signal.go

package main

// Heavily inspired by https://github.com/btcsuite/btcd/blob/master/signal.go

import (
	"os"
	"os/signal"
)

// interruptChannel is used to receive SIGINT (Ctrl+C) signals.
var interruptChannel chan os.Signal

// shutdownRequestChannel is used to request the daemon to shutdown gracefully,
// similar to when receiveing SIGINT.
var shutdownRequestChannel = make(chan struct{})

// addHandlerChannel is used to add an interrupt handler to the list of handlers
// to be invoked on SIGINT (Ctrl+C) signals and shutdown.
var addHandlerChannel = make(chan func())

// mainInterruptHandler listens for SIGINT (Ctrl+C) signals on the
// interruptChannel and shutdown requests on the shutdownRequestChannel, and
// invokes the registered interruptCallbacks accordingly. It also listens for
// callback registration.
// It must be run as a goroutine.
func mainInterruptHandler() {
	// interruptCallbacks is a list of callbacks to invoke when a
	// SIGINT (Ctrl+C) or a shutdown request is received.
	var interruptCallbacks []func()

	// isShutdown is a flag which is used to indicate whether or not
	// the shutdown signal has already been received and hence any future
	// attempts to add a new interrupt handler should invoke them
	// immediately.
	var isShutdown bool

	// shutdown invokes the registered interrupt handlers, then signals the
	// shutdownChannel.
	shutdown := func() {
		// Ignore more than one shutdown signal.
		if isShutdown {
			ltndLog.Infof("Already shutting down...")
			return
		}
		isShutdown = true
		ltndLog.Infof("Shutting down...")

		// Run handlers in LIFO order.
		for i := range interruptCallbacks {
			idx := len(interruptCallbacks) - 1 - i
			callback := interruptCallbacks[idx]
			callback()
		}

		// Signal the main goroutine to shutdown.
		go func() {
			shutdownChannel <- struct{}{}
		}()
	}

	for {
		select {
		case <-interruptChannel:
			ltndLog.Infof("Received SIGINT (Ctrl+C).")
			shutdown()

		case <-shutdownRequestChannel:
			ltndLog.Infof("Received shutdown request.")
			shutdown()

		case handler := <-addHandlerChannel:
			// The shutdown signal has already been received, so
			// just invoke any new handlers immediately.
			if isShutdown {
				handler()
			}

			interruptCallbacks = append(interruptCallbacks, handler)
		}
	}
}

// addInterruptHandler adds a handler to call when a SIGINT (Ctrl+C) or a
// shutdown request is received.
func addInterruptHandler(handler func()) {
	// Create the channel and start the main interrupt handler which invokes
	// all other callbacks and exits if not already done.
	if interruptChannel == nil {
		interruptChannel = make(chan os.Signal, 1)
		signal.Notify(interruptChannel, os.Interrupt)
		go mainInterruptHandler()
	}

	addHandlerChannel <- handler
}