perf record: Fix poll return value propagation

If the perf record command is interrupted in record__mmap_read_all
function, the 'done' is set and err has the latest poll return
value, which is most likely positive number (= number of pollfds
ready to read).

This 'positive err' is then propagated to the exit code, resulting
in not finishing the perf.data header properly, causing following
error in report:

  # perf record -F 50000 -a

  ---
  make the system real busy, so there's more chance
  to interrupt perf in event writing code
  ---

  ^C[ perf record: Woken up 16 times to write data ]
  [ perf record: Captured and wrote 30.292 MB perf.data (~1323468 samples) ]

  # perf report --stdio > /dev/null
  WARNING: The perf.data file's data size field is 0 which is unexpected.
  Was the 'perf record' command properly terminated?

Fixing this by checking for positive poll return value
and setting err to 0.
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1401732126-19465-1-git-send-email-jolsa@kernel.orgSigned-off-by: Jiri Olsa <jolsa@kernel.org>

tools/perf/builtin-record.c

@@ -454,7 +454,11 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
 			if (done)
 				break;
 			err = poll(rec->evlist->pollfd, rec->evlist->nr_fds, -1);
-			if (err < 0 && errno == EINTR)
+			/*
+			 * Propagate error, only if there's any. Ignore positive
+			 * number of returned events and interrupt error.
+			 */
+			if (err > 0 || (err < 0 && errno == EINTR))
 				err = 0;
 			waking++;
 		}