Swap LibevQuicEventBase callback list before processing the callbacks.

Summary:
Previously, the last callback in the list could destroy the LibevQuicEventBase and invalidate the referenced loopCallbackWrappers intrusive list used in the while loop check.

This swaps the list to avoid this corner case.

Reviewed By: mjoras, sharmafb

Differential Revision: D52529616

fbshipit-source-id: 57cbed96c48799bf93d3e12934d82496c5b84ab4

quic/common/events/LibevQuicEventBase.cpp

@@ -114,8 +114,16 @@ void LibevQuicEventBase::scheduleTimeout(
 }
 
 void LibevQuicEventBase::checkCallbacks() {
-  while (!loopCallbackWrappers_.empty()) {
-    loopCallbackWrappers_.front().runLoopCallback();
+  // Running the callbacks in the loop callback list may change the contents of
+  // the list or completely delete the list (with the event base). We swap the
+  // list here to ensure the list survives until the end of the function.
+  folly::IntrusiveList<LoopCallbackWrapper, &LoopCallbackWrapper::listHook_>
+      currentLoopWrappers;
+  loopCallbackWrappers_.swap(currentLoopWrappers);
+  while (!currentLoopWrappers.empty()) {
+    // runLoopCallback first unlinks the callback wrapper from the list.
+    // This allows the callback to schedule itself again on the swapped list.
+    currentLoopWrappers.front().runLoopCallback();
   }
 }