Chain interrupt handlers

Summary: Chain interrupt handlers in then().  User can always choose to set a new one later.

Test Plan: added unittest

Reviewed By: hans@fb.com

Subscribers: doug, folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2091569

Signature: t1:2091569:1432760579:ea7afa9b4e62e4b04b4d5f17950b9b7356a57f09

folly/futures/Future-inl.h

@@ -116,6 +116,7 @@ Future<T>::thenImplementation(F func, detail::argResult<isTry, F, Args...>) {
 
   // wrap these so we can move them into the lambda
   folly::MoveWrapper<Promise<B>> p;
+  p->setInterruptHandler(core_->getInterruptHandler());
   folly::MoveWrapper<F> funcm(std::forward<F>(func));
 
   // grab the Future now before we lose our handle on the Promise

folly/futures/detail/Core.h

@@ -249,6 +249,11 @@ class Core {
     }
   }
 
+  std::function<void(exception_wrapper const&)> getInterruptHandler() {
+    folly::MSLGuard guard(interruptLock_);
+    return interruptHandler_;
+  }
+
   /// Call only from Promise thread
   void setInterruptHandler(std::function<void(exception_wrapper const&)> fn) {
     folly::MSLGuard guard(interruptLock_);

folly/futures/test/TimekeeperTest.cpp

@@ -160,6 +160,15 @@ TEST(Timekeeper, interruptDoesntCrash) {
   f.cancel();
 }
 
+TEST(Timekeeper, chainedInterruptTest) {
+  bool test = false;
+  auto f = futures::sleep(Duration(100)).then([&](){
+    test = true;
+  });
+  f.cancel();
+  f.wait();
+  EXPECT_FALSE(test);
+}
 // TODO(5921764)
 /*
 TEST(Timekeeper, onTimeoutPropagates) {