src: avoid race condition in tracing code

`json_trace_writer_` is protected by `stream_mutex_`,
but one access to it was not guarded by a lock on said mutex.

Refs: https://github.com/nodejs/node/issues/25512

PR-URL: https://github.com/nodejs/node/pull/25624
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/src/tracing/node_trace_writer.cc b/src/tracing/node_trace_writer.cc
index 1d06c94233..5979810668 100644
--- a/src/tracing/node_trace_writer.cc
+++ b/src/tracing/node_trace_writer.cc
@@ -138,8 +138,13 @@ void NodeTraceWriter::FlushPrivate() {
 
 void NodeTraceWriter::Flush(bool blocking) {
   Mutex::ScopedLock scoped_lock(request_mutex_);
-  if (!json_trace_writer_) {
-    return;
+  {
+    // 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;
   }
   int request_id = ++num_write_requests_;
   int err = uv_async_send(&flush_signal_);