fix: Restore empty files when healing (#5257)

HealFile() does not process the case when an empty file is lost in
some disks. Since, Reedsolomon erasure doesn't handle restoring empty
data, HealFile will create empty files similarly to CreateFile().

cmd/erasure-healfile.go

@@ -78,12 +78,14 @@ func (s ErasureStorage) HealFile(staleDisks []StorageAPI, volume, path string, b
 		blocks[i] = make([]byte, chunksize)
 	}
 	var chunkOffset, blockOffset int64
-	for ; blockOffset < size; blockOffset += blocksize {
+
+	// The for loop below is entered when size == 0 and
+	// blockOffset == 0 to allow for reconstructing empty files.
+	for ; blockOffset == 0 || blockOffset < size; blockOffset += blocksize {
 		// last iteration may have less than blocksize data
 		// left, so chunksize needs to be recomputed.
 		if size < blockOffset+blocksize {
-			blocksize = size - blockOffset
-			chunksize = getChunkSize(blocksize, s.dataBlocks)
+			chunksize = getChunkSize(size-blockOffset, s.dataBlocks)
 			for i := range blocks {
 				blocks[i] = blocks[i][:chunksize]
 			}
@@ -121,10 +123,14 @@ func (s ErasureStorage) HealFile(staleDisks []StorageAPI, volume, path string, b
 		// iteration
 		chunkOffset += chunksize
 
-		// reconstruct data - this computes all data and parity shards
+		// reconstruct data - this computes all data and
+		// parity shards - but we skip this step if we are
+		// reconstructing an empty file.
+		if chunksize > 0 {
 			if err = s.ErasureDecodeDataAndParityBlocks(blocks); err != nil {
 				return f, err
 			}
+		}
 
 		// write computed shards as chunks on file in each
 		// stale disk