Fixes tightloop that could happen in the engine with LogQL parser.

I've also added a test to prove the issue was happening and now is fixed.

Fixes #3275
Fixes #3264
Fixes #3020

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

pkg/logql/range_vector.go

@@ -119,6 +119,7 @@ func (r *rangeVectorIterator) load(start, end int64) {
 				var err error
 				metric, err = promql_parser.ParseMetric(lbs)
 				if err != nil {
+					_ = r.iter.Next()
 					continue
 				}
 				r.metrics[lbs] = metric

pkg/logql/range_vector_test.go

@@ -28,8 +28,10 @@ var samples = []logproto.Sample{
 	{Timestamp: time.Unix(100, 1).UnixNano(), Hash: 11, Value: 1.},
 }
 
-var labelFoo, _ = promql_parser.ParseMetric("{app=\"foo\"}")
-var labelBar, _ = promql_parser.ParseMetric("{app=\"bar\"}")
+var (
+	labelFoo, _ = promql_parser.ParseMetric("{app=\"foo\"}")
+	labelBar, _ = promql_parser.ParseMetric("{app=\"bar\"}")
+)
 
 func newSampleIterator() iter.SampleIterator {
 	return iter.NewHeapSampleIterator(context.Background(), []iter.SampleIterator{
@@ -163,3 +165,24 @@ func Test_RangeVectorIterator(t *testing.T) {
 			})
 	}
 }
+
+func Test_RangeVectorIteratorBadLabels(t *testing.T) {
+	badIterator := iter.NewPeekingSampleIterator(
+		iter.NewSeriesIterator(logproto.Series{
+			Labels:  "{badlabels=}",
+			Samples: samples,
+		}))
+	it := newRangeVectorIterator(badIterator, (30 * time.Second).Nanoseconds(),
+		(30 * time.Second).Nanoseconds(), time.Unix(10, 0).UnixNano(), time.Unix(100, 0).UnixNano())
+	ctx, cancel := context.WithCancel(context.Background())
+	go func() {
+		defer cancel()
+		for it.Next() {
+		}
+	}()
+	select {
+	case <-time.After(1 * time.Second):
+		require.Fail(t, "goroutine took too long")
+	case <-ctx.Done():
+	}
+}