blktrace hangs with "-o -" if BLKTRACESETUP fails

I tried to sort through this, but got a bit lost. Possible solution at the
end of the email.

tl;dr: 

# blktrace -o - /dev/sda /dev/sdb /dev/sdc

has to be SIGKILLed if BLKTRACESETUP fails for any or all of the devices
listed. (I simulated this by just catching one of the devices in
setup_buts(), skipping the ioctl, and doing ret++).

This seems to be because with "-o -" on the command line, use_tracer_devpaths()
sees piped_output set, so we call process_trace_bufs which ends up waiting on
(!done) and "done" is never set. i.e.

atexit(exit_tracing)
	wait_tracers
		if (use_tracer_devpaths()) // true because "-o -"
			process_trace_bufs
				while (wait_empty_entries())
					wait_empty_entries
						while (!done ... )
							<loop forever>

I think this can be avoided by just setting "done = 1" before returning
when setup_buts() fails in run_tracers(), but I'm not sure if that's the
right approach. Thoughts?

Thanks,
-Eric

diff --git a/blktrace.c b/blktrace.c
index 3444fbb..038b2cb 100644
--- a/blktrace.c
+++ b/blktrace.c
@@ -2684,8 +2684,10 @@ static int run_tracers(void)
 	if (net_mode == Net_client)
 		printf("blktrace: connecting to %s\n", hostname);
 
-	if (setup_buts())
+	if (setup_buts()) {
+		done = 1;
 		return 1;
+	}
 
 	if (use_tracer_devpaths()) {
 		if (setup_tracer_devpaths())