Systemd readiness notification

This change implements the systemd readiness notification, using
the sd_notify protocol.

See https://www.freedesktop.org/software/systemd/man/sd_notify.html.

cli/sd_notify.go

@@ -0,0 +1,42 @@
+// Copyright 2021 Frédéric Guillot. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package cli // import "miniflux.app/cli"
+
+import (
+	"net"
+	"os"
+)
+
+const (
+	// sdNotifyReady tells the service manager that service startup is
+	// finished, or the service finished loading its configuration.
+	sdNotifyReady = "READY=1"
+)
+
+// sdNotify sends a message to systemd using the sd_notify protocol.
+// See https://www.freedesktop.org/software/systemd/man/sd_notify.html.
+func sdNotify(state string) error {
+	addr := &net.UnixAddr{
+		Net:  "unixgram",
+		Name: os.Getenv("NOTIFY_SOCKET"),
+	}
+
+	if addr.Name == "" {
+		// We're not running under systemd (NOTIFY_SOCKET has not set).
+		return nil
+	}
+
+	conn, err := net.DialUnix(addr.Net, nil, addr)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	if _, err = conn.Write([]byte(state)); err != nil {
+		return err
+	}
+
+	return nil
+}