src: trace_events: fix race with metadata events

Multiple threads may be concurrently adding to the metadata_events list. Protect access with a mutex. Fixes: #24129 PR-URL: #25235 Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: ofrobots

src/tracing/agent.cc

@@ -207,9 +207,17 @@ void Agent::AppendTraceEvent(TraceObject* trace_event) {
  id_writer.second->AppendTraceEvent(trace_event);
 }
 
+void Agent::AddMetadataEvent(std::unique_ptr<TraceObject> event) {
+ Mutex::ScopedLock lock(metadata_events_mutex_);
+ metadata_events_.push_back(std::move(event));
+}
+
 void Agent::Flush(bool blocking) {
- for (const auto& event : metadata_events_)
- AppendTraceEvent(event.get());
+ {
+ Mutex::ScopedLock lock(metadata_events_mutex_);
+ for (const auto& event : metadata_events_)
+ AppendTraceEvent(event.get());
+ }
 
  for (const auto& id_writer : writers_)
  id_writer.second->Flush(blocking);

src/tracing/agent.h

@@ -102,9 +102,7 @@ class Agent {
  // Writes to all writers registered through AddClient().
  void AppendTraceEvent(TraceObject* trace_event);
 
- void AddMetadataEvent(std::unique_ptr<TraceObject> event) {
- metadata_events_.push_back(std::move(event));
- }
+ void AddMetadataEvent(std::unique_ptr<TraceObject> event);
  // Flushes all writers registered through AddClient().
  void Flush(bool blocking);
 
@@ -143,6 +141,8 @@ class Agent {
  ConditionVariable initialize_writer_condvar_;
  uv_async_t initialize_writer_async_;
  std::set<AsyncTraceWriter*> to_be_initialized_;
+
+ Mutex metadata_events_mutex_;
  std::list<std::unique_ptr<TraceObject>> metadata_events_;
 };