fix(log): rotate the log instead of deleting it

src/lib/base/LogOutputters.cpp

@@ -138,7 +138,7 @@ bool FileLogOutputter::write(LogLevel::Level, const QString &message)
 
   if (file.size() > s_logFileSizeLimit) {
     const auto oldFile = QStringLiteral("%1.1").arg(m_fileName);
-    QFile::remove(m_fileName);
+    QFile::remove(oldFile);
     QFile::rename(m_fileName, oldFile);
   }
 

src/unittests/base/CMakeLists.txt

@@ -29,6 +29,14 @@ create_test(
   WORKING_DIRECTORY "${CMAKE_BINARY_DIR}/src/lib/base"
 )
 
+create_test(
+  NAME FileLogOutputterTests
+  DEPENDS base
+  LIBS arch ${extra_libs}
+  SOURCE FileLogOutputterTests.cpp
+  WORKING_DIRECTORY "${CMAKE_BINARY_DIR}/src/lib/base"
+)
+
 create_test(
   NAME BaseExceptionTests
   DEPENDS base

src/unittests/base/FileLogOutputterTests.cpp

@@ -0,0 +1,79 @@
+/*
+ * Deskflow -- mouse and keyboard sharing utility
+ * SPDX-FileCopyrightText: (C) 2026 Synergy App Ltd
+ * SPDX-License-Identifier: GPL-2.0-only WITH LicenseRef-OpenSSL-Exception
+ */
+
+#include "FileLogOutputterTests.h"
+
+#include <QFile>
+#include <QFileInfo>
+#include <QTest>
+
+void FileLogOutputterTests::init()
+{
+  QVERIFY(m_dir.isValid());
+  m_fileName = m_dir.filePath(QStringLiteral("deskflow.log"));
+  QFile::remove(m_fileName);
+  QFile::remove(rotatedFileName());
+}
+
+QString FileLogOutputterTests::rotatedFileName() const
+{
+  return QStringLiteral("%1.1").arg(m_fileName);
+}
+
+QString FileLogOutputterTests::readAll(const QString &fileName)
+{
+  QFile file(fileName);
+  if (!file.open(QFile::ReadOnly))
+    return {};
+  return QString::fromUtf8(file.readAll());
+}
+
+void FileLogOutputterTests::growLogPastSizeLimit(FileLogOutputter &outputter, const QString &marker)
+{
+  const QString padding(s_paddingSize, QLatin1Char('x'));
+
+  QVERIFY(outputter.write(LogLevel::Level::Info, marker));
+
+  // rotating renames the live log away, so its absence is what ends the loop
+  for (int i = 0; i < s_maxPaddingWrites && QFileInfo::exists(m_fileName); ++i)
+    QVERIFY(outputter.write(LogLevel::Level::Info, padding));
+}
+
+void FileLogOutputterTests::write_multipleMessages_appendsEachLine()
+{
+  FileLogOutputter outputter(m_fileName);
+
+  QVERIFY(outputter.write(LogLevel::Level::Info, QStringLiteral("first")));
+  QVERIFY(outputter.write(LogLevel::Level::Info, QStringLiteral("second")));
+
+  QCOMPARE(readAll(m_fileName), QStringLiteral("first\nsecond\n"));
+}
+
+void FileLogOutputterTests::write_sizeLimitExceeded_preservesPreviousLog()
+{
+  FileLogOutputter outputter(m_fileName);
+
+  growLogPastSizeLimit(outputter, QStringLiteral("oldest entry"));
+
+  QVERIFY2(QFileInfo::exists(rotatedFileName()), "log was discarded instead of rotated");
+  QVERIFY(readAll(rotatedFileName()).startsWith(QStringLiteral("oldest entry\n")));
+
+  QVERIFY(outputter.write(LogLevel::Level::Info, QStringLiteral("after rotating")));
+  QCOMPARE(readAll(m_fileName), QStringLiteral("after rotating\n"));
+}
+
+void FileLogOutputterTests::write_rotatedLogExists_replacesIt()
+{
+  FileLogOutputter outputter(m_fileName);
+
+  growLogPastSizeLimit(outputter, QStringLiteral("first batch"));
+  QVERIFY(readAll(rotatedFileName()).startsWith(QStringLiteral("first batch\n")));
+
+  growLogPastSizeLimit(outputter, QStringLiteral("second batch"));
+  QVERIFY(readAll(rotatedFileName()).startsWith(QStringLiteral("second batch\n")));
+}
+
+QTEST_MAIN(FileLogOutputterTests)

src/unittests/base/FileLogOutputterTests.h

@@ -0,0 +1,35 @@
+/*
+ * Deskflow -- mouse and keyboard sharing utility
+ * SPDX-FileCopyrightText: (C) 2026 Synergy App Ltd
+ * SPDX-License-Identifier: GPL-2.0-only WITH LicenseRef-OpenSSL-Exception
+ */
+
+#pragma once
+
+#include "base/LogOutputters.h"
+
+#include <QObject>
+#include <QString>
+#include <QTemporaryDir>
+
+class FileLogOutputterTests : public QObject
+{
+  Q_OBJECT
+
+private Q_SLOTS:
+  void init();
+  void write_multipleMessages_appendsEachLine();
+  void write_sizeLimitExceeded_preservesPreviousLog();
+  void write_rotatedLogExists_replacesIt();
+
+private:
+  QString rotatedFileName() const;
+  void growLogPastSizeLimit(FileLogOutputter &outputter, const QString &marker);
+  static QString readAll(const QString &fileName);
+
+  static constexpr int s_paddingSize = 64 * 1024;
+  static constexpr int s_maxPaddingWrites = 64;
+
+  QTemporaryDir m_dir;
+  QString m_fileName;
+};