Rotate previous log file to '.old' if it exists

src/common/logging/backend.cpp

@@ -144,10 +144,15 @@ void ColorConsoleBackend::Write(const Entry& entry) {
     PrintColoredMessage(entry);
 }
 
-// _SH_DENYWR allows read only access to the file for other programs.
-// It is #defined to 0 on other platforms
-FileBackend::FileBackend(const std::string& filename)
-    : file(filename, "w", _SH_DENYWR), bytes_written(0) {}
+FileBackend::FileBackend(const std::string& filename) : bytes_written(0) {
+    if (FileUtil::Exists(filename)) {
+        FileUtil::Rename(filename, filename + ".old");
+    }
+
+    // _SH_DENYWR allows read only access to the file for other programs.
+    // It is #defined to 0 on other platforms
+    file = FileUtil::IOFile(filename, "w", _SH_DENYWR);
+}
 
 void FileBackend::Write(const Entry& entry) {
     // prevent logs from going over the maximum size (in case its spamming and the user doesn't