Add support for creating an EventBaseManager with a custom backend factory method

Summary:
Add support for creating an EventBaseManager with a custom backend factory method

(Note: this ignores all push blocking failures!)

Reviewed By: kevin-vigor

Differential Revision: D20513736

fbshipit-source-id: 62edc876cd1c0667bd7beb85a3d6fbd9a5a03ebf

folly/io/async/EventBaseBackendBase.h

@@ -115,6 +115,8 @@ class EventBaseEvent {
 class EventBaseBackendBase {
  public:
   using Event = EventBaseEvent;
+  using FactoryFunc =
+      std::function<std::unique_ptr<folly::EventBaseBackendBase>()>;
 
   EventBaseBackendBase() = default;
   virtual ~EventBaseBackendBase() = default;

folly/io/async/EventBaseManager.cpp

@@ -66,7 +66,7 @@ EventBase* EventBaseManager::getEventBase() const {
   // have one?
   auto* info = localStore_.get();
   if (!info) {
-    info = new EventBaseInfo();
+    info = func_ ? new EventBaseInfo(func_()) : new EventBaseInfo();
     localStore_.reset(info);
 
     if (observer_) {

folly/io/async/EventBaseManager.h

@@ -39,6 +39,9 @@ class EventBaseManager {
   // encouraged. You should instead use the global singleton if possible.
   EventBaseManager() {}
 
+  explicit EventBaseManager(folly::EventBaseBackendBase::FactoryFunc func)
+      : func_(func) {}
+
   ~EventBaseManager() {}
 
   explicit EventBaseManager(const std::shared_ptr<EventBaseObserver>& observer)
@@ -111,7 +114,8 @@ class EventBaseManager {
  private:
   struct EventBaseInfo {
     EventBaseInfo(EventBase* evb, bool owned) : eventBase(evb), owned_(owned) {}
-
+    explicit EventBaseInfo(std::unique_ptr<EventBaseBackendBase>&& evb)
+        : eventBase(new EventBase(std::move(evb))), owned_(true) {}
     EventBaseInfo() : eventBase(new EventBase), owned_(true) {}
 
     EventBase* eventBase;
@@ -137,6 +141,8 @@ class EventBaseManager {
     eventBaseSet_.erase(evb);
   }
 
+  folly::EventBaseBackendBase::FactoryFunc func_;
+
   mutable folly::ThreadLocalPtr<EventBaseInfo> localStore_;
 
   // set of "active" EventBase instances