Fix a bug in the SFTP backend that would cause an error when overwriting existing files

internal/adapters/ssh/backend.go

@@ -170,7 +170,7 @@ func (b *Backend) WriteFrom(ctx context.Context, logicalPath string, r io.Reader
 		return storage.Entry{}, storage.NewError(storage.OpWriteFrom, BackendName, logicalPath, storage.ErrConflict, fmt.Errorf("stream size %d does not match expected size %d", written, opts.Size))
 	}
 	if opts.PreferAtomic {
-		if err := b.client.Rename(writePath, nativePath); err != nil {
+		if err := renamePromotedFile(b.client, writePath, nativePath, opts.Overwrite); err != nil {
 			return storage.Entry{}, b.translateError(storage.OpWriteFrom, logicalPath, err)
 		}
 		cleanup = false
@@ -178,6 +178,27 @@ func (b *Backend) WriteFrom(ctx context.Context, logicalPath string, r io.Reader
 	return b.Stat(ctx, logicalPath)
 }
 
+type sftpRenamer interface {
+	PosixRename(oldname, newname string) error
+	Rename(oldname, newname string) error
+	Remove(path string) error
+}
+
+func renamePromotedFile(client sftpRenamer, oldname, newname string, overwrite bool) error {
+	if !overwrite {
+		return client.Rename(oldname, newname)
+	}
+	if err := client.PosixRename(oldname, newname); err == nil {
+		return nil
+	} else if !isReplaceRenameFallbackError(err) {
+		return err
+	}
+	if err := client.Remove(newname); err != nil && !isNotExist(err) {
+		return err
+	}
+	return client.Rename(oldname, newname)
+}
+
 func (b *Backend) Stat(ctx context.Context, logicalPath string) (storage.Entry, error) {
 	if err := ctx.Err(); err != nil {
 		return storage.Entry{}, err
@@ -464,6 +485,14 @@ func isNotExist(err error) bool {
 	return errors.Is(err, fs.ErrNotExist) || errors.Is(err, os.ErrNotExist) || errors.Is(err, sftp.ErrSSHFxNoSuchFile)
 }
 
+func isReplaceRenameFallbackError(err error) bool {
+	if errors.Is(err, sftp.ErrSSHFxFailure) || errors.Is(err, sftp.ErrSSHFxOpUnsupported) {
+		return true
+	}
+	var statusErr *sftp.StatusError
+	return errors.As(err, &statusErr) && (statusErr.FxCode() == sftp.ErrSSHFxFailure || statusErr.FxCode() == sftp.ErrSSHFxOpUnsupported)
+}
+
 func (b *Backend) translateError(op, logicalPath string, err error) error {
 	kind := storage.ErrUnknown
 	switch {