refactor(maildir): extract freshMaildirName for reuse, item 123

DraftStore needs the same filename generation moveMessages() already has,
and duplicating it would duplicate a correctness property rather than a
convenience: the comment records that carrying mbsync's ,U= infix across a
folder boundary produced 'Maildir error: duplicate UID' on real mail.

A pure move with no behaviour change, committed on its own so a bisect can
tell it apart from the feature that needed it. The function gains its own
tests, including the UID-infix case that previously had none.

1 files changed, 41 insertions, 0 deletions

diff --git a/src/maildirname.h b/src/maildirname.h
new file mode 100644
index 0000000..f24bc71
--- /dev/null
+++ b/src/maildirname.h
@@ -0,0 +1,41 @@
+/*
+ * qtmaildir - a Qt6 mail client for notmuch-indexed Maildirs
+ * Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#pragma once
+
+#include <QString>
+
+/// Maildir filename generation, shared by every path that writes a message
+/// file: NotmuchWorker::moveMessages() and DraftStore.
+///
+/// A namespace rather than a class; there is no state beyond a counter.
+namespace MaildirName {
+
+/// A fresh, unique Maildir filename, preserving \p oldName's flag suffix.
+///
+/// A FRESH name, never a reuse. mbsync writes a `,U=<n>` infix that is
+/// meaningful only within one folder, and carrying it across a folder
+/// boundary produced "Maildir error: duplicate UID" on real mail. Only the
+/// `:2,` flag suffix is carried, because the flags describe the message
+/// rather than its position.
+///
+/// Pass an empty string for a message that has no previous name, which is
+/// what a newly composed draft is.
+QString fresh(const QString &oldName);
+
+}  // namespace MaildirName