Avoid infinite loop on duplicate NaN values. (#4275)

Fixes #4254

NaNs don't equal themselves, so a duplicate NaN would
always hit the break statement and never get popped.

We should not be returning multiple data point for the same
timestamp, so don't compare values at all.

Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>

storage/fanout.go

@@ -450,10 +450,10 @@ func (c *mergeIterator) Next() bool {
 		return false
 	}
 
-	currt, currv := c.At()
+	currt, _ := c.At()
 	for len(c.h) > 0 {
-		nextt, nextv := c.h[0].At()
-		if nextt != currt || nextv != currv {
+		nextt, _ := c.h[0].At()
+		if nextt != currt {
 			break
 		}
 

storage/fanout_test.go

@@ -15,6 +15,7 @@ package storage
 
 import (
 	"fmt"
+	"math"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -97,6 +98,16 @@ func TestMergeSeriesSet(t *testing.T) {
 				newMockSeries(labels.FromStrings("foo", "bar"), []sample{{0, 0}, {1, 1}, {2, 2}, {3, 3}}),
 			),
 		},
+		{
+			input: []SeriesSet{newMockSeriesSet(
+				newMockSeries(labels.FromStrings("foo", "bar"), []sample{{0, math.NaN()}}),
+			), newMockSeriesSet(
+				newMockSeries(labels.FromStrings("foo", "bar"), []sample{{0, math.NaN()}}),
+			)},
+			expected: newMockSeriesSet(
+				newMockSeries(labels.FromStrings("foo", "bar"), []sample{{0, math.NaN()}}),
+			),
+		},
 	} {
 		merged := NewMergeSeriesSet(tc.input)
 		for merged.Next() {
@@ -197,6 +208,10 @@ func drainSamples(iter SeriesIterator) []sample {
 	result := []sample{}
 	for iter.Next() {
 		t, v := iter.At()
+		// NaNs can't be compared normally, so substitute for another value.
+		if math.IsNaN(v) {
+			v = -42
+		}
 		result = append(result, sample{t, v})
 	}
 	return result