Dump the last 20 lines of logs to a file when we crash

Fixes: #8467

src/libsync/logger.cpp

@@ -32,6 +32,9 @@
 #include <io.h> // for stdout
 #endif
 
+namespace {
+constexpr int CrashLogSize = 20;
+}
 namespace OCC {
 
 QtMessageHandler s_originalMessageHandler = nullptr;
@@ -52,6 +55,7 @@ static void mirallLogCatcher(QtMsgType type, const QMessageLogContext &ctx, cons
 
     if(type == QtFatalMsg) {
         if (!logger->isNoop()) {
+            logger->dumpCrashLog();
             logger->close();
         }
 #if defined(Q_OS_WIN)
@@ -71,7 +75,8 @@ Logger *Logger::instance()
 Logger::Logger(QObject *parent)
     : QObject(parent)
 {
-    qSetMessagePattern("%{time yyyy-MM-dd hh:mm:ss:zzz} [ %{type} %{category} ]%{if-debug}\t[ %{function} ]%{endif}:\t%{message}");
+    qSetMessagePattern(QStringLiteral("%{time yyyy-MM-dd hh:mm:ss:zzz} [ %{type} %{category} ]%{if-debug}\t[ %{function} ]%{endif}:\t%{message}"));
+    _crashLog.resize(CrashLogSize);
 #ifndef NO_MSG_HANDLER
    s_originalMessageHandler = qInstallMessageHandler(mirallLogCatcher);
 #else
@@ -135,6 +140,8 @@ void Logger::doLog(const QString &msg)
 {
     {
         QMutexLocker lock(&_mutex);
+        _crashLogIndex = (_crashLogIndex + 1) % CrashLogSize;
+        _crashLog[_crashLogIndex] = msg;
         if (_logstream) {
             (*_logstream) << msg << endl;
             if (_doFileFlush)
@@ -267,6 +274,17 @@ void Logger::setLogRules(const QSet<QString> &rules)
     QLoggingCategory::setFilterRules(rules.toList().join(QLatin1Char('\n')));
 }
 
+void Logger::dumpCrashLog()
+{
+    QFile logFile(QDir::tempPath() + QStringLiteral("/" APPLICATION_NAME "-crash.log"));
+    if (logFile.open(QFile::WriteOnly)) {
+        QTextStream out(&logFile);
+        for (int i = 1; i <= CrashLogSize; ++i) {
+            out << _crashLog[(_crashLogIndex + i) % CrashLogSize] << QLatin1Char('\n');
+        }
+    }
+}
+
 static bool compressLog(const QString &originalName, const QString &targetName)
 {
 #ifdef ZLIB_FOUND

src/libsync/logger.h

@@ -95,6 +95,8 @@ public:
     }
     void setLogRules(const QSet<QString> &rules);
 
+    void dumpCrashLog();
+
 signals:
     void logWindowLog(const QString &);
 
@@ -119,6 +121,8 @@ private:
     QString _logDirectory;
     bool _temporaryFolderLogDir = false;
     QSet<QString> _logRules;
+    QVector<QString> _crashLog;
+    int _crashLogIndex = 0;
 };
 
 } // namespace OCC