guard logger QFile access with a mutex

This fixes the following assertion failure in Qt debug builds:
```
ASSERT: "bytes <= bufferSize" in file C:\Users\qt\work\qt\qtbase\src\corelib\tools\qringbuffer.cpp, line 75
```

The message handler is called from the thread that sends the message, so
we need to make sure only one thread can access the file at a time,
since QFile is not inherently threadsafe.

Signed-off-by: Jared Van Bortel <jared@nomic.ai>

gpt4all-chat/src/logger.cpp

@@ -4,6 +4,7 @@
 #include <QDebug>
 #include <QGlobalStatic>
 #include <QIODevice>
+#include <QMutexLocker>
 #include <QStandardPaths>
 
 #include <cstdio>
@@ -62,8 +63,11 @@ void Logger::messageHandler(QtMsgType type, const QMessageLogContext &, const QS
     }
     // Get time and date
     auto timestamp = QDateTime::currentDateTime().toString();
-    // Write message
+
     const std::string out = u"[%1] (%2): %3\n"_s.arg(typeString, timestamp, msg).toStdString();
+
+    // Write message
+    QMutexLocker locker(&logger->m_mutex);
     logger->m_file.write(out.c_str());
     logger->m_file.flush();
     std::cerr << out;

gpt4all-chat/src/logger.h

@@ -2,19 +2,23 @@
 #define LOGGER_H
 
 #include <QFile>
+#include <QMutex>
 #include <QString>
 #include <QtLogging>
 
-class Logger
-{
-    QFile m_file;
-
-    static void messageHandler(QtMsgType type, const QMessageLogContext &context, const QString &msg);
-
+class Logger {
 public:
+    explicit Logger();
+
     static Logger *globalInstance();
 
-    explicit Logger();
+private:
+    static void messageHandler(QtMsgType type, const QMessageLogContext &context, const QString &msg);
+
+private:
+    QFile  m_file;
+    QMutex m_mutex;
+
     friend class MyLogger;
 };