Fix off by one error in concreteSeriesSet (#3262)

storage/remote/read.go

@@ -151,11 +151,11 @@ type concreteSeriesSet struct {
 
 func (c *concreteSeriesSet) Next() bool {
 	c.cur++
-	return c.cur < len(c.series)
+	return c.cur-1 < len(c.series)
 }
 
 func (c *concreteSeriesSet) At() storage.Series {
-	return c.series[c.cur]
+	return c.series[c.cur-1]
 }
 
 func (c *concreteSeriesSet) Err() error {

storage/remote/read_test.go

@@ -20,6 +20,8 @@ import (
 
 	"github.com/prometheus/common/model"
 	"github.com/prometheus/prometheus/pkg/labels"
+	"github.com/prometheus/prometheus/prompb"
+	"github.com/prometheus/prometheus/storage"
 )
 
 func mustNewLabelMatcher(mt labels.MatchType, name, val string) *labels.Matcher {
@@ -92,3 +94,32 @@ func TestAddExternalLabels(t *testing.T) {
 		}
 	}
 }
+
+func TestConcreteSeriesSet(t *testing.T) {
+	series1 := &concreteSeries{
+		labels:  labels.FromStrings("foo", "bar"),
+		samples: []*prompb.Sample{&prompb.Sample{Value: 1, Timestamp: 2}},
+	}
+	series2 := &concreteSeries{
+		labels:  labels.FromStrings("foo", "baz"),
+		samples: []*prompb.Sample{&prompb.Sample{Value: 3, Timestamp: 4}},
+	}
+	c := &concreteSeriesSet{
+		series: []storage.Series{series1, series2},
+	}
+	if !c.Next() {
+		t.Fatalf("Expected Next() to be true.")
+	}
+	if c.At() != series1 {
+		t.Fatalf("Unexpected series returned.")
+	}
+	if !c.Next() {
+		t.Fatalf("Expected Next() to be true.")
+	}
+	if c.At() != series2 {
+		t.Fatalf("Unexpected series returned.")
+	}
+	if c.Next() {
+		t.Fatalf("Expected Next() to be false.")
+	}
+}