perf metrics: Ensure at least 1 id per metric

We may have no events for a metric evaluated to a constant. In such a
case ensure a tool event is at least evaluated for metric parsing and
displaying.

Fixes: 8586d274 ("perf metrics: Don't add all tool events for sharing")
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.garry@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20220618013957.999321-1-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/metricgroup.c

@@ -1372,6 +1372,7 @@ static int parse_ids(bool metric_no_merge, struct perf_pmu *fake_pmu,
 
 	*out_evlist = NULL;
 	if (!metric_no_merge || hashmap__size(ids->ids) == 0) {
+		bool added_event = false;
 		int i;
 		/*
 		 * We may fail to share events between metrics because a tool
@@ -1393,8 +1394,16 @@ static int parse_ids(bool metric_no_merge, struct perf_pmu *fake_pmu,
 				if (!tmp)
 					return -ENOMEM;
 				ids__insert(ids->ids, tmp);
+				added_event = true;
 			}
 		}
+		if (!added_event && hashmap__size(ids->ids) == 0) {
+			char *tmp = strdup("duration_time");
+
+			if (!tmp)
+				return -ENOMEM;
+			ids__insert(ids->ids, tmp);
+		}
 	}
 	ret = metricgroup__build_event_string(&events, ids, modifier,
 					      has_constraint);