activation: add two functions to provide listeners with names

Since v227, systemd can name each listener. This is useful when you
have different socket units activating the same service: you can't
really know in which order the socket will be provided. Systemd adds
an environment variable named LISTEN_FDNAMES for this purpose.

We encapsulate the name in the File object created from the file
descriptor. Unfortunately, once this is turned into a net.Listener,
there doesn't seem to have a way to fetch this information
back. Therefore, two new functions are provided to be able to easily
get named sockets.

The same could be done to PacketConns in the future.

Author: vincentbernat

activation/files.go

@@ -18,6 +18,7 @@ package activation
 import (
 	"os"
 	"strconv"
+	"strings"
 	"syscall"
 )
 
@@ -30,6 +31,7 @@ func Files(unsetEnv bool) []*os.File {
 	if unsetEnv {
 		defer os.Unsetenv("LISTEN_PID")
 		defer os.Unsetenv("LISTEN_FDS")
+		defer os.Unsetenv("LISTEN_FDNAMES")
 	}
 
 	pid, err := strconv.Atoi(os.Getenv("LISTEN_PID"))
@@ -42,10 +44,17 @@ func Files(unsetEnv bool) []*os.File {
 		return nil
 	}
 
+	names := strings.Split(os.Getenv("LISTEN_FDNAMES"), ":")
+
 	files := make([]*os.File, 0, nfds)
 	for fd := listenFdsStart; fd < listenFdsStart+nfds; fd++ {
 		syscall.CloseOnExec(fd)
-		files = append(files, os.NewFile(uintptr(fd), "LISTEN_FD_"+strconv.Itoa(fd)))
+		name := "LISTEN_FD_" + strconv.Itoa(fd)
+		offset := fd - listenFdsStart
+		if offset < len(names) && len(names[offset]) > 0 {
+			name = names[offset]
+		}
+		files = append(files, os.NewFile(uintptr(fd), name))
 	}
 
 	return files

activation/files_test.go

@@ -48,15 +48,15 @@ func TestActivation(t *testing.T) {
 	}
 
 	cmd.Env = os.Environ()
-	cmd.Env = append(cmd.Env, "LISTEN_FDS=2", "FIX_LISTEN_PID=1")
+	cmd.Env = append(cmd.Env, "LISTEN_FDS=2", "LISTEN_FDNAMES=fd1", "FIX_LISTEN_PID=1")
 
 	err := cmd.Run()
 	if err != nil {
 		t.Fatalf(err.Error())
 	}
 
-	correctStringWritten(t, r1, "Hello world")
-	correctStringWritten(t, r2, "Goodbye world")
+	correctStringWritten(t, r1, "Hello world: fd1")
+	correctStringWritten(t, r2, "Goodbye world: LISTEN_FD_4")
 }
 
 func TestActivationNoFix(t *testing.T) {

activation/listeners.go

@@ -37,6 +37,27 @@ func Listeners(unsetEnv bool) ([]net.Listener, error) {
 	return listeners, nil
 }
 
+// ListenersWithNames maps a listener name to a set of net.Listener instances.
+func ListenersWithNames(unsetEnv bool) (map[string][]net.Listener, error) {
+	files := Files(unsetEnv)
+	listeners := map[string][]net.Listener{}
+
+	for _, f := range files {
+		if pc, err := net.FileListener(f); err == nil {
+			current, ok := listeners[f.Name()]
+			if !ok {
+				listeners[f.Name()] = []net.Listener{pc}
+			} else {
+				listeners[f.Name()] = append(current, pc)
+			}
+			if unsetEnv {
+				f.Close()
+			}
+		}
+	}
+	return listeners, nil
+}
+
 // TLSListeners returns a slice containing a net.listener for each matching TCP socket type
 // passed to this process.
 // It uses default Listeners func and forces TCP sockets handlers to use TLS based on tlsConfig.
@@ -58,3 +79,26 @@ func TLSListeners(unsetEnv bool, tlsConfig *tls.Config) ([]net.Listener, error)
 
 	return listeners, err
 }
+
+// TLSListenersWithNames maps a listener name to a net.Listener with
+// the associated TLS configuration.
+func TLSListenersWithNames(unsetEnv bool, tlsConfig *tls.Config) (map[string][]net.Listener, error) {
+	listeners, err := ListenersWithNames(unsetEnv)
+
+	if listeners == nil || err != nil {
+		return nil, err
+	}
+
+	if tlsConfig != nil && err == nil {
+		for _, ll := range listeners {
+			// Activate TLS only for TCP sockets
+			for i, l := range ll {
+				if l.Addr().Network() == "tcp" {
+					ll[i] = tls.NewListener(l, tlsConfig)
+				}
+			}
+		}
+	}
+
+	return listeners, err
+}

activation/listeners_test.go

@@ -73,14 +73,14 @@ func TestListeners(t *testing.T) {
 	r2.Write([]byte("Hi"))
 
 	cmd.Env = os.Environ()
-	cmd.Env = append(cmd.Env, "LISTEN_FDS=2", "FIX_LISTEN_PID=1")
+	cmd.Env = append(cmd.Env, "LISTEN_FDS=2", "LISTEN_FDNAMES=fd1:fd2", "FIX_LISTEN_PID=1")
 
