Disallow InlineExecutor in coro::Task

Reviewed By: davidtgoldblatt

Differential Revision: D16530636

fbshipit-source-id: 0ea2b9b628f3c41f87a3d31e0b4c1dc71101f639

folly/executors/GlobalExecutor.cpp

@@ -67,10 +67,18 @@ class GlobalExecutor {
   Function<std::unique_ptr<ExecutorBase>()> constructDefault_;
 };
 
+// aka InlineExecutor
+class DefaultCPUExecutor : public Executor {
+ public:
+  FOLLY_NOINLINE void add(Func f) override {
+    f();
+  }
+};
+
 Singleton<GlobalExecutor<Executor>> gGlobalCPUExecutor([] {
   return new GlobalExecutor<Executor>(
       // Default global CPU executor is an InlineExecutor.
-      [] { return std::make_unique<InlineExecutor>(); });
+      [] { return std::make_unique<DefaultCPUExecutor>(); });
 });
 
 Singleton<GlobalExecutor<IOExecutor>> gGlobalIOExecutor([] {

folly/experimental/coro/Task.h

@@ -318,6 +318,15 @@ class FOLLY_NODISCARD TaskWithExecutor {
       auto& promise = coro_.promise();
       DCHECK(!promise.continuation_);
       DCHECK(promise.executor_);
+      DCHECK(!dynamic_cast<folly::InlineExecutor*>(promise.executor_.get()))
+          << "InlineExecutor is not safe and is not supported for coro::Task. "
+          << "If you need to run a task inline in a unit-test, you should use "
+          << "coro::blockingWait instead.";
+      DCHECK(!dynamic_cast<folly::QueuedImmediateExecutor*>(
+          promise.executor_.get()))
+          << "QueuedImmediateExecutor is not safe and is not supported for coro::Task. "
+          << "If you need to run a task inline in a unit-test, you should use "
+          << "coro::blockingWait instead.";
 
       promise.continuation_ = continuation;
       promise.executor_->add(