FiberManager - executeOnFiber

Summary:
This diff implements a new API to schedule a function on a new fiber and block the existing fiber till completion.

The API should be used sparingly to deal with fiber-stack-overflow issues

Reviewed By: A5he

Differential Revision: D22074021

fbshipit-source-id: 893e578b1acd6dd5cca34187fc8431202cdb73da

folly/fibers/async/Collect.h

@@ -83,6 +83,19 @@ collectAll(Ts&&... tasks) {
   return Async(folly::unwrapTryTuple(std::move(tuple)));
 }
 
+/*
+ * Run an async-annotated functor on a new fiber, blocking the current fiber.
+ *
+ * Should be used sparingly to reset the fiber stack usage and avoid fiber stack
+ * overflows
+ */
+template <typename F>
+auto executeOnNewFiber(F&& func) {
+  DCHECK(detail::onFiber());
+  return futureWait(detail::addFiberFuture(
+      std::forward<F>(func), FiberManager::getFiberManager()));
+}
+
 } // namespace async
 } // namespace fibers
 } // namespace folly

folly/fibers/async/test/AsyncTest.cpp

@@ -259,12 +259,18 @@ TEST(FiberManager, asyncFiberManager) {
   }
 
   {
-    bool completed = false;
+    bool outerCompleted = false;
     async::executeOnFiberAndWait([&]() -> async::Async<void> {
-      completed = true;
+      bool innerCompleted = false;
+      async::await(async::executeOnNewFiber([&]() -> async::Async<void> {
+        innerCompleted = true;
+        return {};
+      }));
+      EXPECT_TRUE(innerCompleted);
+      outerCompleted = true;
       return {};
     });
-    EXPECT_TRUE(completed);
+    EXPECT_TRUE(outerCompleted);
   }
 
   {