Make sure to not inline in runInMainContext

Summary: Currently if compiler decides to inline fiber function, we'll end up allocating variables on fiber stack even though we use `runInMainContext`.

Reviewed By: andriigrynenko

Differential Revision: D18842065

fbshipit-source-id: 847c4f95c44ef49f5c7537af878ca8340f2b7b16

folly/fibers/FiberManagerInternal-inl.h

@@ -46,6 +46,11 @@ inline FiberManager::Options preprocessOptions(FiberManager::Options opts) {
   return opts;
 }
 
+template <class F>
+FOLLY_NOINLINE invoke_result_t<F> runNoInline(F&& func) {
+  return func();
+}
+
 } // namespace
 
 inline void FiberManager::ensureLoopScheduled() {
@@ -512,7 +517,7 @@ void FiberManager::addTaskFinallyEager(F&& func, G&& finally) {
 template <typename F>
 invoke_result_t<F> FiberManager::runInMainContext(F&& func) {
   if (UNLIKELY(activeFiber_ == nullptr)) {
-    return func();
+    return runNoInline(std::forward<F>(func));
   }
 
   typedef invoke_result_t<F> Result;
@@ -607,5 +612,15 @@ typename FirstArgOf<F>::type::value_type inline await(F&& func) {
 
   return Promise<Result, BatonT>::await(std::forward<F>(func));
 }
+
+template <typename F>
+invoke_result_t<F> inline runInMainContext(F&& func) {
+  auto fm = FiberManager::getFiberManagerUnsafe();
+  if (UNLIKELY(fm == nullptr)) {
+    return runNoInline(std::forward<F>(func));
+  }
+  return fm->runInMainContext(std::forward<F>(func));
+}
+
 } // namespace fibers
 } // namespace folly

folly/fibers/FiberManagerInternal.h

@@ -675,13 +675,7 @@ typename FirstArgOf<F>::type::value_type inline await(F&& func);
  * @return value returned by func().
  */
 template <typename F>
-invoke_result_t<F> inline runInMainContext(F&& func) {
-  auto fm = FiberManager::getFiberManagerUnsafe();
-  if (UNLIKELY(fm == nullptr)) {
-    return func();
-  }
-  return fm->runInMainContext(std::forward<F>(func));
-}
+invoke_result_t<F> inline runInMainContext(F&& func);
 
 /**
  * Returns a refference to a fiber-local context for given Fiber. Should be

folly/fibers/test/FibersTest.cpp

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#include <array>
 #include <atomic>
 #include <thread>
 #include <vector>
@@ -982,6 +983,39 @@ TEST(FiberManager, runInMainContext) {
   EXPECT_TRUE(checkRan);
 }
 
+namespace {
+
+FOLLY_NOINLINE int runHugeStackInMainContext(bool& checkRan) {
+  auto ret = runInMainContext([&checkRan]() {
+    std::array<unsigned char, 1 << 20> buf;
+    buf.fill(42);
+    checkRan = true;
+    return buf[time(nullptr) % buf.size()];
+  });
+  EXPECT_TRUE(checkRan);
+  EXPECT_EQ(42, ret);
+  return ret;
+}
+
+} // namespace
+
+TEST(FiberManager, runInMainContextNoInline) {
+  FiberManager::Options opts;
+  opts.recordStackEvery = 1;
+  FiberManager manager(std::make_unique<SimpleLoopController>(), opts);
+  auto& loopController =
+      dynamic_cast<SimpleLoopController&>(manager.loopController());
+
+  bool checkRan = false;
+  manager.addTask([&] { return runHugeStackInMainContext(checkRan); });
+
+  EXPECT_TRUE(manager.stackHighWatermark() < 100000);
+
+  loopController.loop([&]() { loopController.stop(); });
+
+  EXPECT_TRUE(checkRan);
+}
+
 TEST(FiberManager, addTaskFinally) {
   FiberManager manager(std::make_unique<SimpleLoopController>());
   auto& loopController =