sftp: Allow password entry

This was a bit tricky: We start the ssh binary, but we want it to ignore
SIGINT. In contrast, restic itself should process SIGINT and clean up
properly. Before, we used `setsid()` to give the ssh process its own
process group, but that means it cannot prompt the user for a password
because the tty is gone.

So, now we're passing in two functions that ignore SIGINT just before
the ssh process is started and re-install it after start.

cmd/restic/global.go

@@ -470,7 +470,7 @@ func open(s string, opts options.Options) (restic.Backend, error) {
 	case "local":
 		be, err = local.Open(cfg.(local.Config))
 	case "sftp":
-		be, err = sftp.Open(cfg.(sftp.Config))
+		be, err = sftp.Open(cfg.(sftp.Config), SuspendSignalHandler, InstallSignalHandler)
 	case "s3":
 		be, err = s3.Open(cfg.(s3.Config))
 	case "gs":
@@ -522,7 +522,7 @@ func create(s string, opts options.Options) (restic.Backend, error) {
 	case "local":
 		return local.Create(cfg.(local.Config))
 	case "sftp":
-		return sftp.Create(cfg.(sftp.Config))
+		return sftp.Create(cfg.(sftp.Config), SuspendSignalHandler, InstallSignalHandler)
 	case "s3":
 		return s3.Create(cfg.(s3.Config))
 	case "gs":

internal/backend/sftp/layout_test.go

@@ -46,7 +46,7 @@ func TestLayout(t *testing.T) {
 				Command: fmt.Sprintf("%q -e", sftpServer),
 				Path:    repo,
 				Layout:  test.layout,
-			})
+			}, nil, nil)
 			if err != nil {
 				t.Fatal(err)
 			}

internal/backend/sftp/sftp.go

@@ -36,7 +36,7 @@ var _ restic.Backend = &SFTP{}
 
 const defaultLayout = "default"
 
-func startClient(program string, args ...string) (*SFTP, error) {
+func startClient(preExec, postExec func(), program string, args ...string) (*SFTP, error) {
 	debug.Log("start client %v %v", program, args)
 	// Connect to a remote host and request the sftp subsystem via the 'ssh'
 	// command.  This assumes that passwordless login is correctly configured.
@@ -55,9 +55,6 @@ func startClient(program string, args ...string) (*SFTP, error) {
 		}
 	}()
 
-	// ignore signals sent to the parent (e.g. SIGINT)
-	cmd.SysProcAttr = ignoreSigIntProcAttr()
-
 	// get stdin and stdout
 	wr, err := cmd.StdinPipe()
 	if err != nil {
@@ -68,11 +65,19 @@ func startClient(program string, args ...string) (*SFTP, error) {
 		return nil, errors.Wrap(err, "cmd.StdoutPipe")
 	}
 
+	if preExec != nil {
+		preExec()
+	}
+
 	// start the process
 	if err := cmd.Start(); err != nil {
 		return nil, errors.Wrap(err, "cmd.Start")
 	}
 
+	if postExec != nil {
+		postExec()
+	}
+
 	// wait in a different goroutine
 	ch := make(chan error, 1)
 	go func() {
@@ -104,8 +109,9 @@ func (r *SFTP) clientError() error {
 }
 
 // Open opens an sftp backend as described by the config by running
-// "ssh" with the appropriate arguments (or cfg.Command, if set).
+// "ssh" with the appropriate arguments (or cfg.Command, if set). The function
-func Open(cfg Config) (*SFTP, error) {
+// preExec is run just before, postExec just after starting a program.
+func Open(cfg Config, preExec, postExec func()) (*SFTP, error) {
 	debug.Log("open backend with config %#v", cfg)
 
 	cmd, args, err := buildSSHCommand(cfg)
@@ -113,7 +119,7 @@ func Open(cfg Config) (*SFTP, error) {
 		return nil, err
 	}
 
-	sftp, err := startClient(cmd, args...)
+	sftp, err := startClient(preExec, postExec, cmd, args...)
 	if err != nil {
 		debug.Log("unable to start program: %v", err)
 		return nil, err
@@ -225,15 +231,16 @@ func buildSSHCommand(cfg Config) (cmd string, args []string, err error) {
 	return cmd, args, nil
 }
 
-// Create creates an sftp backend as described by the config by running
+// Create creates an sftp backend as described by the config by running "ssh"
-// "ssh" with the appropriate arguments (or cfg.Command, if set).
+// with the appropriate arguments (or cfg.Command, if set). The function
-func Create(cfg Config) (*SFTP, error) {
+// preExec is run just before, postExec just after starting a program.
+func Create(cfg Config, preExec, postExec func()) (*SFTP, error) {
 	cmd, args, err := buildSSHCommand(cfg)
 	if err != nil {
 		return nil, err
 	}
 
-	sftp, err := startClient(cmd, args...)
+	sftp, err := startClient(preExec, postExec, cmd, args...)
 	if err != nil {
 		debug.Log("unable to start program: %v", err)
 		return nil, err
@@ -261,7 +268,7 @@ func Create(cfg Config) (*SFTP, error) {
 	}
 
 	// open backend
-	return Open(cfg)
+	return Open(cfg, preExec, postExec)
 }
 
 // Location returns this backend's location (the directory name).

internal/backend/sftp/sftp_test.go

@@ -51,13 +51,13 @@ func newTestSuite(t testing.TB) *test.Suite {
 		// CreateFn is a function that creates a temporary repository for the tests.
 		Create: func(config interface{}) (restic.Backend, error) {
 			cfg := config.(sftp.Config)
-			return sftp.Create(cfg)
+			return sftp.Create(cfg, nil, nil)
 		},
 
 		// OpenFn is a function that opens a previously created temporary repository.
 		Open: func(config interface{}) (restic.Backend, error) {
 			cfg := config.(sftp.Config)
-			return sftp.Open(cfg)
+			return sftp.Open(cfg, nil, nil)
 		},
 
 		// CleanupFn removes data created during the tests.

internal/backend/sftp/sftp_unix.go

@@ -1,13 +0,0 @@
-// +build !windows
-
-package sftp
-
-import (
-	"syscall"
-)
-
-// ignoreSigIntProcAttr returns a syscall.SysProcAttr that
-// disables SIGINT on parent.
-func ignoreSigIntProcAttr() *syscall.SysProcAttr {
-	return &syscall.SysProcAttr{Setsid: true}
-}

internal/backend/sftp/sftp_windows.go

@@ -1,11 +0,0 @@
-package sftp
-
-import (
-	"syscall"
-)
-
-// ignoreSigIntProcAttr returns a default syscall.SysProcAttr
-// on Windows.
-func ignoreSigIntProcAttr() *syscall.SysProcAttr {
-	return &syscall.SysProcAttr{}
-}