s3/local backend: Fix error for overwriting files

backend/local/local.go

@@ -171,16 +171,19 @@ func (b *Local) Save(h backend.Handle, p []byte) (err error) {
 
 	f := filename(b.p, h.Type, h.Name)
 
-	// create directories if necessary, ignore errors
-	if h.Type == backend.Data {
-		os.MkdirAll(filepath.Dir(f), backend.Modes.Dir)
-	}
-
 	// test if new path already exists
 	if _, err := os.Stat(f); err == nil {
 		return fmt.Errorf("Rename(): file %v already exists", f)
 	}
 
+	// create directories if necessary, ignore errors
+	if h.Type == backend.Data {
+		err = os.MkdirAll(filepath.Dir(f), backend.Modes.Dir)
+		if err != nil {
+			return err
+		}
+	}
+
 	err = os.Rename(tmpfile.Name(), f)
 	debug.Log("local.Save", "save %v: rename %v -> %v: %v",
 		h, filepath.Base(tmpfile.Name()), filepath.Base(f), err)

backend/s3/s3.go

@@ -2,6 +2,7 @@ package s3
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"strings"
 
@@ -103,6 +104,13 @@ func (be S3Backend) Save(h backend.Handle, p []byte) (err error) {
 
 	path := s3path(h.Type, h.Name)
 
+	// Check key does not already exist
+	_, err = be.client.StatObject(be.bucketname, path)
+	if err == nil {
+		debug.Log("s3.blob.Finalize()", "%v already exists", h)
+		return errors.New("key already exists")
+	}
+
 	<-be.connChan
 	defer func() {
 		be.connChan <- struct{}{}