Support running on demand systemd socket activation

cmd/rest-server/listener_unix.go

@@ -0,0 +1,45 @@
+// +build !windows
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"net"
+
+	"github.com/coreos/go-systemd/activation"
+)
+
+// findListener tries to find a listener via systemd socket activation. If that
+// fails, it tries to create a listener on addr.
+func findListener(addr string) (listener net.Listener, err error) {
+	// try systemd socket activation
+	listeners, err := activation.Listeners()
+	if err != nil {
+		panic(err)
+	}
+
+	switch len(listeners) {
+	case 0:
+		// no listeners found, listen manually
+		listener, err = net.Listen("tcp", addr)
+		if err != nil {
+			return nil, fmt.Errorf("listen on %v failed: %w", addr, err)
+		}
+
+		log.Printf("start server on %v", addr)
+		return listener, nil
+
+	case 1:
+		// one listener supplied by systemd, use that one
+		//
+		// for testing, run rest-server with systemd-socket-activate as follows:
+		//
+		//    systemd-socket-activate -l 8080 ./rest-server
+		log.Printf("systemd socket activation mode")
+		return listeners[0], nil
+
+	default:
+		return nil, fmt.Errorf("got %d listeners from systemd, expected one", len(listeners))
+	}
+}

cmd/rest-server/listener_windows.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"net"
+)
+
+// findListener creates a listener.
+func findListener(addr string) (listener net.Listener, err error) {
+	// listen manually
+	listener, err = net.Listen("tcp", addr)
+	if err != nil {
+		return nil, fmt.Errorf("listen on %v failed: %w", addr, err)
+	}
+
+	log.Printf("start server on %v", addr)
+	return listener, nil
+}

cmd/rest-server/main.go

@@ -129,16 +129,17 @@ func runRoot(cmd *cobra.Command, args []string) error {
 	if err != nil {
 		return err
 	}
-	if !enabledTLS {
-		log.Printf("Starting server on %s\n", server.Listen)
-		err = http.ListenAndServe(server.Listen, handler)
-	} else {
 
-		log.Println("TLS enabled")
-		log.Printf("Private key: %s", privateKey)
-		log.Printf("Public key(certificate): %s", publicKey)
-		log.Printf("Starting server on %s\n", server.Listen)
-		err = http.ListenAndServeTLS(server.Listen, publicKey, privateKey, handler)
+	listener, err := findListener(server.Listen)
+	if err != nil {
+		return fmt.Errorf("unable to listen: %w", err)
+	}
+
+	if !enabledTLS {
+		err = http.Serve(listener, handler)
+	} else {
+		log.Printf("TLS enabled, private key %s, pubkey %v", privateKey, publicKey)
+		err = http.ServeTLS(listener, handler, publicKey, privateKey)
 	}
 
 	return err