streamdata: optionally rename from alternate

This is useful for migration, or external downloads.

Signed-off-by: Yonah <contrib@gensokyo.uk>

streamdata.go

@@ -143,6 +143,14 @@ func (c *ChannelMismatchError) Error() string {
 		strconv.FormatUint(c.Want, 10)
 }
 
+// RenameFrom is a pathname returned by a function passed to [Channel.Add] to
+// rename from this pathname instead of the managed transaction file.
+type RenameFrom string
+
+func (pathname RenameFrom) Error() string {
+	return "requesting rename from " + strconv.Quote(string(pathname))
+}
+
 // Add adds a [VOD] and its corresponding asset to the on-disk representation.
 func (c *Channel) Add(ident *Ident, f func(v *VOD, w io.Writer) error) error {
 	if ident == nil || f == nil {
@@ -167,8 +175,20 @@ func (c *Channel) Add(ident *Ident, f func(v *VOD, w io.Writer) error) error {
 		err = closeErr
 	}
 	if err != nil {
-		_ = c.root.Remove(channelPathPending)
-		return err
+		var rf RenameFrom
+		if !errors.As(err, &rf) {
+			_ = c.root.Remove(channelPathPending)
+			return err
+		}
+		if err = c.root.Remove(channelPathPending); err != nil {
+			return err
+		}
+		if err = os.Rename(string(rf), path.Join(
+			c.root.Name(),
+			channelPathPending,
+		)); err != nil {
+			return err
+		}
 	}
 
 	if err = c.root.Chmod(channelPathPending, 0444); err != nil {