streamdata: optionally rename from alternate

This is useful for migration, or external downloads.

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

streamdata_test.go

@@ -194,7 +194,7 @@ func TestChannelBadMetadata(t *testing.T) {
 	})
 }
 
-func TestChannelAdd(t *testing.T) {
+func TestChannel(t *testing.T) {
 	t.Parallel()
 
 	d := t.TempDir()
@@ -334,6 +334,28 @@ func TestChannelAdd(t *testing.T) {
 		t.Errorf("Perm: %#o", fi.Mode().Perm())
 	}
 
+	rf := streamdata.RenameFrom(path.Join(d, "alternate"))
+	if err := os.WriteFile(string(rf), []byte{0}, 0); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Add(&streamdata.Ident{Channel: 0xcafe}, func(*streamdata.VOD, io.Writer) error {
+		return rf
+	}); err != nil {
+		t.Fatalf("Add: error = %v", err)
+	}
+	if _, err := os.Stat(string(rf)); !errors.Is(err, os.ErrNotExist) {
+		t.Fatalf("Stat: error = %v", err)
+	}
+	if data, err := os.ReadFile(path.Join(
+		d,
+		"vod",
+		(&streamdata.Ident{Channel: 0xcafe}).String()+".mp4",
+	)); err != nil {
+		t.Fatal(err)
+	} else if string(data) != "\x00" {
+		t.Errorf("(rf) Add: %#v", data)
+	}
+
 	if err := os.Chmod(path.Join(d, "vod", wantIdent), 0); err != nil {
 		t.Fatal(err)
 	}