backup: Fix shutdown hang when running in the background

On shutdown the backup commands waits for the terminal output goroutine
to stop. However while running in the background the goroutine ignored
the canceled context.

changelog/unreleased/pull-3152

@@ -0,0 +1,8 @@
+Bugfix: On Linux `backup` hangs during exit when run as a background job
+
+On Linux, when running in the background restic failed to stop the terminal
+output of the `backup` command after it had completed. This caused restic to
+hang until moved to the foreground. This has been fixed.
+
+https://github.com/restic/restic/pull/3152
+https://forum.restic.net/t/restic-alpine-container-cron-hangs-epoll-pwait/3334

internal/ui/termstatus/status.go

@@ -96,11 +96,9 @@ func (t *Terminal) run(ctx context.Context) {
 	for {
 		select {
 		case <-ctx.Done():
-			if IsProcessBackground(t.fd) {
-				// ignore all messages, do nothing, we are in the background process group
-				continue
+			if !IsProcessBackground(t.fd) {
+				t.undoStatus(len(status))
 			}
-			t.undoStatus(len(status))
 
 			return