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.

2 files changed, 94 insertions, 0 deletions

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 58d5659..15f9955 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -69,6 +69,7 @@ add_qtmaildir_test(busyindicator)
 add_qtmaildir_test(tagstrip)
 add_qtmaildir_test(messagedetailsdialog)
 add_qtmaildir_test(markdownrenderer)
+add_qtmaildir_test(maildirname)
 add_qtmaildir_test(translations)
 # Asserts on the tracked .ts rather than the generated .qm: an untranslated
 # string is dropped by lrelease, so it is invisible in the .qm and shows up
diff --git a/tests/test_maildirname.cpp b/tests/test_maildirname.cpp
new file mode 100644
index 0000000..dcc8fab
--- /dev/null
+++ b/tests/test_maildirname.cpp
@@ -0,0 +1,93 @@
+/*
+ * 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 <QSet>
+#include <QTest>
+
+class TestMaildirName : public QObject
+{
+    Q_OBJECT
+
+private slots:
+    void aFreshNameIsUniquePerCall();
+    void theFlagSuffixIsPreserved();
+    void anEmptyFlagSuffixIsPreserved();
+    void aNameWithNoSuffixGetsNone();
+    void theUidInfixIsNotCarriedAcross();
+};
+
+// Two messages written in the same second must not collide, which a
+// timestamp alone does not guarantee, and that is what the counter is for.
+void TestMaildirName::aFreshNameIsUniquePerCall()
+{
+    QSet<QString> names;
+    for (int i = 0; i < 100; ++i)
+        names.insert(MaildirName::fresh(QStringLiteral("1234.M1P1Q1.host")));
+
+    QVERIFY2(names.size() == 100,
+             qPrintable(QStringLiteral("expected 100 unique names, got %1")
+                            .arg(names.size())));
+}
+
+// The flags say whether a message is read, flagged or draft, and losing them
+// on a move silently marks mail unread again.
+void TestMaildirName::theFlagSuffixIsPreserved()
+{
+    const QString name = MaildirName::fresh(QStringLiteral("1234.M1P1Q1.host:2,FS"));
+    QVERIFY2(name.endsWith(QStringLiteral(":2,FS")),
+             qPrintable(QStringLiteral("generated name did not preserve flags: %1")
+                            .arg(name)));
+}
+
+// `:2,` with no flags is not the same as no suffix at all, it says the flags
+// are known and empty.
+void TestMaildirName::anEmptyFlagSuffixIsPreserved()
+{
+    const QString name = MaildirName::fresh(QStringLiteral("1234.M1P1Q1.host:2,"));
+    QVERIFY2(name.endsWith(QStringLiteral(":2,")),
+             qPrintable(QStringLiteral("generated name did not preserve empty flag suffix: %1")
+                            .arg(name)));
+}
+
+// A suffix must not be invented.
+void TestMaildirName::aNameWithNoSuffixGetsNone()
+{
+    const QString name = MaildirName::fresh(QStringLiteral("1234.M1P1Q1.host"));
+    QVERIFY2(!name.contains(QStringLiteral(":2,")),
+             qPrintable(QStringLiteral("generated name invented a flag suffix: %1")
+                            .arg(name)));
+}
+
+// This is the reason the function exists; carrying mbsync's `,U=` infix
+// across a folder boundary produced "Maildir error: duplicate UID" on real
+// mail.
+void TestMaildirName::theUidInfixIsNotCarriedAcross()
+{
+    const QString name = MaildirName::fresh(QStringLiteral("1234.M1P1Q1.host,U=42:2,S"));
+    QVERIFY2(!name.contains(QStringLiteral("U=42")),
+             qPrintable(QStringLiteral("generated name carried the UID infix across: %1")
+                            .arg(name)));
+    QVERIFY2(name.endsWith(QStringLiteral(":2,S")),
+             qPrintable(QStringLiteral("generated name did not preserve flags: %1")
+                            .arg(name)));
+}
+
+QTEST_MAIN(TestMaildirName)
+#include "test_maildirname.moc"