lnd+signal: implement systemd notify

This adds support for notifying systemd about the state of LND. It
notifies systemd just before waiting for wallet password or, if
`wallet-password-file` was specified, right after unlocking the wallet.

This means that "ready" represents RPC being available for intended use.
It's intentional, so that client services can use `After=` in `systemd`
configuration to avoid misleading error messages about missing files or
refused connections.

Part of #4470

signal/signal.go

@@ -7,10 +7,13 @@ package signal
 
 import (
 	"errors"
+	"fmt"
 	"os"
 	"os/signal"
 	"sync/atomic"
 	"syscall"
+
+	"github.com/coreos/go-systemd/daemon"
 )
 
 var (
@@ -19,8 +22,78 @@ var (
 	started int32
 )
 
+// systemdNotifyReady notifies systemd about LND being ready, logs the result of
+// the operation or possible error. Besides logging, systemd being unavailable
+// is ignored.
+func systemdNotifyReady() error {
+	notified, err := daemon.SdNotify(false, daemon.SdNotifyReady)
+	if err != nil {
+		err := fmt.Errorf("failed to notify systemd %v (if you aren't "+
+			"running systemd clear the environment variable "+
+			"NOTIFY_SOCKET)", err)
+		log.Error(err)
+
+		// The SdNotify doc says it's common to ignore the
+		// error. We don't want to ignore it because if someone
+		// set up systemd to wait for initialization other
+		// processes would get stuck.
+		return err
+	}
+	if notified {
+		log.Info("Systemd was notified about our readiness")
+	} else {
+		log.Info("We're not running within systemd")
+	}
+	return nil
+}
+
+// systemdNotifyStop notifies systemd that LND is stopping and logs error if
+// the notification failed. It also logs if the notification was actually sent.
+// Systemd being unavailable is intentionally ignored.
+func systemdNotifyStop() {
+	notified, err := daemon.SdNotify(false, daemon.SdNotifyStopping)
+
+	// Just log - we're stopping anyway.
+	if err != nil {
+		log.Errorf("Failed to notify systemd: %v", err)
+	}
+	if notified {
+		log.Infof("Systemd was notified about stopping")
+	}
+}
+
+// Notifier handles notifications about status of LND.
+type Notifier struct {
+	// notifiedReady remembers whether Ready was sent to avoid sending it
+	// multiple times.
+	notifiedReady bool
+}
+
+// NotifyReady notifies other applications that RPC is ready.
+func (notifier *Notifier) NotifyReady(walletUnlocked bool) error {
+	if !notifier.notifiedReady {
+		err := systemdNotifyReady()
+		if err != nil {
+			return err
+		}
+		notifier.notifiedReady = true
+	}
+	if walletUnlocked {
+		_, _ = daemon.SdNotify(false, "STATUS=Wallet unlocked")
+	} else {
+		_, _ = daemon.SdNotify(false, "STATUS=Wallet locked")
+	}
+
+	return nil
+}
+
+// notifyStop notifies other applications that LND is stopping.
+func (notifier *Notifier) notifyStop() {
+	systemdNotifyStop()
+}
+
 // Interceptor contains channels and methods regarding application shutdown
-// and interrupt signals
+// and interrupt signals.
 type Interceptor struct {
 	// interruptChannel is used to receive SIGINT (Ctrl+C) signals.
 	interruptChannel chan os.Signal
@@ -33,11 +106,16 @@ type Interceptor struct {
 	shutdownRequestChannel chan struct{}
 
 	// quit is closed when instructing the main interrupt handler to exit.
+	// Note that to avoid losing notifications, only shutdown func may
+	// close this channel.
 	quit chan struct{}
+
+	// Notifier handles sending shutdown notifications.
+	Notifier Notifier
 }
 
 // Intercept starts the interception of interrupt signals and returns an `Interceptor` instance.
-// Note that any previous active interceptor must be stopped before a new one can be created
+// Note that any previous active interceptor must be stopped before a new one can be created.
 func Intercept() (Interceptor, error) {
 	if !atomic.CompareAndSwapInt32(&started, 0, 1) {
 		return Interceptor{}, errors.New("intercept already started")
@@ -85,6 +163,7 @@ func (c *Interceptor) mainInterruptHandler() {
 		}
 		isShutdown = true
 		log.Infof("Shutting down...")
+		c.Notifier.notifyStop()
 
 		// Signal the main interrupt handler to exit, and stop accept
 		// post-facto requests.