Support running on demand systemd socket activation

changelog/unreleased/issue-126

@@ -0,0 +1,7 @@
+Feature: Allow running rest-server via systemd socket activation
+
+We've added the option to have systemd create the listening socket and start the rest-server on demand.
+
+https://github.com/restic/rest-server/issues/126
+https://github.com/restic/rest-server/pull/151
+https://github.com/restic/rest-server/pull/127

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
 	}
+
+	listener, err := findListener(server.Listen)
+	if err != nil {
+		return fmt.Errorf("unable to listen: %w", err)
+	}
+
 	if !enabledTLS {
-		log.Printf("Starting server on %s\n", server.Listen)
-		err = http.ListenAndServe(server.Listen, handler)
+		err = http.Serve(listener, 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)
+		log.Printf("TLS enabled, private key %s, pubkey %v", privateKey, publicKey)
+		err = http.ServeTLS(listener, handler, publicKey, privateKey)
 	}
 
 	return err

examples/systemd/rest-server.service

@@ -3,6 +3,9 @@ Description=Rest Server
 After=syslog.target
 After=network.target
 
+# if you want to use socket activation, make sure to require the socket here
+#Requires=rest-server.socket
+
 [Service]
 Type=simple
 # You may prefer to use a different user or group on your system.

examples/systemd/rest-server.socket

@@ -0,0 +1,5 @@
+[Socket]
+ListenStream = 8080
+
+[Install]
+WantedBy = sockets.target

go.mod

@@ -3,6 +3,7 @@ module github.com/restic/rest-server
 go 1.14
 
 require (
+	github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf
 	github.com/felixge/httpsnoop v1.0.2 // indirect
 	github.com/gorilla/handlers v1.5.1
 	github.com/miolini/datacounter v1.0.2

go.sum

@@ -65,6 +65,8 @@ github.com/cncf/udpa/go v0.0.0-20191209042840-269d4d468f6f/go.mod h1:M8M6+tZqaGX
 github.com/cncf/udpa/go v0.0.0-20200629203442-efcf912fb354/go.mod h1:WmhPx2Nbnhtbo57+VJT5O0JRkEi1Wbu0z5j0R8u5Hbk=
 github.com/cncf/udpa/go v0.0.0-20201120205902-5459f2c99403/go.mod h1:WmhPx2Nbnhtbo57+VJT5O0JRkEi1Wbu0z5j0R8u5Hbk=
 github.com/coreos/go-semver v0.3.0/go.mod h1:nnelYz7RCh+5ahJtPPxZlU+153eP4D4r3EedlOD2RNk=
+github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf h1:iW4rZ826su+pqaw19uhpSCzhj44qo35pNgKFGqzDKkU=
+github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf/go.mod h1:F5haX7vjVVG0kc13fIWeqUViNPyEJxv/OmvnBo0Yme4=
 github.com/coreos/go-systemd/v22 v22.3.2/go.mod h1:Y58oyj3AT4RCenI/lSvhwexgC+NSVTIJ3seZv2GcEnc=
 github.com/cpuguy83/go-md2man/v2 v2.0.0/go.mod h1:maD7wRr/U5Z6m/iR4s+kqSMx2CaBsrgA7czyZG/E6dU=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=