Add detach helper to folly::SemiFuture for no executor detach

Summary: Adds a strongly discouraged discard helper for cases where we know there is no deferred work.

Reviewed By: yfeldblum

Differential Revision: D22248947

fbshipit-source-id: 26265c33d9cd10848ab730f9c5f0356eb843025b

folly/futures/Future-inl.h

@@ -2559,6 +2559,16 @@ void detachOnGlobalCPUExecutor(folly::SemiFuture<T>&& fut) {
   detachOn(folly::getGlobalCPUExecutor(), std::move(fut));
 }
 
+template <class T>
+void detachWithoutExecutor(folly::SemiFuture<T>&& fut) {
+  auto executor = futures::detail::stealDeferredExecutor(fut);
+  // Fail if we try to detach a SemiFuture with deferred work
+  DCHECK(executor.get() == nullptr);
+  if (executor) {
+    executor.get()->detach();
+  }
+}
+
 } // namespace futures
 
 template <class Clock>

folly/futures/Future.h

@@ -451,11 +451,20 @@ template <typename T>
 futures::detail::DeferredWrapper stealDeferredExecutor(SemiFuture<T>& future);
 } // namespace detail
 
+// Detach the SemiFuture by scheduling work onto exec.
 template <class T>
 void detachOn(folly::Executor::KeepAlive<> exec, folly::SemiFuture<T>&& fut);
 
+// Detach the SemiFuture by detaching work onto the global CPU executor.
 template <class T>
 void detachOnGlobalCPUExecutor(folly::SemiFuture<T>&& fut);
+
+// Detach the SemiFuture with no executor.
+// NOTE: If there is deferred work of any sort on this SemiFuture
+// will leak and not be run.
+// Use at your own risk.
+template <class T>
+void detachWithoutExecutor(folly::SemiFuture<T>&& fut);
 } // namespace futures
 
 /// The interface (along with Future) for the consumer-side of a

folly/futures/test/FutureTest.cpp

@@ -1441,7 +1441,7 @@ TEST(Future, NoThrow) {
 }
 
 TEST(Future, DetachTest) {
-  folly::Baton<> b1, b2;
+  folly::Baton<> b1, b2, b3;
   folly::ManualExecutor exec;
   std::atomic<int> result(0);
 
@@ -1456,10 +1456,22 @@ TEST(Future, DetachTest) {
         b2.post();
       }));
 
+  // This will run correctly, but cleanly detach because we know for sure
+  // this is safe
+  folly::futures::detachWithoutExecutor( //
+      makeSemiFuture()
+          .via(&exec)
+          .thenValue([&](auto&&) {
+            result++;
+            b3.post();
+          })
+          .semi());
+
   exec.drain();
   b1.wait();
   b2.wait();
-  EXPECT_TRUE(result == 2);
+  b3.wait();
+  EXPECT_TRUE(result == 3);
 }
 
 #if FOLLY_FUTURE_USING_FIBER