Fix data race on PerformanceObserver entry buffer

Summary:
Add mutex to protect `PerformanceObserver::buffer_` and `didScheduleFlushBuffer_`, which can be accessed concurrently from a background thread (`handleEntry`) and JS thread (`takeRecords`).

Fixes T263429319.

Changelog: [Internal]

Differential Revision: D99820359

Author: huntie

packages/react-native/ReactCommon/react/performance/timeline/PerformanceObserver.cpp

@@ -25,16 +25,21 @@ void PerformanceObserver::handleEntry(const PerformanceEntry& entry) {
       return;
     }
 
-    buffer_.push_back(entry);
+    {
+      std::lock_guard lock(bufferMutex_);
+      buffer_.push_back(entry);
+    }
     scheduleFlushBuffer();
   }
 }
 
 std::vector<PerformanceEntry> PerformanceObserver::takeRecords() {
   std::vector<PerformanceEntry> result;
-  buffer_.swap(result);
-
-  didScheduleFlushBuffer_ = false;
+  {
+    std::lock_guard lock(bufferMutex_);
+    buffer_.swap(result);
+    didScheduleFlushBuffer_ = false;
+  }
 
   return result;
 }
@@ -87,9 +92,16 @@ void PerformanceObserver::disconnect() noexcept {
 }
 
 void PerformanceObserver::scheduleFlushBuffer() {
-  if (!didScheduleFlushBuffer_) {
-    didScheduleFlushBuffer_ = true;
+  bool shouldSchedule = false;
+  {
+    std::lock_guard lock(bufferMutex_);
+    if (!didScheduleFlushBuffer_) {
+      didScheduleFlushBuffer_ = true;
+      shouldSchedule = true;
+    }
+  }
 
+  if (shouldSchedule) {
     callback_();
   }
 }

packages/react-native/ReactCommon/react/performance/timeline/PerformanceObserver.h

@@ -14,6 +14,7 @@
 
 #include <functional>
 #include <memory>
+#include <mutex>
 #include <unordered_set>
 #include <vector>
 
@@ -126,6 +127,7 @@ class PerformanceObserver : public std::enable_shared_from_this<PerformanceObser
 
   /// https://www.w3.org/TR/event-timing/#sec-modifications-perf-timeline
   HighResDuration durationThreshold_ = DEFAULT_DURATION_THRESHOLD;
+  std::mutex bufferMutex_;
   std::vector<PerformanceEntry> buffer_;
   bool didScheduleFlushBuffer_ = false;
   bool requiresDroppedEntries_ = false;