lib/cache: add PutErr to put a value with an error into the cache

lib/cache/cache.go

@@ -124,8 +124,8 @@ func (c *Cache) Unpin(key string) {
 	c.addPin(key, -1)
 }
 
-// Put puts a value named key into the cache
-func (c *Cache) Put(key string, value interface{}) {
+// PutErr puts a value named key with err into the cache
+func (c *Cache) PutErr(key string, value interface{}, err error) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	if c.noCache() {
@@ -134,11 +134,17 @@ func (c *Cache) Put(key string, value interface{}) {
 	entry := &cacheEntry{
 		value: value,
 		key:   key,
+		err:   err,
 	}
 	c.used(entry)
 	c.cache[key] = entry
 }
 
+// Put puts a value named key into the cache
+func (c *Cache) Put(key string, value interface{}) {
+	c.PutErr(key, value, nil)
+}
+
 // GetMaybe returns the key and true if found, nil and false if not
 func (c *Cache) GetMaybe(key string) (value interface{}, found bool) {
 	c.mu.Lock()