backend: move backend implementation helpers to util package

This removes code that is only used within a backend implementation from
the backend package. The latter now only contains code that also has
external users.

internal/backend/util/foreground_unix.go

@@ -0,0 +1,72 @@
+//go:build !aix && !solaris && !windows
+// +build !aix,!solaris,!windows
+
+package util
+
+import (
+	"os"
+	"os/exec"
+	"os/signal"
+
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/errors"
+
+	"golang.org/x/sys/unix"
+)
+
+func tcsetpgrp(fd int, pid int) error {
+	// IoctlSetPointerInt silently casts to int32 internally,
+	// so this assumes pid fits in 31 bits.
+	return unix.IoctlSetPointerInt(fd, unix.TIOCSPGRP, pid)
+}
+
+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 {
+		debug.Log("unable to open tty: %v", err)
+		bg = func() error {
+			return nil
+		}
+		return bg, cmd.Start()
+	}
+
+	signal.Ignore(unix.SIGTTIN)
+	signal.Ignore(unix.SIGTTOU)
+
+	// run the command in its own process group
+	cmd.SysProcAttr = &unix.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 := unix.Getpgrp()
+	err = tcsetpgrp(int(tty.Fd()), cmd.Process.Pid)
+	if err != nil {
+		_ = tty.Close()
+		return nil, err
+	}
+
+	bg = func() error {
+		signal.Reset(unix.SIGTTIN)
+		signal.Reset(unix.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
+}