perf/live-mode: Handle payload-less events

Some events, such as the PERF_RECORD_FINISHED_ROUND event consist of
only an event header and no data.  In this case, a 0-length payload
will be read, and the 0 return value will be wrongly interpreted as an
'unexpected end of event stream'.

This patch allows for proper handling of data-less events by skipping
0-length reads.
Signed-off-by: Tom Zanussi <tzanussi@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Masami Hiramatsu <mhiramat@redhat.com>
LKML-Reference: <1273038527.6383.51.camel@tropicana>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

tools/perf/util/session.c

@@ -696,7 +696,9 @@ static int __perf_session__process_pipe_events(struct perf_session *self,
 	p = &event;
 	p += sizeof(struct perf_event_header);
 
-	err = do_read(self->fd, p, size - sizeof(struct perf_event_header));
+	if (size - sizeof(struct perf_event_header)) {
+		err = do_read(self->fd, p,
+			      size - sizeof(struct perf_event_header));
 		if (err <= 0) {
 			if (err == 0) {
 				pr_err("unexpected end of event stream\n");
@@ -706,6 +708,7 @@ static int __perf_session__process_pipe_events(struct perf_session *self,
 			pr_err("failed to read event data\n");
 			goto out_err;
 		}
+	}
 
 	if (size == 0 ||
 	    (skip = perf_session__process_event(self, &event, ops,