fixup! src: avoid race condition in tracing code

src/tracing/node_trace_writer.cc

@@ -139,6 +139,9 @@ void NodeTraceWriter::FlushPrivate() {
 void NodeTraceWriter::Flush(bool blocking) {
   Mutex::ScopedLock scoped_lock(request_mutex_);
   {
+    // We need to lock the mutexes here in a nested fashion; stream_mutex_
+    // protects json_trace_writer_, and without request_mutex_ there might be
+    // a time window in which the stream state changes?
     Mutex::ScopedLock stream_mutex_lock(stream_mutex_);
     if (!json_trace_writer_)
       return;

src/tracing/node_trace_writer.h

@@ -49,6 +49,7 @@ class NodeTraceWriter : public AsyncTraceWriter {
   // as well as json_trace_writer_.
   Mutex stream_mutex_;
   // Prevents concurrent R/W on state related to write requests.
+  // If both mutexes are locked, request_mutex_ has to be locked first.
   Mutex request_mutex_;
   // Allows blocking calls to Flush() to wait on a condition for
   // trace events to be written to disk.