Fix IsProcessBackground on Linux with stdin redirection

The previous implementation assumed that stdin was a terminal.
It now checks the terminal's fd.

internal/ui/termstatus/background.go

@@ -4,6 +4,6 @@ package termstatus
 
 // IsProcessBackground reports whether the current process is running in the
 // background. Not implemented for this platform.
-func IsProcessBackground() bool {
+func IsProcessBackground(uintptr) bool {
 	return false
 }

internal/ui/termstatus/background_linux.go

@@ -1,21 +1,23 @@
 package termstatus
 
 import (
-	"syscall"
-	"unsafe"
-
 	"github.com/restic/restic/internal/debug"
+
+	"golang.org/x/sys/unix"
 )
 
-// IsProcessBackground reports whether the current process is running in the background.
-func IsProcessBackground() bool {
-	var pid int
-	_, _, err := syscall.Syscall(syscall.SYS_IOCTL, uintptr(syscall.Stdin), syscall.TIOCGPGRP, uintptr(unsafe.Pointer(&pid)))
-
-	if err != 0 {
+// IsProcessBackground reports whether the current process is running in the
+// background. fd must be a file descriptor for the terminal.
+func IsProcessBackground(fd uintptr) bool {
+	bg, err := isProcessBackground(fd)
+	if err != nil {
 		debug.Log("Can't check if we are in the background. Using default behaviour. Error: %s\n", err.Error())
 		return false
 	}
-
-	return pid != syscall.Getpgrp()
+	return bg
+}
+
+func isProcessBackground(fd uintptr) (bool, error) {
+	pid, err := unix.IoctlGetInt(int(fd), unix.TIOCGPGRP)
+	return pid != unix.Getpgrp(), err
 }

internal/ui/termstatus/background_linux_test.go

@@ -0,0 +1,19 @@
+package termstatus
+
+import (
+	"os"
+	"testing"
+
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestIsProcessBackground(t *testing.T) {
+	tty, err := os.Open("/dev/tty")
+	if err != nil {
+		t.Skipf("can't open terminal: %v", err)
+	}
+	defer tty.Close()
+
+	_, err = isProcessBackground(tty.Fd())
+	rtest.OK(t, err)
+}

internal/ui/termstatus/status.go

@@ -95,7 +95,7 @@ func (t *Terminal) run(ctx context.Context) {
 	for {
 		select {
 		case <-ctx.Done():
-			if IsProcessBackground() {
+			if IsProcessBackground(t.fd) {
 				// ignore all messages, do nothing, we are in the background process group
 				continue
 			}
@@ -104,7 +104,7 @@ func (t *Terminal) run(ctx context.Context) {
 			return
 
 		case msg := <-t.msg:
-			if IsProcessBackground() {
+			if IsProcessBackground(t.fd) {
 				// ignore all messages, do nothing, we are in the background process group
 				continue
 			}
@@ -136,7 +136,7 @@ func (t *Terminal) run(ctx context.Context) {
 			}
 
 		case stat := <-t.status:
-			if IsProcessBackground() {
+			if IsProcessBackground(t.fd) {
 				// ignore all messages, do nothing, we are in the background process group
 				continue
 			}