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

Fixes: #8467

changelog/unreleased/8467

@@ -0,0 +1,7 @@
+Enhancement: Attach the last 20 log lines to a crash report
+
+We now save the last 20 lines of log to a tempoary file.
+This file is then part of a crash report.
+
+https://github.com/owncloud/client/issues/8467
+https://github.com/owncloud/client/pull/8469

src/libsync/logger.cpp

@@ -31,6 +31,9 @@
 #include <io.h> // for stdout
 #endif
 
+namespace {
+constexpr int CrashLogSize = 20;
+}
 namespace OCC {
 
 static void mirallLogCatcher(QtMsgType type, const QMessageLogContext &ctx, const QString &message)
@@ -45,6 +48,7 @@ static void mirallLogCatcher(QtMsgType type, const QMessageLogContext &ctx, cons
 
     if(type == QtFatalMsg) {
         if (!logger->isNoop()) {
+            logger->dumpCrashLog();
             logger->close();
         }
 #if defined(Q_OS_WIN)
@@ -69,6 +73,7 @@ Logger::Logger(QObject *parent)
     , _logDebug(false)
 {
     qSetMessagePattern(QStringLiteral("%{time MM-dd hh:mm:ss:zzz} [ %{type} %{category} ]%{if-debug}\t[ %{function} ]%{endif}:\t%{message}"));
+    _crashLog.resize(CrashLogSize);
 #ifndef NO_MSG_HANDLER
     qInstallMessageHandler(mirallLogCatcher);
 #else
@@ -132,6 +137,8 @@ void Logger::doLog(const QString &msg)
 {
     {
         QMutexLocker lock(&_mutex);
+        _crashLogIndex = (_crashLogIndex + 1) % CrashLogSize;
+        _crashLog[_crashLogIndex] = msg;
         if (_logstream) {
             (*_logstream) << msg << endl;
             if (_doFileFlush)
@@ -246,6 +253,17 @@ void Logger::setLogRules(const QSet<QString> &rules)
     QLoggingCategory::setFilterRules(tmp);
 }
 
+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

@@ -89,6 +89,8 @@ class OWNCLOUDSYNC_EXPORT Logger : public QObject
     }
     void setLogRules(const QSet<QString> &rules);
 
+    void dumpCrashLog();
+
 signals:
     void logWindowLog(const QString &);
 
@@ -113,6 +115,8 @@ public slots:
     QString _logDirectory;
     bool _temporaryFolderLogDir = false;
     QSet<QString> _logRules;
+    QVector<QString> _crashLog;
+    int _crashLogIndex = 0;
 };
 
 } // namespace OCC