change IOExecutorThreadPool such that observer logic can be overriden

Summary: We want to have a custom `IOExecutorThreadPool` that performs different logic for handling observers being adding or not. This change makes it so the functions and structs necessary to do so can be properly overriden.

Reviewed By: Gownta

Differential Revision: D32101852

fbshipit-source-id: 2f1c6bb4c00d41630891aa72b8e519708cb1a40f

folly/executors/IOThreadPoolExecutor.h

@@ -88,7 +88,7 @@ class IOThreadPoolExecutor : public ThreadPoolExecutor, public IOExecutor {
 
   folly::EventBaseManager* getEventBaseManager();
 
- private:
+ protected:
   struct alignas(Thread) IOThread : public Thread {
     IOThread(IOThreadPoolExecutor* pool)
         : Thread(pool), shouldRun(true), pendingTasks(0) {}
@@ -98,6 +98,7 @@ class IOThreadPoolExecutor : public ThreadPoolExecutor, public IOExecutor {
     std::mutex eventBaseShutdownMutex_;
   };
 
+ private:
   ThreadPtr makeThread() override;
   std::shared_ptr<IOThread> pickThread();
   void threadRun(ThreadPtr thread) override;

folly/executors/ThreadPoolExecutor.h

@@ -172,8 +172,8 @@ class ThreadPoolExecutor : public DefaultKeepAliveExecutor {
     virtual ~Observer() = default;
   };
 
-  void addObserver(std::shared_ptr<Observer>);
-  void removeObserver(std::shared_ptr<Observer>);
+  virtual void addObserver(std::shared_ptr<Observer>);
+  virtual void removeObserver(std::shared_ptr<Observer>);
 
   void setThreadDeathTimeout(std::chrono::milliseconds timeout) {
     threadTimeout_ = timeout;