fix race handling bug in futures interrupt-handler

Summary:
Setting the interrupt-handler races with setting the interrupt-exception. This is expected to occur on occasion and should be handled in both `setInterruptHandler` and `raise`. The latter is correct but the former has a bug when, if the race happens, a moved-from interrupt-handler is invoked. The fix is to be sure to invoke a non-moved-from handler.

For consistency, always performs precisely one copy or move of the handler and always invokes the handler as an lvalue-ref-to-non-const.

Reviewed By: iahs

Differential Revision: D29974165

fbshipit-source-id: faf083e97c042fda0622801fc94bbc6fbca910fd

folly/futures/detail/Core.h

@@ -436,11 +436,12 @@ class CoreBase {
     if (hasResult()) {
       return;
     }
+    handler_type* handler = nullptr;
     auto interrupt = interrupt_.load(std::memory_order_acquire);
     switch (interrupt & InterruptMask) {
       case InterruptInitial: { // store the handler
         assert(!interrupt);
-        auto handler = new handler_type(static_cast<F&&>(fn));
+        handler = new handler_type(static_cast<F&&>(fn));
         auto exchanged = folly::atomic_compare_exchange_strong_explicit(
             &interrupt_,
             &interrupt,
@@ -451,7 +452,6 @@ class CoreBase {
           return;
         }
         // lost the race!
-        delete handler;
         if (interrupt & InterruptHasHandler) {
           terminate_with<std::logic_error>("set-interrupt-handler race");
         }
@@ -466,7 +466,14 @@ class CoreBase {
         }
         auto pointer = interrupt & ~InterruptMask;
         auto object = reinterpret_cast<exception_wrapper*>(pointer);
-        fn(as_const(*object));
+        if (handler) {
+          handler->handle(*object);
+          delete handler;
+        } else {
+          // mimic constructing and invoking a handler: 1 copy; non-const invoke
+          auto fn_ = static_cast<F&&>(fn);
+          fn_(as_const(*object));
+        }
         delete object;
         return;
       }