Add SSH master connection feature

main.go

@@ -34,11 +34,12 @@ var _ interface {
 } = new(rootCmd)
 
 type rootCmd struct {
-	skipSync     bool
-	syncBack     bool
-	printVersion bool
-	bindAddr     string
-	sshFlags     string
+	skipSync          bool
+	syncBack          bool
+	printVersion      bool
+	noReuseConnection bool
+	bindAddr          string
+	sshFlags          string
 }
 
 func (c *rootCmd) Spec() cli.CommandSpec {
@@ -53,6 +54,7 @@ func (c *rootCmd) RegisterFlags(fl *flag.FlagSet) {
 	fl.BoolVar(&c.skipSync, "skipsync", false, "skip syncing local settings and extensions to remote host")
 	fl.BoolVar(&c.syncBack, "b", false, "sync extensions back on termination")
 	fl.BoolVar(&c.printVersion, "version", false, "print version information and exit")
+	fl.BoolVar(&c.noReuseConnection, "no-reuse-connection", false, "do not reuse SSH connection via control socket")
 	fl.StringVar(&c.bindAddr, "bind", "", "local bind address for SSH tunnel, in [HOST][:PORT] syntax (default: 127.0.0.1)")
 	fl.StringVar(&c.sshFlags, "ssh-flags", "", "custom SSH flags")
 }
@@ -76,10 +78,11 @@ func (c *rootCmd) Run(fl *flag.FlagSet) {
 	}
 
 	err := sshCode(host, dir, options{
-		skipSync: c.skipSync,
-		sshFlags: c.sshFlags,
-		bindAddr: c.bindAddr,
-		syncBack: c.syncBack,
+		skipSync:        c.skipSync,
+		sshFlags:        c.sshFlags,
+		bindAddr:        c.bindAddr,
+		syncBack:        c.syncBack,
+		reuseConnection: !c.noReuseConnection,
 	})
 
 	if err != nil {
@@ -101,7 +104,7 @@ Environment variables:
 More info: https://github.com/cdr/sshcode
 
 Arguments:
-%vHOST is passed into the ssh command. Valid formats are '<ip-address>' or 'gcp:<instance-name>'. 
+%vHOST is passed into the ssh command. Valid formats are '<ip-address>' or 'gcp:<instance-name>'.
 %vDIR is optional.`,
 		helpTab, vsCodeConfigDirEnv,
 		helpTab, vsCodeExtensionsDirEnv,

sshcode.go

@@ -12,6 +12,7 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/pkg/browser"
@@ -21,18 +22,23 @@ import (
 
 const codeServerPath = "~/.cache/sshcode/sshcode-server"
 
+const (
+	sshDirectory               = "~/.ssh"
+	sshDirectoryUnsafeModeMask = 0022
+	sshControlPath             = sshDirectory + "/control-%h-%p-%r"
+)
+
 type options struct {
-	skipSync   bool
-	syncBack   bool
-	noOpen     bool
-	bindAddr   string
-	remotePort string
-	sshFlags   string
+	skipSync        bool
+	syncBack        bool
+	noOpen          bool
+	reuseConnection bool
+	bindAddr        string
+	remotePort      string
+	sshFlags        string
 }
 
 func sshCode(host, dir string, o options) error {
-	flog.Info("ensuring code-server is updated...")
-
 	host, extraSSHFlags, err := parseHost(host)
 	if err != nil {
 		return xerrors.Errorf("failed to parse host IP: %w", err)
@@ -53,6 +59,24 @@ func sshCode(host, dir string, o options) error {
 		return xerrors.Errorf("failed to find available remote port: %w", err)
 	}
 
+	// Check the SSH directory's permissions and warn the user if it is not safe.
+	o.reuseConnection = checkSSHDirectory(sshDirectory, o.reuseConnection)
+
+	// Start SSH master connection socket. This prevents multiple password prompts from appearing as authentication
+	// only happens on the initial connection.
+	if o.reuseConnection {
+		flog.Info("starting SSH master connection...")
+		newSSHFlags, cancel, err := startSSHMaster(o.sshFlags, sshControlPath, host)
+		defer cancel()
+		if err != nil {
+			flog.Error("failed to start SSH master connection: %v", err)
+			o.reuseConnection = false
+		} else {
+			o.sshFlags = newSSHFlags
+		}
+	}
+
+	flog.Info("ensuring code-server is updated...")
 	dlScript := downloadScript(codeServerPath)
 
 	// Downloads the latest code-server and allows it to be executed.
@@ -147,8 +171,8 @@ func sshCode(host, dir string, o options) error {
 	case <-c:
 	}
 
+	flog.Info("shutting down")
 	if !o.syncBack || o.skipSync {
-		flog.Info("shutting down")
 		return nil
 	}
 
@@ -167,6 +191,24 @@ func sshCode(host, dir string, o options) error {
 	return nil
 }
 