-	out, err := cmd.Output()
+	out, err := cmd.CombinedOutput()
 	if err != nil {
 		println(string(out))
 		t.Fatalf(err.Error())
 	}
 
-	correctStringWrittenNet(t, r1, "Hello world")
-	correctStringWrittenNet(t, r2, "Goodbye world")
+	correctStringWrittenNet(t, r1, "Hello world: fd1")
+	correctStringWrittenNet(t, r2, "Goodbye world: fd2")
 }

activation/packetconns_test.go

@@ -56,7 +56,7 @@ func TestPacketConns(t *testing.T) {
 	r2.Write([]byte("Hi"))
 
 	cmd.Env = os.Environ()
-	cmd.Env = append(cmd.Env, "LISTEN_FDS=2", "FIX_LISTEN_PID=1")
+	cmd.Env = append(cmd.Env, "LISTEN_FDS=2", "LISTEN_FDNAMES=fd1:fd2", "FIX_LISTEN_PID=1")
 
 	out, err := cmd.CombinedOutput()
 	if err != nil {

examples/activation/activation.go

@@ -42,19 +42,19 @@ func main() {
 		panic("No files")
 	}
 
-	if os.Getenv("LISTEN_PID") == "" || os.Getenv("LISTEN_FDS") == "" {
+	if os.Getenv("LISTEN_PID") == "" || os.Getenv("LISTEN_FDS") == "" || os.Getenv("LISTEN_FDNAMES") == "" {
 		panic("Should not unset envs")
 	}
 
 	files = activation.Files(true)
 
-	if os.Getenv("LISTEN_PID") != "" || os.Getenv("LISTEN_FDS") != "" {
+	if os.Getenv("LISTEN_PID") != "" || os.Getenv("LISTEN_FDS") != "" || os.Getenv("LISTEN_FDNAMES") != "" {
 		panic("Can not unset envs")
 	}
 
 	// Write out the expected strings to the two pipes
-	files[0].Write([]byte("Hello world"))
-	files[1].Write([]byte("Goodbye world"))
+	files[0].Write([]byte("Hello world: " + files[0].Name()))
+	files[1].Write([]byte("Goodbye world: " + files[1].Name()))
 
 	return
 }

examples/activation/listen.go

@@ -42,25 +42,25 @@ func main() {
 		panic("No listeners")
 	}
 
-	if os.Getenv("LISTEN_PID") == "" || os.Getenv("LISTEN_FDS") == "" {
+	if os.Getenv("LISTEN_PID") == "" || os.Getenv("LISTEN_FDS") == "" || os.Getenv("LISTEN_FDNAMES") == "" {
 		panic("Should not unset envs")
 	}
 
-	listeners, err := activation.Listeners(true)
+	listenersWithNames, err := activation.ListenersWithNames(true)
 	if err != nil {
 		panic(err)
 	}
 
-	if os.Getenv("LISTEN_PID") != "" || os.Getenv("LISTEN_FDS") != "" {
+	if os.Getenv("LISTEN_PID") != "" || os.Getenv("LISTEN_FDS") != "" || os.Getenv("LISTEN_FDNAMES") != "" {
 		panic("Can not unset envs")
 	}
 
-	c0, _ := listeners[0].Accept()
-	c1, _ := listeners[1].Accept()
+	c0, _ := listenersWithNames["fd1"][0].Accept()
+	c1, _ := listenersWithNames["fd2"][0].Accept()
 
 	// Write out the expected strings to the two pipes
-	c0.Write([]byte("Hello world"))
-	c1.Write([]byte("Goodbye world"))
+	c0.Write([]byte("Hello world: fd1"))
+	c1.Write([]byte("Goodbye world: fd2"))
 
 	return
 }

examples/activation/udpconn.go

@@ -43,7 +43,7 @@ func main() {
 		panic("No packetConns")
 	}
 
-	if os.Getenv("LISTEN_PID") == "" || os.Getenv("LISTEN_FDS") == "" {
+	if os.Getenv("LISTEN_PID") == "" || os.Getenv("LISTEN_FDS") == "" || os.Getenv("LISTEN_FDNAMES") == "" {
 		panic("Should not unset envs")
 	}
 
@@ -52,7 +52,7 @@ func main() {
 		panic(err)
 	}
 
-	if os.Getenv("LISTEN_PID") != "" || os.Getenv("LISTEN_FDS") != "" {
+	if os.Getenv("LISTEN_PID") != "" || os.Getenv("LISTEN_FDS") != "" || os.Getenv("LISTEN_FDNAMES") != "" {
 		panic("Can not unset envs")
 	}