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diff --git a/src/calendarwriter.h b/src/calendarwriter.h
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+/*
+ * 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 <QByteArray>
+#include <QString>
+
+#include <optional>
+
+/// Every write to the calendar vdir goes through here.
+///
+/// One function covers create, edit, delete and every undo of those, because
+/// each is the same act: the file must currently be X, and must become Y,
+/// where either may be "absent". vdirsyncer writes the same files from cron,
+/// so "must currently be X" is the stale check that stops a save from
+/// silently discarding what a sync just pulled.
+namespace CalendarWriter {
+
+enum class Result { Ok, Stale, IoError };
+
+/// `expected` nullopt: the file must not exist. `newText` nullopt: delete it.
+/// A write goes to a hidden temporary in the same directory and is renamed
+/// into place, so vdirsyncer, which lists only *.ics, never reads half a file.
+Result replace(const QString &path, const std::optional<QByteArray> &expected,
+               const std::optional<QByteArray> &newText, QString *error);
+
+}  // namespace CalendarWriter