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, 80 insertions, 0 deletions

diff --git a/src/maildirname.cpp b/src/maildirname.cpp
new file mode 100644
index 0000000..6263aec
--- /dev/null
+++ b/src/maildirname.cpp
@@ -0,0 +1,80 @@
+/*
+ * 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.
+ */
+
+#include "maildirname.h"
+
+#include <QCoreApplication>
+#include <QDateTime>
+#include <QHostInfo>
+
+namespace MaildirName {
+
+/// A fresh Maildir filename for a message being moved between folders,
+/// preserving only its `:2,<flags>` suffix.
+///
+/// mbsync's manual is explicit about why this exists, under "the more
+/// efficient default UID mapping scheme": "it is important that the MUA
+/// renames files when moving them between Maildir folders", and "the general
+/// expectation is that a completely new filename is generated as if the
+/// message was new".
+///
+/// The `,U=<n>` infix mbsync writes is its per-folder IMAP UID. Carrying it
+/// into another folder makes it a claim about a folder the file is no longer
+/// in; moving a message out and back then reinserts a UID the server has
+/// since reassigned, and mbsync refuses the folder with `Maildir error:
+/// duplicate UID`. Measured on real mail, four collisions in one folder from
+/// a single move-and-restore.
+///
+/// The FLAGS are kept, deliberately, and that is not a contradiction of
+/// "as if the message was new". They record seen, flagged and replied, and
+/// `maildir.synchronize_flags` is true, so notmuch reads them back as tags:
+/// dropping them would mark every deleted message unread and lose Important
+/// on the way to the trash. Only the unique part is regenerated.
+QString fresh(const QString &oldName)
+{
+    // The `:2,` suffix, when there is one. `info` is everything from the
+    // separator on, so an empty-flag `:2,` is preserved as faithfully as
+    // `:2,FS`.
+    QString info;
+    const int sep = oldName.indexOf(QStringLiteral(":2,"));
+    if (sep >= 0)
+        info = oldName.mid(sep);
+
+    // The conventional left-to-right unique part: time, a per-process counter,
+    // the pid, the host. The counter is what makes two messages moved in the
+    // same second distinct, which a timestamp alone does not guarantee.
+    static quint64 counter = 0;
+    const qint64 now = QDateTime::currentSecsSinceEpoch();
+    const QString host = QHostInfo::localHostName().isEmpty()
+                             ? QStringLiteral("localhost")
+                             : QHostInfo::localHostName();
+
+    return QStringLiteral("%1.M%2P%3Q%4.%5%6")
+        .arg(now)
+        .arg(QDateTime::currentMSecsSinceEpoch() % 1000)
+        .arg(QCoreApplication::applicationPid())
+        .arg(++counter)
+        // A `/` or a `:` in a hostname would break the path or the flag
+        // separator. Neither is legal in a hostname, so this is belt and
+        // braces rather than a known case.
+        .arg(QString(host).replace(QLatin1Char('/'), QLatin1Char('_'))
+                 .replace(QLatin1Char(':'), QLatin1Char('_')))
+        .arg(info);
+}
+
+}  // namespace MaildirName