sftp: Prompt for password, don't terminate on SIGINT

This is a follow-up on fb9729fdb9, which
runs the `ssh` in its own process group and selects that process group
as the foreground group. After the sftp connection is established,
restic switches back to the previous foreground process group.

This allows `ssh` to prompt for the password, but it won't receive
the interrupt signal (SIGINT, ^C) later on, because it is not in the
foreground process group any more, allowing a clean tear down.

internal/backend/sftp/foreground_unix.go

@@ -0,0 +1,73 @@
+// +build !windows
+
+package sftp
+
+import (
+	"os"
+	"os/exec"
+	"os/signal"
+	"syscall"
+	"unsafe"
+
+	"github.com/restic/restic/internal/errors"
+)
+
+func tcsetpgrp(fd int, pid int) error {
+	_, _, errno := syscall.RawSyscall(syscall.SYS_IOCTL, uintptr(fd),
+		uintptr(syscall.TIOCSPGRP), uintptr(unsafe.Pointer(&pid)))
+	if errno == 0 {
+		return nil
+	}
+
+	return errno
+}
+
+// startForeground runs cmd in the foreground, by temporarily switching to the
+// new process group created for cmd. The returned function `bg` switches back
+// to the previous process group.
+func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
+	// open the TTY, we need the file descriptor
+	tty, err := os.OpenFile("/dev/tty", os.O_RDWR, 0)
+	if err != nil {
+		return nil, errors.Wrap(err, "open TTY")
+	}
+
+	signal.Ignore(syscall.SIGTTIN)
+	signal.Ignore(syscall.SIGTTOU)
+
+	// run the command in its own process group
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Setpgid: true,
+	}
+
+	// start the process
+	err = cmd.Start()
+	if err != nil {
+		_ = tty.Close()
+		return nil, errors.Wrap(err, "cmd.Start")
+	}
+
+	// move the command's process group into the foreground
+	prev := syscall.Getpgrp()
+	err = tcsetpgrp(int(tty.Fd()), cmd.Process.Pid)
+	if err != nil {
+		_ = tty.Close()
+		return nil, err
+	}
+
+	bg = func() error {
+		signal.Reset(syscall.SIGTTIN)
+		signal.Reset(syscall.SIGTTOU)
+
+		// reset the foreground process group
+		err = tcsetpgrp(int(tty.Fd()), prev)
+		if err != nil {
+			_ = tty.Close()
+			return err
+		}
+
+		return tty.Close()
+	}
+
+	return bg, nil
+}

internal/backend/sftp/foreground_windows.go

@@ -0,0 +1,21 @@
+package sftp
+
+import (
+	"os/exec"
+
+	"github.com/restic/restic/internal/errors"
+)
+
+// startForeground runs cmd in the foreground, by temporarily switching to the
+// new process group created for cmd. The returned function `bg` switches back
+// to the previous process group.
+func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
+	// just start the process and hope for the best
+	err = cmd.Start()
+	if err != nil {
+		return nil, errors.Wrap(err, "cmd.Start")
+	}
+
+	bg = func() error { return nil }
+	return bg, nil
+}

internal/backend/sftp/sftp.go

@@ -69,8 +69,8 @@ func startClient(preExec, postExec func(), program string, args ...string) (*SFT
 		preExec()
 	}
 
-	// start the process
-	if err := cmd.Start(); err != nil {
+	bg, err := startForeground(cmd)
+	if err != nil {
 		return nil, errors.Wrap(err, "cmd.Start")
 	}
 
@@ -92,6 +92,11 @@ func startClient(preExec, postExec func(), program string, args ...string) (*SFT
 		return nil, errors.Errorf("unable to start the sftp session, error: %v", err)
 	}
 
+	err = bg()
+	if err != nil {
+		return nil, errors.Wrap(err, "bg")
+	}
+
 	return &SFTP{c: client, cmd: cmd, result: ch}, nil
 }