Mitigate lock contention in BlockingWaitExecutor

Summary: `BlockingWaitExecutor::add` (and similarly `WaitExecutor`) posts the baton while holding the queue lock. If these happen in two different threads, the `drive()` thread wakes up while the lock is still held; the `add()` thread may be descheduled before the lock is released, causing the other thread to have to wait (and possibly sleep) for a while.

Reviewed By: andriigrynenko

Differential Revision: D25814620

fbshipit-source-id: 07d3e9afdc4647e68f996e3f0a1c007fd1596611

folly/experimental/coro/BlockingWait.h

@@ -306,9 +306,12 @@ class BlockingWaitExecutor final : public folly::DrivableExecutor {
   }
 
   void add(Func func) override {
+    bool empty;
+    {
       auto wQueue = queue_.wlock();
-    bool empty = wQueue->empty();
+      empty = wQueue->empty();
       wQueue->push_back(std::move(func));
+    }
     if (empty) {
       baton_.post();
     }

folly/futures/Future-inl.h

@@ -478,12 +478,15 @@ FutureBase<T>::thenImplementation(
 class WaitExecutor final : public folly::Executor {
  public:
   void add(Func func) override {
+    bool empty;
+    {
       auto wQueue = queue_.wlock();
       if (wQueue->detached) {
         return;
       }
-    bool empty = wQueue->funcs.empty();
+      empty = wQueue->funcs.empty();
       wQueue->funcs.push_back(std::move(func));
+    }
     if (empty) {
       baton_.post();
     }