From 93886d84172503c30a58c60a90d0f3be3a9903c0 Mon Sep 17 00:00:00 2001
From: Callum Styan <callumstyan@gmail.com>
Date: Thu, 2 Sep 2021 11:08:05 -0700
Subject: [PATCH] Fix div by 0 panic is resize function. (#9286)

Signed-off-by: Callum Styan <callumstyan@gmail.com>
---
 tsdb/exemplar.go      |  2 +-
 tsdb/exemplar_test.go | 10 +++++++++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/tsdb/exemplar.go b/tsdb/exemplar.go
index c7cefaba0..1e09da21e 100644
--- a/tsdb/exemplar.go
+++ b/tsdb/exemplar.go
@@ -279,7 +279,7 @@ func (ce *CircularExemplarStorage) Resize(l int64) int {
 
 	migrated := 0
 
-	if l > 0 {
+	if l > 0 && len(oldBuffer) > 0 {
 		// Rewind previous next index by count with wrap-around.
 		// This math is essentially looking at nextIndex, where we would write the next exemplar to,
 		// and find the index in the old exemplar buffer that we should start migrating exemplars from.
diff --git a/tsdb/exemplar_test.go b/tsdb/exemplar_test.go
index 8fcaf6d2e..eb95daa34 100644
--- a/tsdb/exemplar_test.go
+++ b/tsdb/exemplar_test.go
@@ -413,7 +413,7 @@ func TestResize(t *testing.T) {
 			expectedMigrated:  50,
 		},
 		{
-			name:              "Zero",
+			name:              "ShrinkToZero",
 			startSize:         100,
 			newCount:          0,
 			expectedSeries:    []int{},
@@ -436,6 +436,14 @@ func TestResize(t *testing.T) {
 			notExpectedSeries: []int{},
 			expectedMigrated:  0,
 		},
+		{
+			name:              "GrowFromZero",
+			startSize:         0,
+			newCount:          10,
+			expectedSeries:    []int{},
+			notExpectedSeries: []int{},
+			expectedMigrated:  0,
+		},
 	}
 
 	for _, tc := range testCases {