pv/display: handle error of tcgetpgrp() in pv_in_foreground()

Show pv progress bar even if no terminal is set, e.g., in a busybox
init script. The description of pv_in_forground() states it will
return true "if we aren't outputting to a terminal". However, this
is not the case since tcgetpgrg() will return an error and set ERRNO
to ENOTTY if the output fd is not an tty. We now handle this error
correctly and pv_in_foreground() returns also true in that case.

Signed-off-by: Michael Weiß <michael.weiss@aisec.fraunhofer.de>

src/pv/display.c

@@ -48,6 +48,10 @@ bool pv_in_foreground(void)
 
 	our_process_group = getpgrp();
 	tty_process_group = tcgetpgrp(STDERR_FILENO);
+
+	if (tty_process_group == -1 && errno == ENOTTY)
+		return true;
+
 	if (our_process_group == tty_process_group)
 		return true;