From 78efdc6d6b283efeab27fc1ed77b37d9b8339e84 Mon Sep 17 00:00:00 2001
From: Brian Brazil <brian.brazil@robustperception.io>
Date: Mon, 18 Jun 2018 17:34:08 +0100
Subject: [PATCH] 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      |  6 +++---
 storage/fanout_test.go | 15 +++++++++++++++
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/storage/fanout.go b/storage/fanout.go
index 3282871597..3ab994391c 100644
--- a/storage/fanout.go
+++ b/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
 		}
 
diff --git a/storage/fanout_test.go b/storage/fanout_test.go
index f6298c859a..3328673efb 100644
--- a/storage/fanout_test.go
+++ b/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