DPL: fix race condition with the delivery of region events (#11961)

* DPL: fix race condition with the delivery of region events

* Update Framework/Core/src/DataProcessingDevice.cxx

Author: ktf

Framework/Core/src/DataProcessingDevice.cxx

@@ -956,9 +956,12 @@ void DataProcessingDevice::InitTask()
       LOG(detail) << "ptr: " << info.ptr;
       LOG(detail) << "size: " << info.size;
       LOG(detail) << "flags: " << info.flags;
-      context.expectedRegionCallbacks -= 1;
+      // Now we check for pending events with the mutex,
+      // so the lines below are atomic.
       pendingRegionInfos.push_back(info);
-      // We always want to handle these on the main loop
+      context.expectedRegionCallbacks -= 1;
+      // We always want to handle these on the main loop,
+      // so we awake it.
       ServiceRegistryRef ref{registry};
       uv_async_send(ref.get<DeviceState>().awakeMainThread);
     });
@@ -990,11 +993,17 @@ void DataProcessingDevice::InitTask()
   // We will get there.
   this->fillContext(mServiceRegistry.get<DataProcessorContext>(ServiceRegistry::globalDeviceSalt()), deviceContext);
 
+  auto hasPendingEvents = [&mutex = mRegionInfoMutex, &pendingRegionInfos = mPendingRegionInfos](DeviceContext& deviceContext) {
+    std::lock_guard<std::mutex> lock(mutex);
+    return (pendingRegionInfos.empty() == false) || deviceContext.expectedRegionCallbacks > 0;
+  };
   /// We now run an event loop also in InitTask. This is needed to:
   /// * Make sure region registration callbacks are invoked
   /// on the main thread.
   /// * Wait for enough callbacks to be delivered before moving to START
-  while (deviceContext.expectedRegionCallbacks > 0 && uv_run(state.loop, UV_RUN_ONCE)) {
+  while (hasPendingEvents(deviceContext)) {
+    // Wait for the callback to signal its done, so that we do not busy wait.
+    uv_run(state.loop, UV_RUN_ONCE);
     // Handle callbacks if any
     {
       std::lock_guard<std::mutex> lock(mRegionInfoMutex);