chunk cache adds function ReadChunkAt

weed/util/chunk_cache/chunk_cache_in_memory.go

@@ -1,9 +1,8 @@
 package chunk_cache
 
 import (
-	"time"
-
 	"github.com/karlseguin/ccache/v2"
+	"time"
 )
 
 // a global cache for recently accessed file chunks
@@ -45,6 +44,21 @@ func (c *ChunkCacheInMemory) getChunkSlice(fileId string, offset, length uint64)
 	return data[offset : int(offset)+wanted], nil
 }
 
+func (c *ChunkCacheInMemory) readChunkAt(buffer []byte, fileId string, offset uint64) (int, error) {
+	item := c.cache.Get(fileId)
+	if item == nil {
+		return 0, nil
+	}
+	data := item.Value().([]byte)
+	item.Extend(time.Hour)
+	wanted := min(len(buffer), len(data)-int(offset))
+	if wanted < 0 {
+		return 0, ErrorOutOfBounds
+	}
+	n := copy(buffer, data[offset:int(offset)+wanted])
+	return n, nil
+}
+
 func (c *ChunkCacheInMemory) SetChunk(fileId string, data []byte) {
 	localCopy := make([]byte, len(data))
 	copy(localCopy, data)