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/xl-v1-metadata.go

@@ -146,7 +146,10 @@ type xlMetaV1 struct {
 // XL metadata constants.
 const (
 	// XL meta version.
-	xlMetaVersion = "1.0.0"
+	xlMetaVersion = "1.0.1"
+
+	// XL meta version.
+	xlMetaVersion100 = "1.0.0"
 
 	// XL meta format string.
 	xlMetaFormat = "xl"
@@ -173,7 +176,38 @@ func newXLMetaV1(object string, dataBlocks, parityBlocks int) (xlMeta xlMetaV1)
 // IsValid - tells if the format is sane by validating the version
 // string and format style.
 func (m xlMetaV1) IsValid() bool {
-	return m.Version == xlMetaVersion && m.Format == xlMetaFormat
+	return isXLMetaValid(m.Version, m.Format)
+}
+
+// Verifies if the backend format metadata is sane by validating
+// the version string and format style.
+func isXLMetaValid(version, format string) bool {
+	return ((version == xlMetaVersion || version == xlMetaVersion100) &&
+		format == xlMetaFormat)
+}
+
+// Converts metadata to object info.
+func (m xlMetaV1) ToObjectInfo(bucket, object string) ObjectInfo {
+	objInfo := ObjectInfo{
+		IsDir:           false,
+		Bucket:          bucket,
+		Name:            object,
+		Size:            m.Stat.Size,
+		ModTime:         m.Stat.ModTime,
+		ContentType:     m.Meta["content-type"],
+		ContentEncoding: m.Meta["content-encoding"],
+	}
+
+	// Extract etag from metadata.
+	objInfo.ETag = extractETag(m.Meta)
+
+	// etag/md5Sum has already been extracted. We need to
+	// remove to avoid it from appearing as part of
+	// response headers. e.g, X-Minio-* or X-Amz-*.
+	objInfo.UserDefined = cleanMetaETag(m.Meta)
+
+	// Success.
+	return objInfo
 }
 
 // objectPartIndex - returns the index of matching object part number.