func OnSignal(sig <-chan struct{}, procfunc gp.ProcessFunc) gp.Process {
	return gp.Go(func(proc gp.Process) {
		for {
			select {
			case <-sig:
				select {
				case <-proc.Go(procfunc).Closed(): // spin it out as a child, and wait till it's done.
				case <-proc.Closing(): // we're told to close
					return
				}
			case <-proc.Closing(): // we're told to close
				return
			}
		}
	})
}
func OnSignal(sig <-chan struct{}, procfunc gp.ProcessFunc) gp.Process {
	return gp.Go(func(proc gp.Process) {
		for {
			select {
			case <-sig:
				select {
				case <-proc.Go(procfunc).Closed(): // spin it out as a child, and wait till it's done.
				case <-proc.Closing(): // we're told to close
					return
				}
			case <-proc.Closing(): // we're told to close
				return
			}
		}
	})
}
// OnSignal calls the given ProcessFunc every time the signal fires, and waits for it to exit.
// This is sequentially rate limited, only one call will be in-flight at a time.
//
//  sig := make(chan struct{})
//  p := periodicproc.OnSignal(sig, func(proc goprocess.Process) {
//  	fmt.Println("fire!")
//  	<-time.After(time.Second) // delays sequential execution by 1 second
//  })
//
//  sig<- struct{}
//  sig<- struct{}
//  sig<- struct{}
//
//  // Output:
//  // fire!
//  // fire!
//  // fire!