `interval` seconds between executions.

If a process with the same `key` is already running, `task_repeat()` does not start a second worker and instead returns the PID of the existing one.

:see
>task