Fix MemcachedClient Get() function (#500)

* Fix MemcacheClient get function

When there was a cache miss after Get() the item was nil and we were
using its key attribute to wrap an error with more information.

This was wrong and now the error is using the input key string.

Also added a unit test for this scenario.

* fix go imports

pkg/chunk/cache/memcached_client.go

@@ -205,7 +205,7 @@ func (c *MemcachedClient) Delete(key string) error {
 func (c *MemcachedClient) Get(key string) (*memcache.Item, error) {
 	item, err := c.client.Get(key)
 	if err != nil {
-		return nil, c.wrapErrWithServer(item.Key, err)
+		return nil, c.wrapErrWithServer(key, err)
 	}
 	return item, nil
 }

pkg/chunk/cache/memcached_client_test.go

@@ -7,8 +7,15 @@ package cache_test
 
 import (
 	"sync"
+	"testing"
+	"time"
 
 	"github.com/bradfitz/gomemcache/memcache"
+	"github.com/go-kit/log"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/stretchr/testify/assert"
+
+	"github.com/grafana/mimir/pkg/chunk/cache"
 )
 
 type mockMemcachedBasicClient struct {
@@ -42,3 +49,18 @@ func (m *mockMemcachedBasicClient) Set(item *memcache.Item) error {
 	m.contents[item.Key] = item.Value
 	return nil
 }
+
+func TestMemcachedClient_Get_Error(t *testing.T) {
+	m := cache.NewMemcachedClient(
+		cache.MemcachedClientConfig{
+			UpdateInterval: time.Second,
+		},
+		"test",
+		prometheus.NewRegistry(),
+		log.NewNopLogger(),
+	)
+	t.Cleanup(m.Stop)
+	item, err := m.Get("foo")
+	assert.Error(t, err)
+	assert.Nil(t, item)
+}