erasure-readfile: Use chunk size to read from each disk for a block. (#1949)

A block of data is split into data chunk and each data chunk is
written to each disk.  Previously block size was used to read data
chunk which returns corrupted data.

This patch fixes the issue by reading chunk sized data from each disk
and assembles a block.

Fixes #1939

erasure-readfile.go

@@ -43,6 +43,9 @@ func erasureReadFile(writer io.Writer, disks []StorageAPI, volume string, path s
 	// Get block info for given offset, length and block size.
 	startBlock, bytesToSkip, endBlock := getBlockInfo(offset, length, eInfo.BlockSize)
 
+	// Data chunk size on each block.
+	chunkSize := eInfo.BlockSize / int64(eInfo.DataBlocks)
+
 	for block := startBlock; block <= endBlock; block++ {
 		// Allocate encoded blocks up to storage disks.
 		enBlocks := make([][]byte, len(disks))
@@ -52,7 +55,7 @@ func erasureReadFile(writer io.Writer, disks []StorageAPI, volume string, path s
 		var noReconstruct bool // Set for no reconstruction.
 
 		// Keep how many bytes are read for this block.
-		// In most cases, last block in the file is shorter than eInfo.BlockSize.
+		// In most cases, last block in the file is shorter than chunkSize
 		lastReadSize := int64(0)
 
 		// Read from all the disks.
@@ -66,14 +69,14 @@ func erasureReadFile(writer io.Writer, disks []StorageAPI, volume string, path s
 			}
 
 			// Initialize chunk slice and fill the data from each parts.
-			enBlocks[blockIndex] = make([]byte, eInfo.BlockSize)
+			enBlocks[blockIndex] = make([]byte, chunkSize)
 
 			// Read the necessary blocks.
-			n, err := disk.ReadFile(volume, path, block*eInfo.BlockSize, enBlocks[blockIndex])
+			n, err := disk.ReadFile(volume, path, block*chunkSize, enBlocks[blockIndex])
 			if err != nil {
 				enBlocks[blockIndex] = nil
-			} else if n < eInfo.BlockSize {
+			} else if n < chunkSize {
-				// As the data we got is smaller than eInfo.BlockSize, keep only required chunk slice
+				// As the data we got is smaller than chunk size, keep only required chunk slice
 				enBlocks[blockIndex] = append([]byte{}, enBlocks[blockIndex][:n]...)
 			}