+// expandPath returns an expanded version of path.
+func expandPath(path string) string {
+	path = filepath.Clean(os.ExpandEnv(path))
+
+	// Replace tilde notation in path with the home directory. You can't replace the first instance of `~` in the
+	// string with the homedir as having a tilde in the middle of a filename is valid.
+	homedir := os.Getenv("HOME")
+	if homedir != "" {
+		if path == "~" {
+			path = homedir
+		} else if strings.HasPrefix(path, "~/") {
+			path = filepath.Join(homedir, path[2:])
+		}
+	}
+
+	return filepath.Clean(path)
+}
+
 func parseBindAddr(bindAddr string) (string, error) {
 	if !strings.Contains(bindAddr, ":") {
 		bindAddr += ":"
@@ -263,6 +305,100 @@ func randomPort() (string, error) {
 	return "", xerrors.Errorf("max number of tries exceeded: %d", maxTries)
 }
 
+// checkSSHDirectory performs sanity and safety checks on sshDirectory, and
+// returns a new value for o.reuseConnection depending on the checks.
+func checkSSHDirectory(sshDirectory string, reuseConnection bool) bool {
+	sshDirectoryMode, err := os.Lstat(expandPath(sshDirectory))
+	if err != nil {
+		if reuseConnection {
+			flog.Info("failed to stat %v directory, disabling connection reuse feature: %v", sshDirectory, err)
+		}
+		reuseConnection = false
+	} else {
+		if !sshDirectoryMode.IsDir() {
+			if reuseConnection {
+				flog.Info("%v is not a directory, disabling connection reuse feature", sshDirectory)
+			} else {
+				flog.Info("warning: %v is not a directory", sshDirectory)
+			}
+			reuseConnection = false
+		}
+		if sshDirectoryMode.Mode().Perm()&sshDirectoryUnsafeModeMask != 0 {
+			flog.Info("warning: the %v directory has unsafe permissions, they should only be writable by "+
+				"the owner (and files inside should be set to 0600)", sshDirectory)
+		}
+	}
+	return reuseConnection
+}
+
+// startSSHMaster starts an SSH master connection and waits for it to be ready.
+// It returns a new set of SSH flags for child SSH processes to use.
+func startSSHMaster(sshFlags string, sshControlPath string, host string) (string, func(), error) {
+	ctx, cancel := context.WithCancel(context.Background())
+
+	newSSHFlags := fmt.Sprintf(`%v -o "ControlPath=%v"`, sshFlags, sshControlPath)
+
+	// -MN means "start a master socket and don't open a session, just connect".
+	sshCmdStr := fmt.Sprintf(`exec ssh %v -MNq %v`, newSSHFlags, host)
+	sshMasterCmd := exec.CommandContext(ctx, "sh", "-c", sshCmdStr)
+	sshMasterCmd.Stdin = os.Stdin
+	sshMasterCmd.Stderr = os.Stderr
+
+	// Gracefully stop the SSH master.
+	stopSSHMaster := func() {
+		if sshMasterCmd.Process != nil {
+			if sshMasterCmd.ProcessState != nil && sshMasterCmd.ProcessState.Exited() {
+				return
+			}
+			err := sshMasterCmd.Process.Signal(syscall.SIGTERM)
+			if err != nil {
+				flog.Error("failed to send SIGTERM to SSH master process: %v", err)
+			}
+		}
+		cancel()
+	}
+
+	// Start ssh master and wait. Waiting prevents the process from becoming a zombie process if it dies before
+	// sshcode does, and allows sshMasterCmd.ProcessState to be populated.
+	err := sshMasterCmd.Start()
+	go sshMasterCmd.Wait()
+	if err != nil {
+		return "", stopSSHMaster, err
+	}
+	err = checkSSHMaster(sshMasterCmd, newSSHFlags, host)
+	if err != nil {
+		stopSSHMaster()
+		return "", stopSSHMaster, xerrors.Errorf("SSH master wasn't ready on time: %w", err)
+	}
+	return newSSHFlags, stopSSHMaster, nil
+}
+
+// checkSSHMaster polls every second for 30 seconds to check if the SSH master
+// is ready.
+func checkSSHMaster(sshMasterCmd *exec.Cmd, sshFlags string, host string) error {
+	var (
+		maxTries = 30
+		sleepDur = time.Second
+		err      error
+	)
+	for i := 0; i < maxTries; i++ {
+		// Check if the master is running.
+		if sshMasterCmd.Process == nil || (sshMasterCmd.ProcessState != nil && sshMasterCmd.ProcessState.Exited()) {
+			return xerrors.Errorf("SSH master process is not running")
+		}
+
+		// Check if it's ready.
+		sshCmdStr := fmt.Sprintf(`ssh %v -O check %v`, sshFlags, host)
+		sshCmd := exec.Command("sh", "-c", sshCmdStr)
+		err = sshCmd.Run()
+		if err == nil {
+			return nil
+		}
+		time.Sleep(sleepDur)
+	}
+	return xerrors.Errorf("max number of tries exceeded: %d", maxTries)
+}
+
 func syncUserSettings(sshFlags string, host string, back bool) error {
 	localConfDir, err := configDir()
 	if err != nil {