Check for existence of thread during getEventBase

Summary: During shutdown there's a race condition in IOThreadPoolExecutor::getEventBase where a thread is created in ensureActiveThreads, and then destroyed before pickThread() is called. This adds the same guard that exists in add().

Reviewed By: yfeldblum

Differential Revision: D17955227

fbshipit-source-id: 042cb5c42056a9da4578571fb10936bbc477c4ff

folly/executors/IOThreadPoolExecutor.cpp

@@ -141,6 +141,9 @@ IOThreadPoolExecutor::pickThread() {
 EventBase* IOThreadPoolExecutor::getEventBase() {
   ensureActiveThreads();
   SharedMutex::ReadHolder r{&threadListLock_};
+  if (threadList_.get().empty()) {
+    throw std::runtime_error("No threads available");
+  }
   return pickThread()->eventBase;
 }