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>

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

@@ -104,9 +104,7 @@ 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);
 
@@ -145,6 +143,8 @@ 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_;
 };