Allow building without systemd dependency

sdk/unix_listener.go

@@ -10,9 +10,7 @@ import (
 	"github.com/docker/go-connections/sockets"
 )
 
-const (
-	pluginSockDir = "/run/docker/plugins"
-)
+const pluginSockDir = "/run/docker/plugins"
 
 func newUnixListener(pluginName string, gid int) (net.Listener, string, error) {
 	path, err := fullSocketAddress(pluginName)

sdk/unix_listener_nosystemd.go

@@ -0,0 +1,10 @@
+// +build linux freebsd
+// +build nosystemd
+
+package sdk
+
+import "net"
+
+func setupSocketActivation() (net.Listener, error) {
+	return nil, nil
+}

sdk/unix_listener_systemd.go

@@ -0,0 +1,45 @@
+// +build linux freebsd
+// +build !nosystemd
+
+package sdk
+
+import (
+	"fmt"
+	"net"
+	"os"
+
+	"github.com/coreos/go-systemd/activation"
+)
+
+// isRunningSystemd checks whether the host was booted with systemd as its init
+// system. This functions similarly to systemd's `sd_booted(3)`: internally, it
+// checks whether /run/systemd/system/ exists and is a directory.
+// http://www.freedesktop.org/software/systemd/man/sd_booted.html
+//
+// Copied from github.com/coreos/go-systemd/util.IsRunningSystemd
+func isRunningSystemd() bool {
+	fi, err := os.Lstat("/run/systemd/system")
+	if err != nil {
+		return false
+	}
+	return fi.IsDir()
+}
+
+func setupSocketActivation() (net.Listener, error) {
+	if !isRunningSystemd() {
+		return nil, nil
+	}
+	listenFds := activation.Files(false)
+	if len(listenFds) > 1 {
+		return nil, fmt.Errorf("expected only one socket from systemd, got %d", len(listenFds))
+	}
+	var listener net.Listener
+	if len(listenFds) == 1 {
+		l, err := net.FileListener(listenFds[0])
+		if err != nil {
+			return nil, err
+		}
+		listener = l
+	}
+	return listener, nil
+}