support systemd notifications

Fixes #1733

distrib/systemd/ergo.service

 # After=network.target mysql.service
 
 [Service]
-Type=simple
+Type=notify
 User=ergo
 WorkingDirectory=/home/ergo
 ExecStart=/home/ergo/ergo run --conf /home/ergo/ircd.yaml
 ExecReload=/bin/kill -HUP $MAINPID
 Restart=on-failure
 LimitNOFILE=1048576
+NotifyAccess=main
 # Uncomment this for a hidden service:
 # PrivateNetwork=true
 

go.mod

 	github.com/go-test/deep v1.0.6 // indirect
 	github.com/gorilla/websocket v1.4.2
 	github.com/goshuirc/irc-go v0.0.0-20210318074529-bdc2c2cd2fef // indirect
+	github.com/okzk/sdnotify v0.0.0-20180710141335-d9becc38acbd
 	github.com/onsi/ginkgo v1.12.0 // indirect
 	github.com/onsi/gomega v1.9.0 // indirect
 	github.com/oragono/confusables v0.0.0-20201108231250-4ab98ab61fb1 // indirect

go.sum

 github.com/goshuirc/irc-go v0.0.0-20210318074529-bdc2c2cd2fef/go.mod h1:q/JhvvKLmif3y9q8MDQM+gRCnjEKnu5ClF298TTXJug=
 github.com/hpcloud/tail v1.0.0 h1:nfCOvKYfkgYP8hkirhJocXT2+zOD8yUNjXaWfTlyFKI=
 github.com/hpcloud/tail v1.0.0/go.mod h1:ab1qPbhIpdTxEkNHXyeSf5vhxWSCs/tWer42PpOxQnU=
+github.com/okzk/sdnotify v0.0.0-20180710141335-d9becc38acbd h1:+iAPaTbi1gZpcpDwe/BW1fx7Xoesv69hLNGPheoyhBs=
+github.com/okzk/sdnotify v0.0.0-20180710141335-d9becc38acbd/go.mod h1:4soZNh0zW0LtYGdQ416i0jO0EIqMGcbtaspRS4BDvRQ=
 github.com/onsi/ginkgo v1.6.0/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
 github.com/onsi/ginkgo v1.12.0 h1:Iw5WCbBcaAAd0fpRb1c9r5YCylv4XDoCSigm1zLevwU=
 github.com/onsi/ginkgo v1.12.0/go.mod h1:oUhWkIvk5aDxtKvDDuw8gItl8pKl42LzjC9KZE0HfGg=

irc/server.go

 	"unsafe"
 
 	"github.com/ergochat/irc-go/ircfmt"
+	"github.com/okzk/sdnotify"
 
 	"github.com/ergochat/ergo/irc/caps"
 	"github.com/ergochat/ergo/irc/connection_limits"
 
 // Shutdown shuts down the server.
 func (server *Server) Shutdown() {
+	sdnotify.Stopping()
+
 	//TODO(dan): Make sure we disallow new nicks
 	for _, client := range server.clients.AllClients() {
 		client.Notice("Server is shutting down")
 	server.rehashMutex.Lock()
 	defer server.rehashMutex.Unlock()
 
+	sdnotify.Reloading()
+
 	config, err := LoadConfig(server.configFilename)
 	if err != nil {
 		server.logger.Error("server", "failed to load config file", err.Error())
 		server.logger.Info("server", "Proxied IPs will be accepted from", strings.Join(config.Server.ProxyAllowedFrom, ", "))
 	}
 
-	// we are now open for business
 	err = server.setupListeners(config)
 	// send other config warnings
 	if config.Accounts.RequireSasl.Enabled && config.Accounts.Registration.Enabled {
 		server.logger.Warning("server", "Warning: although require-sasl is enabled, users can still register accounts. If your server is not intended to be public, you must set accounts.registration.enabled to false.")
 	}
 
+	// we are now open for business
+	sdnotify.Ready()
+
 	if !initial {
 		// push new info to all of our clients
 		for _, sClient := range server.clients.AllClients() {

vendor/github.com/okzk/sdnotify/LICENSE

+The MIT License (MIT)
+
+Copyright (c) 2016 okzk
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

vendor/github.com/okzk/sdnotify/README.md

+# sdnotify
+
+sd_notify utility for golang.
+
+## Installation
+
+    go get github.com/okzk/sdnotify
+
+## Example
+
+see [sample/main.go](sample/main.go)
+
+## License
+
+MIT

vendor/github.com/okzk/sdnotify/notify.go

+// +build !linux
+
+package sdnotify
+
+// SdNotify sends a specified string to the systemd notification socket.
+func SdNotify(state string) error {
+	// do nothing
+	return nil
+}

vendor/github.com/okzk/sdnotify/notify_linux.go

+package sdnotify
+
+import (
+	"net"
+	"os"
+)
+
+// SdNotify sends a specified string to the systemd notification socket.
+func SdNotify(state string) error {
+	name := os.Getenv("NOTIFY_SOCKET")
+	if name == "" {
+		return ErrSdNotifyNoSocket
+	}
+
+	conn, err := net.DialUnix("unixgram", nil, &net.UnixAddr{Name: name, Net: "unixgram"})
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	_, err = conn.Write([]byte(state))
+	return err
+}

vendor/github.com/okzk/sdnotify/util.go

+package sdnotify
+
+import (
+	"errors"
+	"fmt"
+)
+
+// ErrSdNotifyNoSocket is the error returned when the NOTIFY_SOCKET does not exist.
+var ErrSdNotifyNoSocket = errors.New("No socket")
+
+// Ready sends READY=1 to the systemd notify socket.
+func Ready() error {
+	return SdNotify("READY=1")
+}
+
+// Stopping sends STOPPING=1 to the systemd notify socket.
+func Stopping() error {
+	return SdNotify("STOPPING=1")
+}
+
+// Reloading sends RELOADING=1 to the systemd notify socket.
+func Reloading() error {
+	return SdNotify("RELOADING=1")
+}
+
+// Errno sends ERRNO=? to the systemd notify socket.
+func Errno(errno int) error {
+	return SdNotify(fmt.Sprintf("ERRNO=%d", errno))
+}
+
+// Status sends STATUS=? to the systemd notify socket.
+func Status(status string) error {
+	return SdNotify("STATUS=" + status)
+}
+
+// Watchdog sends WATCHDOG=1 to the systemd notify socket.
+func Watchdog() error {
+	return SdNotify("WATCHDOG=1")
+}

vendor/modules.txt

 github.com/gorilla/websocket
 # github.com/goshuirc/irc-go v0.0.0-20210318074529-bdc2c2cd2fef
 ## explicit
+# github.com/okzk/sdnotify v0.0.0-20180710141335-d9becc38acbd
+## explicit
+github.com/okzk/sdnotify
 # github.com/onsi/ginkgo v1.12.0
 ## explicit
 # github.com/onsi/gomega v1.9.0