Wrap the global timeline event handler callback in a std::atomic (#32073)

Author: jason-simmons

fml/trace_event.cc

@@ -19,7 +19,7 @@ namespace tracing {
 
 namespace {
 AsciiTrie gAllowlist;
-TimelineEventHandler gTimelineEventHandler;
+std::atomic<TimelineEventHandler> gTimelineEventHandler;
 
 inline void FlutterTimelineEvent(const char* label,
                                  int64_t timestamp0,
@@ -28,9 +28,11 @@ inline void FlutterTimelineEvent(const char* label,
                                  intptr_t argument_count,
                                  const char** argument_names,
                                  const char** argument_values) {
-  if (gTimelineEventHandler && gAllowlist.Query(label)) {
-    gTimelineEventHandler(label, timestamp0, timestamp1_or_async_id, type,
-                          argument_count, argument_names, argument_values);
+  TimelineEventHandler handler =
+      gTimelineEventHandler.load(std::memory_order_relaxed);
+  if (handler && gAllowlist.Query(label)) {
+    handler(label, timestamp0, timestamp1_or_async_id, type, argument_count,
+            argument_names, argument_values);
   }
 }
 }  // namespace

fml/trace_event.h

@@ -147,13 +147,13 @@ using TraceIDArg = int64_t;
 
 void TraceSetAllowlist(const std::vector<std::string>& allowlist);
 
-using TimelineEventHandler = std::function<void(const char*,
-                                                int64_t,
-                                                int64_t,
-                                                Dart_Timeline_Event_Type,
-                                                intptr_t,
-                                                const char**,
-                                                const char**)>;
+typedef void (*TimelineEventHandler)(const char*,
+                                     int64_t,
+                                     int64_t,
+                                     Dart_Timeline_Event_Type,
+                                     intptr_t,
+                                     const char**,
+                                     const char**);
 
 void TraceSetTimelineEventHandler(TimelineEventHandler handler);