cache generalization

make LRU pinning API no-op for non-pinned caches
make sure all operations on zero-size cache are sane

common/cache/lru.go

@@ -161,6 +161,9 @@ func NewLRUWithInitialCapacity(initialCapacity, maxSize int) Cache {
 
 // Get retrieves the value stored under the given key
 func (c *lru) Get(key interface{}) interface{} {
+	if c.maxSize == 0 { //
+		return nil
+	}
 	c.mut.Lock()
 	defer c.mut.Unlock()
 
@@ -210,6 +213,9 @@ func (c *lru) PutIfNotExist(key interface{}, value interface{}) (interface{}, er
 
 // Delete deletes a key, value pair associated with a key
 func (c *lru) Delete(key interface{}) {
+	if c.maxSize == 0 {
+		return
+	}
 	c.mut.Lock()
 	defer c.mut.Unlock()
 
@@ -221,7 +227,7 @@ func (c *lru) Delete(key interface{}) {
 
 // Release decrements the ref count of a pinned element.
 func (c *lru) Release(key interface{}) {
-	if !c.pin {
+	if c.maxSize == 0 || !c.pin {
 		return
 	}
 	c.mut.Lock()
@@ -244,8 +250,11 @@ func (c *lru) Size() int {
 }
 
 // Put puts a new value associated with a given key, returning the existing value (if present)
-// allowUpdate flag is used to control overwrite behavior if the value exists
+// allowUpdate flag is used to control overwrite behavior if the value exists.
 func (c *lru) putInternal(key interface{}, value interface{}, allowUpdate bool) (interface{}, error) {
+	if c.maxSize == 0 {
+		return nil, nil
+	}
 	c.mut.Lock()
 	defer c.mut.Unlock()
 

common/cache/lru_test.go

@@ -303,3 +303,20 @@ func TestIterator(t *testing.T) {
 	it.Close()
 	assert.Equal(t, expected, actual)
 }
+
+func TestZeroSizeCache(t *testing.T) {
+	cache := NewLRU(0)
+	_, err := cache.PutIfNotExist("A", t)
+	assert.NoError(t, err)
+	assert.Equal(t, nil, cache.Get("A"))
+	assert.Equal(t, 0, cache.Size())
+	it := cache.Iterator()
+	assert.False(t, it.HasNext())
+	it.Close()
+	cache.Release("A")
+	cache.Delete("A")
+	v, err := cache.PutIfNotExist("A", t)
+	assert.Equal(t, v, t)
+	assert.Nil(t, err)
+	assert.Equal(t, 0, cache.Size())
+}