Fix blockingWait to schedule task on the DrivableExecutor

Summary: The first portion of the task (before the first suspension point) was run inline.

Reviewed By: yfeldblum

Differential Revision: D21460275

fbshipit-source-id: 31d50e15d5890fbc7127dfff424a4e814dcb2320

folly/experimental/coro/BlockingWait.h

@@ -222,10 +222,11 @@ class BlockingWaitTask {
     folly::Try<detail::lift_lvalue_reference_t<T>> result;
     auto& promise = coro_.promise();
     promise.setTry(&result);
-    {
-      RequestContextScopeGuard guard{RequestContext::saveContext()};
-      coro_.resume();
-    }
+    executor->add(
+        [coro = coro_, rctx = RequestContext::saveContext()]() mutable {
+          RequestContextScopeGuard guard{std::move(rctx)};
+          coro.resume();
+        });
     while (!promise.done()) {
       executor->drive();
     }

folly/experimental/coro/test/BlockingWaitTest.cpp

@@ -255,8 +255,13 @@ TEST_F(BlockingWaitTest, WaitTaskInFiber) {
   auto& fm = folly::fibers::getFiberManager(evb);
 
   auto future = fm.addTaskFuture([&] {
-    return folly::coro::blockingWait(folly::coro::co_invoke(
-        [&]() -> folly::coro::Task<int> { co_return co_await promise; }));
+    return folly::coro::blockingWait(
+        folly::coro::co_invoke([&]() -> folly::coro::Task<int> {
+          EXPECT_FALSE(folly::fibers::onFiber());
+          auto ret = co_await promise;
+          EXPECT_FALSE(folly::fibers::onFiber());
+          co_return ret;
+        }));
   });
 
   evb.loopOnce();