Add ability to run Collector as Windows Service

[james-bebbington]

When running the Collector on Windows, users will generally want to run this as a Windows service. It's not possible to just run a .exe directly as a Windows service, so some minor modifications need to be made to the startup/service code.

Suggested implementation:

Add a separate main method on Windows (using build flags):

• Check if the executable is running interactively & if so, run the Collector as normal
• Otherwise, if the executable has been invoked from service control manager, implement the Handler interface as a wrapper around service.Application, and call svc.Run on this handler
  • In order to make minimal changes to existing code, make the signalsChannel, that is used to be notified of OS SIGTERM signals, available for this wrapper to use when the Windows svc.Stop or svc.Shutdown signals are received (see example code below).
  • Allow a zap hook to be supplied to service.newLogger() so that logging can be redirected to the Windows Event Viewer instead of being lost. We may want to limit this to INFO+ (or WARNING+) logs.

Sample code:

type WindowsService struct {}

func (m *WindowsService) Execute(args []string, requests <-chan svc.ChangeRequest, changes chan<- svc.Status) (ssec bool, errno uint32) {
	...

	go func() {
		err = app.Start()
		if err != nil {
			log.Fatalf("application run finished with error: %v", err)
		}
	}()

	changes <- svc.Status{State: svc.Running, Accepts: svc.AcceptStop | svc.AcceptShutdown}

	for req := range requests {
		switch req.Cmd {
		case svc.Interrogate:
			changes <- req.CurrentStatus
		case svc.Stop, svc.Shutdown:
			app.signalsChannel <- syscall.SIGTERM
			changes <- svc.Status{State: svc.StopPending}
			return
		default:
			log.Fatalf(fmt.Sprintf("unexpected control request #%d", req))
		}
	}
}

[james-bebbington]

(can assign this to me)