Let FutureSplitter hold a KeepAlive

Summary: [Folly] Let `FutureSplitter` hold a `Executor::KeepAlive<>` rather than a raw `Executor*`.

Reviewed By: andriigrynenko

Differential Revision: D15835712

fbshipit-source-id: c0d5b4271212708657f550d5df51d4272e33fc44

folly/futures/FutureSplitter.h

@@ -79,7 +79,7 @@ class FutureSplitter {
 
  private:
   std::shared_ptr<SharedPromise<T>> promise_;
-  Executor* e_ = nullptr;
+  Executor::KeepAlive<> e_;
 
   static Executor* getExecutorFrom(Future<T>& f) {
     // If the passed future had a null executor, use an inline executor

folly/futures/test/FutureSplitterTest.cpp

@@ -15,6 +15,8 @@
  */
 
 #include <folly/futures/FutureSplitter.h>
+
+#include <folly/executors/ManualExecutor.h>
 #include <folly/portability/GTest.h>
 
 using namespace folly;
@@ -174,3 +176,19 @@ TEST(FutureSplitter, splitFutureFailure) {
   EXPECT_TRUE(f2.isReady());
   EXPECT_TRUE(f2.hasException());
 }
+
+TEST(FutureSplitter, lifetime) {
+  struct ManualExecutorWithPriority : folly::ManualExecutor {
+    void addWithPriority(Func func, int8_t) override {
+      add(std::move(func));
+    }
+  };
+  ManualExecutorWithPriority ex;
+  auto ka = folly::ExecutorWithPriority::create(
+      folly::getKeepAliveToken(ex), folly::Executor::MID_PRI);
+  auto split = folly::splitFuture(folly::via(std::move(ka), [] { return 3; }));
+  ex.drain();
+  auto fut = split.getFuture().thenValue([](auto i) { return i + 1; });
+  ex.drain();
+  EXPECT_EQ(4, fut.value());
+}