Use systemd notify socket to report process status

It has all sorts of benefits due to the service manager being aware of
the starting/running/stopping state, see systemd.service(5)

On top of that, start-up errors are reported using STATUS= key, so they
will be easier to see in the 'systemctl status' output.

dist/systemd/maddy.service

@@ -6,6 +6,9 @@ Documentation=https://github.com/foxcpp/maddy/wiki
 After=network.target
 
 [Service]
+Type=notify
+NotifyAccess=main
+
 # For systemd before 235. Assumes pre-existing user & group.
 User=maddy
 Group=maddy

dist/systemd/maddy@.service

@@ -3,6 +3,9 @@ Description=maddy mail server (using %i.conf)
 After=network.target
 
 [Service]
+Type=notify
+NotifyAccess=main
+
 # For systemd before 235. Assumes pre-existing user & group.
 User=maddy
 Group=maddy

module_main.go

@@ -55,8 +55,12 @@ func moduleMain(cfg []config.Node) error {
 		return err
 	}
 
+	systemdStatus(SDReady, "Listening for incoming connections...")
+
 	handleSignals()
 
+	systemdStatus(SDStopping, "Waiting for running transactions to complete...")
+
 	for _, inst := range insts {
 		if closer, ok := inst.(io.Closer); ok {
 			if err := closer.Close(); err != nil {

run.go

@@ -43,6 +43,7 @@ func Run() int {
 	var err error
 	log.DefaultLogger.Out, err = LogOutputOption(strings.Split(*logTargets, " "))
 	if err != nil {
+		systemdStatusErr(err)
 		log.Println(err)
 		return 2
 	}
@@ -51,18 +52,21 @@ func Run() int {
 
 	f, err := os.Open(*configPath)
 	if err != nil {
-		log.Printf("cannot open %q: %v\n", *configPath, err)
+		systemdStatusErr(err)
+		log.Println(err)
 		return 2
 	}
 	defer f.Close()
 
 	cfg, err := parser.Read(f, *configPath)
 	if err != nil {
-		log.Printf("cannot parse %q: %v\n", *configPath, err)
+		systemdStatusErr(err)
+		log.Println(err)
 		return 2
 	}
 
 	if err := moduleMain(cfg); err != nil {
+		systemdStatusErr(err)
 		log.Println(err)
 		return 2
 	}

signal_nonposix.go

@@ -16,7 +16,7 @@ func handleSignals() os.Signal {
 
 	s := <-sig
 	go func() {
-		s := waitForSignal()
+		s := handleSignals()
 		log.Printf("forced shutdown due to signal (%v)!", s)
 		os.Exit(1)
 	}()

systemd.go

@@ -0,0 +1,110 @@
+//+build linux
+
+package maddy
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"strings"
+	"syscall"
+
+	"github.com/foxcpp/maddy/log"
+)
+
+type SDStatus string
+
+const (
+	SDReady    = "READY=1"
+	SDStopping = "STOPPING=1"
+)
+
+var (
+	ErrNoNotifySock = errors.New("no systemd socket")
+)
+
+func sdNotifySock() (*net.UnixConn, error) {
+	sockAddr := os.Getenv("NOTIFY_SOCKET")
+	if sockAddr == "" {
+		return nil, ErrNoNotifySock
+	}
+	if strings.HasPrefix(sockAddr, "@") {
+		sockAddr = "\x00" + sockAddr[1:]
+	}
+
+	return net.DialUnix("unixgram", nil, &net.UnixAddr{
+		Name: sockAddr,
+		Net:  "unixgram",
+	})
+}
+
+func setScmPassCred(sock *net.UnixConn) error {
+	sConn, err := sock.SyscallConn()
+	if err != nil {
+		return err
+	}
+	if err := sConn.Control(func(fd uintptr) {
+		syscall.SetsockoptByte(int(fd), syscall.SOL_SOCKET, syscall.SO_PASSCRED, 1)
+	}); err != nil {
+		return err
+	}
+	return nil
+}
+
+func systemdStatus(status SDStatus, desc string) {
+	sock, err := sdNotifySock()
+	if err != nil {
+		if err != ErrNoNotifySock {
+			log.Println("systemd: failed to acquire notify socket:", err)
+		}
+		return
+	}
+	defer sock.Close()
+
+	if err := setScmPassCred(sock); err != nil {
+		log.Println("failed to set SCM_PASSCRED on the socket for communication with systemd:", err)
+	}
+
+	if desc != "" {
+		if _, err := io.WriteString(sock, fmt.Sprintf("%s\nSTATUS=%s", status, desc)); err != nil {
+			log.Println("systemd: I/O error:", err)
+		}
+		log.Debugf(`systemd: %s STATUS="%s"`, status, desc)
+	} else {
+		if _, err := io.WriteString(sock, string(status)); err != nil {
+			log.Println("systemd: I/O error:", err)
+		}
+		log.Debugf(`systemd: %s`, status)
+	}
+}
+
+func systemdStatusErr(reportedErr error) {
+	sock, err := sdNotifySock()
+	if err != nil {
+		if err != ErrNoNotifySock {
+			log.Println("systemd: failed to acquire notify socket:", err)
+		}
+		return
+	}
+	defer sock.Close()
+
+	if err := setScmPassCred(sock); err != nil {
+		log.Println("systemd: failed to set SCM_PASSCRED on the socket:", err)
+	}
+
+	var errno syscall.Errno
+	if errors.As(reportedErr, &errno) {
+		log.Debugf(`systemd: ERRNO=%d STATUS="%v"`, errno, reportedErr)
+		if _, err := io.WriteString(sock, fmt.Sprintf("ERRNO=%d\nSTATUS=%v", errno, reportedErr)); err != nil {
+			log.Println("systemd: I/O error:", err)
+		}
+		return
+	}
+
+	if _, err := io.WriteString(sock, fmt.Sprintf("STATUS=%v\n", reportedErr)); err != nil {
+		log.Println("systemd: I/O error:", err)
+	}
+	log.Debugf(`systemd: STATUS="%v"`, reportedErr)
+}

systemd_nonlinux.go

@@ -0,0 +1,14 @@
+//+build !linux
+
+package maddy
+
+type SDStatus string
+
+const (
+	SDReady    = "READY=1"
+	SDStopping = "STOPPING=1"
+)
+
+func systemdStatus(SDStatus, string) {}
+
+func systemdStatusErr(error) {}