From 30fc82852aa5a96881eacdd307886c478b0e57d7 Mon Sep 17 00:00:00 2001
From: Thomas Jackson <jacksontj.89@gmail.com>
Date: Mon, 20 Jul 2020 11:08:46 -0700
Subject: [PATCH] Handle empty result sets in merge logic

Fixes #329
---
 pkg/promhttputil/merge.go      |  7 ++++
 pkg/promhttputil/merge_test.go | 58 ++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+)

diff --git a/pkg/promhttputil/merge.go b/pkg/promhttputil/merge.go
index fb3b76eae..f1b11fa65 100644
--- a/pkg/promhttputil/merge.go
+++ b/pkg/promhttputil/merge.go
@@ -181,6 +181,13 @@ func MergeSampleStream(antiAffinityBuffer model.Time, a, b *model.SampleStream)
 		return nil, fmt.Errorf("cannot merge mismatch fingerprints")
 	}
 
+	// if either set of values are empty, return the one with data
+	if len(a.Values) == 0 {
+		return b, nil
+	} else if len(b.Values) == 0 {
+		return a, nil
+	}
+
 	// TODO: really there should be a library method for this in prometheus IMO
 	// At this point we have 2 sorted lists of datapoints which we need to merge
 	newValues := make([]model.SamplePair, 0, len(a.Values))
diff --git a/pkg/promhttputil/merge_test.go b/pkg/promhttputil/merge_test.go
index c76a3a9b8..50ac9b56c 100644
--- a/pkg/promhttputil/merge_test.go
+++ b/pkg/promhttputil/merge_test.go
@@ -509,6 +509,64 @@ func TestMergeValues(t *testing.T) {
 			}),
 			antiAffinity: model.Time(2),
 		},
+
+		{
+			name: "Matrix merge empty A",
+			a: model.Matrix([]*model.SampleStream{
+				{
+					model.Metric(model.LabelSet{model.MetricNameLabel: model.LabelValue("hosta")}),
+					[]model.SamplePair{},
+				},
+			}),
+			b: model.Matrix([]*model.SampleStream{
+				{
+					model.Metric(model.LabelSet{model.MetricNameLabel: model.LabelValue("hosta")}),
+					[]model.SamplePair{{
+						model.Time(100),
+						model.SampleValue(10),
+					}},
+				},
+			}),
+			r: model.Matrix([]*model.SampleStream{
+				{
+					model.Metric(model.LabelSet{model.MetricNameLabel: model.LabelValue("hosta")}),
+					[]model.SamplePair{{
+						model.Time(100),
+						model.SampleValue(10),
+					}},
+				},
+			}),
+			antiAffinity: model.Time(2),
+		},
+
+		{
+			name: "Matrix merge empty A",
+			a: model.Matrix([]*model.SampleStream{
+				{
+					model.Metric(model.LabelSet{model.MetricNameLabel: model.LabelValue("hosta")}),
+					[]model.SamplePair{{
+						model.Time(100),
+						model.SampleValue(10),
+					}},
+				},
+			}),
+			b: model.Matrix([]*model.SampleStream{
+				{
+					model.Metric(model.LabelSet{model.MetricNameLabel: model.LabelValue("hosta")}),
+					[]model.SamplePair{},
+				},
+			}),
+			r: model.Matrix([]*model.SampleStream{
+				{
+					model.Metric(model.LabelSet{model.MetricNameLabel: model.LabelValue("hosta")}),
+					[]model.SamplePair{{
+						model.Time(100),
+						model.SampleValue(10),
+					}},
+				},
+			}),
+			antiAffinity: model.Time(2),
+		},
 	}
 
 	for _, test := range tests {