fix cache logic

Author: roseduan

segment.go

@@ -328,7 +328,7 @@ func (seg *segment) readInternal(blockNumber uint32, chunkOffset int64) ([]byte,
 			// cache the block, so that the next time it can be read from the cache.
 			// if the block size is smaller than blockSize, it means that the block is not full,
 			// so we will not cache it.
-			if seg.cache != nil && size == blockSize {
+			if seg.cache != nil && size == blockSize && len(cachedBlock) == 0 {
 				cacheBlock := make([]byte, blockSize)
 				copy(cacheBlock, bh.block)
 				seg.cache.Add(seg.getCacheKey(blockNumber), cacheBlock)