3 files changed, 108 insertions, 0 deletions
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 8af8374..e70039e 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -46,6 +46,7 @@ add_library(qtmaildir_lib STATIC
     signatures.cpp
     repeatrule.cpp
     calendarstore.cpp
+    calendarwriter.cpp
 )
 
 target_include_directories(qtmaildir_lib
diff --git a/src/calendarwriter.cpp b/src/calendarwriter.cpp
new file mode 100644
index 0000000..4c81a37
--- /dev/null
+++ b/src/calendarwriter.cpp
@@ -0,0 +1,64 @@
+/*
+ * 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 "calendarwriter.h"
+
+#include <QFile>
+#include <QFileInfo>
+#include <QSaveFile>
+
+namespace CalendarWriter {
+
+Result replace(const QString &path, const std::optional<QByteArray> &expected,
+               const std::optional<QByteArray> &newText, QString *error)
+{
+    QFile current(path);
+    const bool exists = current.exists();
+    if (expected.has_value() != exists)
+        return Result::Stale;
+    if (exists) {
+        if (!current.open(QIODevice::ReadOnly)) {
+            *error = current.errorString();
+            return Result::IoError;
+        }
+        if (current.readAll() != *expected)
+            return Result::Stale;
+        current.close();
+    }
+
+    if (!newText) {
+        if (!QFile::remove(path)) {
+            *error = QFile(path).errorString();
+            return Result::IoError;
+        }
+        return Result::Ok;
+    }
+
+    // QSaveFile is the platform's atomic write: a temporary in the same
+    // directory, renamed on commit(). Its temporary is named "<name>.XXXXXX",
+    // which does not end in .ics, so vdirsyncer never lists it.
+    QSaveFile file(path);
+    if (!file.open(QIODevice::WriteOnly) || file.write(*newText) != newText->size()
+        || !file.commit()) {
+        *error = file.errorString();
+        return Result::IoError;
+    }
+    return Result::Ok;
+}
+
+}  // namespace CalendarWriter
diff --git a/src/calendarwriter.h b/src/calendarwriter.h
new file mode 100644
index 0000000..0bcca09
--- /dev/null
+++ b/src/calendarwriter.h
@@ -0,0 +1,43 @@
+/*
+ * 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