notify: Send all sd_notify signals from main caddy process (#4060)

Initial sd_notify support was added in #3963, but that sent signals from
both cmdRun and cmdReload.  This approach has two drawbacks:

- Reloads initiated via the API do not send signals.
- The signals are sent from different processes, which requires the
  `NotifyAccess=exec` directive in the unit file.

This change moves the NotifyReloading and NotifyReadiness invocations to
Load, which address both of those drawbacks.  It also adds a
complimentary NotifyStopping method which is invoked from handleStop.
All the notify methods are defined in a notify package to avoid an
import loop.

notify/notify.go

@@ -0,0 +1,30 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package notify
+
+// NotifyReadiness notifies process manager of readiness.
+func NotifyReadiness() error {
+	return notifyReadiness()
+}
+
+// NotifyReloading notifies process manager of reloading.
+func NotifyReloading() error {
+	return notifyReloading()
+}
+
+// NotifyStopping notifies process manager of stopping.
+func NotifyStopping() error {
+	return notifyStopping()
+}

notify/notify_linux.go

@@ -0,0 +1,80 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package notify
+
+import (
+	"io"
+	"net"
+	"os"
+	"strings"
+)
+
+// The documentation about this IPC protocol is available here:
+// https://www.freedesktop.org/software/systemd/man/sd_notify.html
+
+func sdNotify(path, payload string) error {
+	socketAddr := &net.UnixAddr{
+		Name: path,
+		Net:  "unixgram",
+	}
+
+	conn, err := net.DialUnix(socketAddr.Net, nil, socketAddr)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	if _, err := io.Copy(conn, strings.NewReader(payload)); err != nil {
+		return err
+	}
+	return nil
+}
+
+// notifyReadiness notifies systemd that caddy has finished its
+// initialization routines.
+func notifyReadiness() error {
+	val, ok := os.LookupEnv("NOTIFY_SOCKET")
+	if !ok || val == "" {
+		return nil
+	}
+	if err := sdNotify(val, "READY=1"); err != nil {
+		return err
+	}
+	return nil
+}
+
+// notifyReloading notifies systemd that caddy is reloading its config.
+func notifyReloading() error {
+	val, ok := os.LookupEnv("NOTIFY_SOCKET")
+	if !ok || val == "" {
+		return nil
+	}
+	if err := sdNotify(val, "RELOADING=1"); err != nil {
+		return err
+	}
+	return nil
+}
+
+// notifyStopping notifies systemd that caddy is stopping.
+func notifyStopping() error {
+	val, ok := os.LookupEnv("NOTIFY_SOCKET")
+	if !ok || val == "" {
+		return nil
+	}
+	if err := sdNotify(val, "STOPPING=1"); err != nil {
+		return err
+	}
+	return nil
+}

notify/notify_other.go

@@ -0,0 +1,29 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !linux
+
+package notify
+
+func notifyReadiness() error {
+	return nil
+}
+
+func notifyReloading() error {
+	return nil
+}
+
+func notifyStopping() error {
+	return nil
+}