Check that fiber suspends are not occuring in exception catch blocks

Summary:
Fiber suspends are incompatible with C++ exception handling. We should enforce that callers are not suspending fibers while handling exceptions.
See https://fb.facebook.com/groups/560979627394613/permalink/1119586164867287/

NOTE: This check currently fires not just for baton.wait() (and all things based on baton), but also for calling runInMainContext. I'm unsure of the safety of this.

Reviewed By: yfeldblum

Differential Revision: D6695110

fbshipit-source-id: cf36edbab9f0573cbf639bf79d0c267681ac5914

folly/Portability.h

@@ -350,6 +350,13 @@ constexpr auto kMscVer = _MSC_VER;
 #else
 constexpr auto kMscVer = 0;
 #endif
+
+// TODO: Remove when removing support for gcc4.9
+#if __GLIBCXX__ && __GLIBCXX__ == 20150123
+constexpr auto kIsGlib49 = true;
+#else
+constexpr auto kIsGlib49 = false;
+#endif
 } // namespace folly
 
 // Define FOLLY_USE_CPP14_CONSTEXPR to be true if the compiler's C++14

folly/fibers/Fiber.cpp

@@ -170,6 +170,9 @@ void Fiber::preempt(State state) {
     DCHECK_EQ(fiberManager_.activeFiber_, this);
     DCHECK_EQ(state_, RUNNING);
     DCHECK_NE(state, RUNNING);
+    DCHECK(!std::current_exception());
+    // std::uncaught_exception is broken in libstdc++ v4.9
+    DCHECK(kIsGlib49 || !std::uncaught_exception());
 
     state_ = state;