api: Introduce metadata update APIs to update only metadata (#11962)

Current implementation heavily relies on readAllFileInfo
but with the advent of xl.meta inlined with data, we cannot
easily avoid reading data when we are only interested is
updating metadata, this leads to invariably write
amplification during metadata updates, repeatedly reading
data when we are only interested in updating metadata.

This PR ensures that we implement a metadata only update
API at storage layer, that handles updates to metadata alone
for any given version - given the version is valid and
present.

This helps reduce the chattiness for following calls..

- PutObjectTags
- DeleteObjectTags
- PutObjectLegalHold
- PutObjectRetention
- ReplicateObject (updates metadata on replication status)

cmd/xl-storage-format-v2.go

@@ -624,6 +624,75 @@ func (z *xlMetaV2) AppendTo(dst []byte) ([]byte, error) {
 	return append(dst, z.data...), nil
 }
 
+// UpdateObjectVersion updates metadata and modTime for a given
+// versionID, NOTE: versionID must be valid and should exist -
+// and must not be a DeleteMarker or legacy object, if no
+// versionID is specified 'null' versionID is updated instead.
+//
+// It is callers responsibility to set correct versionID, this
+// function shouldn't be further extended to update immutable
+// values such as ErasureInfo, ChecksumInfo.
+//
+// Metadata is only updated to new values, existing values
+// stay as is, if you wish to update all values you should
+// update all metadata freshly before calling this function
+// in-case you wish to clear existing metadata.
+func (z *xlMetaV2) UpdateObjectVersion(fi FileInfo) error {
+	if fi.VersionID == "" {
+		// this means versioning is not yet
+		// enabled or suspend i.e all versions
+		// are basically default value i.e "null"
+		fi.VersionID = nullVersionID
+	}
+
+	var uv uuid.UUID
+	var err error
+	if fi.VersionID != "" && fi.VersionID != nullVersionID {
+		uv, err = uuid.Parse(fi.VersionID)
+		if err != nil {
+			return err
+		}
+	}
+
+	for i, version := range z.Versions {
+		if !version.Valid() {
+			return errFileCorrupt
+		}
+		switch version.Type {
+		case LegacyType:
+			if version.ObjectV1.VersionID == fi.VersionID {
+				return errMethodNotAllowed
+			}
+		case ObjectType:
+			if version.ObjectV2.VersionID == uv {
+				for k, v := range fi.Metadata {
+					if strings.HasPrefix(strings.ToLower(k), ReservedMetadataPrefixLower) {
+						if v == "" {
+							delete(z.Versions[i].ObjectV2.MetaSys, k)
+						} else {
+							z.Versions[i].ObjectV2.MetaSys[k] = []byte(v)
+						}
+					} else {
+						if v == "" {
+							delete(z.Versions[i].ObjectV2.MetaUser, k)
+						} else {
+							z.Versions[i].ObjectV2.MetaUser[k] = v
+						}
+					}
+				}
+				if !fi.ModTime.IsZero() {
+					z.Versions[i].ObjectV2.ModTime = fi.ModTime.UnixNano()
+				}
+				return nil
+			}
+		case DeleteType:
+			return errMethodNotAllowed
+		}
+	}
+
+	return errFileVersionNotFound
+}
+
 // AddVersion adds a new version
 func (z *xlMetaV2) AddVersion(fi FileInfo) error {
 	if fi.VersionID == "" {