fs/erasure: Rename meta 'md5Sum' as 'etag'. (#4319)

This PR also does backend format change to 1.0.1
from 1.0.0.  Backward compatible changes are still
kept to read the 'md5Sum' key. But all new objects
will be stored with the same details under 'etag'.

Fixes #4312

cmd/fs-v1.go

@@ -712,12 +712,12 @@ func (fs fsObjects) PutObject(bucket string, object string, size int64, data io.
 
 	newMD5Hex := hex.EncodeToString(md5Writer.Sum(nil))
 	// Update the md5sum if not set with the newly calculated one.
-	if len(metadata["md5Sum"]) == 0 {
-		metadata["md5Sum"] = newMD5Hex
+	if len(metadata["etag"]) == 0 {
+		metadata["etag"] = newMD5Hex
 	}
 
 	// md5Hex representation.
-	md5Hex := metadata["md5Sum"]
+	md5Hex := metadata["etag"]
 	if md5Hex != "" {
 		if newMD5Hex != md5Hex {
 			// Returns md5 mismatch.
@@ -849,8 +849,12 @@ func (fs fsObjects) getObjectETag(bucket, entry string) (string, error) {
 		}
 	}
 
-	fsMetaMap := parseFSMetaMap(fsMetaBuf)
-	return fsMetaMap["md5Sum"], nil
+	// Check if FS metadata is valid, if not return error.
+	if !isFSMetaValid(parseFSVersion(fsMetaBuf), parseFSFormat(fsMetaBuf)) {
+		return "", toObjectErr(traceError(errCorruptedFormat), bucket, entry)
+	}
+
+	return extractETag(parseFSMetaMap(fsMetaBuf)), nil
 }
 
 // ListObjects - list all objects at prefix upto maxKeys., optionally delimited by '/'. Maintains the list pool
@@ -901,8 +905,8 @@ func (fs fsObjects) ListObjects(bucket, prefix, marker, delimiter string, maxKey
 		// Protect reading `fs.json`.
 		objectLock := globalNSMutex.NewNSLock(bucket, entry)
 		objectLock.RLock()
-		var md5Sum string
-		md5Sum, err = fs.getObjectETag(bucket, entry)
+		var etag string
+		etag, err = fs.getObjectETag(bucket, entry)
 		objectLock.RUnlock()
 		if err != nil {
 			return ObjectInfo{}, err
@@ -922,7 +926,7 @@ func (fs fsObjects) ListObjects(bucket, prefix, marker, delimiter string, maxKey
 			Size:    fi.Size(),
 			ModTime: fi.ModTime(),
 			IsDir:   fi.IsDir(),
-			MD5Sum:  md5Sum,
+			ETag:    etag,
 		}, nil
 	}