bug fix in the data received from cache processing (#6002)

The patch addresses #3745.

The cache should return the exact amount of data requested by the buffer.
By construction of the cache it is always all requested data range
or we have error happening.

The old use of minsize miscalculate the requested data size,
if non zero offset is requested.

weed/util/chunk_cache/chunk_cache.go

@@ -57,7 +57,7 @@ func (c *TieredChunkCache) ReadChunkAt(data []byte, fileId string, offset uint64
 		if err != nil {
 			glog.Errorf("failed to read from memcache: %s", err)
 		}
-		if n >= int(minSize) {
+		if n == int(len(data)) {
 			return n, nil
 		}
 	}
@@ -65,24 +65,24 @@ func (c *TieredChunkCache) ReadChunkAt(data []byte, fileId string, offset uint64
 	fid, err := needle.ParseFileIdFromString(fileId)
 	if err != nil {
 		glog.Errorf("failed to parse file id %s", fileId)
-		return n, nil
+		return 0, nil
 	}
 
 	if minSize <= c.onDiskCacheSizeLimit0 {
 		n, err = c.diskCaches[0].readChunkAt(data, fid.Key, offset)
-		if n >= int(minSize) {
+		if n == int(len(data)) {
 			return
 		}
 	}
 	if minSize <= c.onDiskCacheSizeLimit1 {
 		n, err = c.diskCaches[1].readChunkAt(data, fid.Key, offset)
-		if n >= int(minSize) {
+		if n == int(len(data)) {
 			return
 		}
 	}
 	{
 		n, err = c.diskCaches[2].readChunkAt(data, fid.Key, offset)
-		if n >= int(minSize) {
+		if n == int(len(data)) {
 			return
 		}
 	}