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, 3 insertions, 1 deletions

diff --git a/src/tracing/node_trace_writer.h b/src/tracing/node_trace_writer.h
index a91176ad49..f412587ab9 100644
--- a/src/tracing/node_trace_writer.h
+++ b/src/tracing/node_trace_writer.h
@@ -45,9 +45,11 @@ class NodeTraceWriter : public AsyncTraceWriter {
   // Triggers callback to close async objects, ending the tracing thread.
   uv_async_t exit_signal_;
   // Prevents concurrent R/W on state related to serialized trace data
-  // before it's written to disk, namely stream_ and total_traces_.
+  // before it's written to disk, namely stream_ and total_traces_
+  // 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.