Fix bug with calling addTaskEager from another FiberManager

Summary:
The current code is calling `runInMainContext` on the FiberManager that we're scheduling to, which will do nothing if we're calling it from another fiber manager. This will assume fiber stack as a main stack which is incorrect.

This diff fixes it to call `folly::fibers::runInMainContext` that would properly call `runInMainContext` on the active FiberManager.

Reviewed By: andriigrynenko

Differential Revision: D25116215

fbshipit-source-id: 1d13f436a0599c077e7531dc1fe75f3a143e3478

folly/fibers/FiberManagerInternal-inl.h

@@ -307,7 +307,7 @@ inline void FiberManager::runEagerFiber(Fiber* fiber) {
 }
 
 inline void FiberManager::runEagerFiberImpl(Fiber* fiber) {
-  runInMainContext([&] {
+  folly::fibers::runInMainContext([&] {
     auto prevCurrentFiber = std::exchange(currentFiber_, fiber);
     SCOPE_EXIT { currentFiber_ = prevCurrentFiber; };
     runFibersHelper([&] { runReadyFiber(fiber); });

folly/fibers/test/FibersTest.cpp

@@ -2619,6 +2619,27 @@ TEST(FiberManager, addTaskEagerNested) {
   EXPECT_TRUE(secondTaskDone);
 }
 
+TEST(FiberManager, addTaskEagerNestedFiberManager) {
+  folly::EventBase evb;
+  auto& fm = getFiberManager(evb);
+  FiberManager::Options opts;
+  opts.stackSize *= 2;
+  auto& fm2 = getFiberManager(evb, FiberManager::FrozenOptions(opts));
+
+  bool eagerTaskDone{false};
+
+  fm.addTask([&] {
+    fm2.addTaskEager([&] {
+      EXPECT_FALSE(fm.hasActiveFiber());
+      eagerTaskDone = true;
+    });
+  });
+
+  evb.loop();
+
+  EXPECT_TRUE(eagerTaskDone);
+}
+
 TEST(FiberManager, swapWithException) {
   folly::EventBase evb;
   auto& fm = getFiberManager(evb);