add support for systemd notify

internal/notify/notify.go

@@ -0,0 +1,19 @@
+// Package notify handles notifying the operating system of the program's state.
+//
+// For linux based operating systems, this is done through the systemd socket
+// set by "NOTIFY_SOCKET" environment variable.
+//
+// Currently, no other operating systems are supported.
+package notify
+
+func Readiness() error {
+	return readiness()
+}
+
+func Reloading() error {
+	return reloading()
+}
+
+func Stopping() error {
+	return stopping()
+}

internal/notify/notify_linux.go

@@ -0,0 +1,48 @@
+package notify
+
+import (
+	"io"
+	"net"
+	"os"
+	"strings"
+)
+
+func notify(path string, r io.Reader) error {
+	s := &net.UnixAddr{
+		Name: path,
+		Net:  "unixgram",
+	}
+	c, err := net.DialUnix(s.Net, nil, s)
+	if err != nil {
+		return err
+	}
+	defer c.Close()
+
+	if _, err := io.Copy(c, r); err != nil {
+		return err
+	}
+	return nil
+}
+
+func socketNotify(payload string) error {
+	v, ok := os.LookupEnv("NOTIFY_SOCKET")
+	if !ok || v == "" {
+		return nil
+	}
+	if err := notify(v, strings.NewReader(payload)); err != nil {
+		return err
+	}
+	return nil
+}
+
+func readiness() error {
+	return socketNotify("READY=1")
+}
+
+func reloading() error {
+	return socketNotify("RELOADING=1")
+}
+
+func stopping() error {
+	return socketNotify("STOPPING=1")
+}

internal/notify/notify_other.go

@@ -0,0 +1,15 @@
+// +build !linux
+
+package notify
+
+func readiness() error {
+	return nil
+}
+
+func reloading() error {
+	return nil
+}
+
+func stopping() error {
+	return nil
+}