union: support metadata

backend/union/entry.go

@@ -2,6 +2,7 @@ package union
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -34,9 +35,8 @@ type entry interface {
 	candidates() []upstream.Entry
 }
 
-// UnWrap returns the Object that this Object is wrapping or
-// nil if it isn't wrapping anything
-func (o *Object) UnWrap() *upstream.Object {
+// UnWrapUpstream returns the upstream Object that this Object is wrapping
+func (o *Object) UnWrapUpstream() *upstream.Object {
 	return o.Object
 }
 
@@ -140,6 +140,42 @@ func (o *Object) SetModTime(ctx context.Context, t time.Time) error {
 	return errs.Err()
 }
 
+// GetTier returns storage tier or class of the Object
+func (o *Object) GetTier() string {
+	do, ok := o.Object.Object.(fs.GetTierer)
+	if !ok {
+		return ""
+	}
+	return do.GetTier()
+}
+
+// ID returns the ID of the Object if known, or "" if not
+func (o *Object) ID() string {
+	do, ok := o.Object.Object.(fs.IDer)
+	if !ok {
+		return ""
+	}
+	return do.ID()
+}
+
+// MimeType returns the content type of the Object if known
+func (o *Object) MimeType(ctx context.Context) (mimeType string) {
+	if do, ok := o.Object.Object.(fs.MimeTyper); ok {
+		mimeType = do.MimeType(ctx)
+	}
+	return mimeType
+}
+
+// SetTier performs changing storage tier of the Object if
+// multiple storage classes supported
+func (o *Object) SetTier(tier string) error {
+	do, ok := o.Object.Object.(fs.SetTierer)
+	if !ok {
+		return errors.New("underlying remote does not support SetTier")
+	}
+	return do.SetTier(tier)
+}
+
 // ModTime returns the modification date of the directory
 // It returns the latest ModTime of all candidates
 func (d *Directory) ModTime(ctx context.Context) (t time.Time) {
@@ -164,3 +200,8 @@ func (d *Directory) Size() (s int64) {
 	}
 	return s
 }
+
+// Check the interfaces are satisfied
+var (
+	_ fs.FullObject = (*Object)(nil)
+)