Switch to HeapTimekeeper as default Timekeeper

Summary:
See D49060468 for the analysis.

The old timekeeper is kept behind a kill-switch just in case.

Reviewed By: dmm-fb

Differential Revision: D49061304

fbshipit-source-id: b7dc4c41d42f3dd03e5505aaa69ab08b0c6890e5

folly/futures/Future.cpp

@@ -18,7 +18,14 @@
 
 #include <folly/Likely.h>
 #include <folly/Singleton.h>
+#include <folly/futures/HeapTimekeeper.h>
 #include <folly/futures/ThreadWheelTimekeeper.h>
+#include <folly/portability/GFlags.h>
+
+FOLLY_GFLAGS_DEFINE_bool(
+    folly_futures_use_thread_wheel_timekeeper,
+    false,
+    "Use ThreadWheelTimekeeper for the default Future timekeeper singleton");
 
 namespace folly {
 namespace futures {
@@ -81,9 +88,14 @@ SemiFuture<Unit> wait(std::shared_ptr<fibers::Baton> baton) {
 namespace detail {
 
 namespace {
-Singleton<Timekeeper, TimekeeperSingletonTag> gTimekeeperSingleton([] {
-  return new ThreadWheelTimekeeper;
-});
+Singleton<Timekeeper, TimekeeperSingletonTag> gTimekeeperSingleton(
+    []() -> Timekeeper* {
+      if (FLAGS_folly_futures_use_thread_wheel_timekeeper) {
+        return new ThreadWheelTimekeeper;
+      } else {
+        return new HeapTimekeeper;
+      }
+    });
 } // namespace
 
 std::shared_ptr<Timekeeper> getTimekeeperSingleton() {

folly/futures/test/HeapTimekeeperTest.cpp

@@ -22,4 +22,13 @@ namespace folly {
 INSTANTIATE_TYPED_TEST_SUITE_P(
     HeapTimekeeperTest, TimekeeperTest, HeapTimekeeper);
 
+TEST(TimekeeperSingletonTest, ExpectedType) {
+  // This is just to check that the un-mocked default timekeeper singleton
+  // Implementation is covered by some instantiation of the test suite. If the
+  // default implementation is changed this test should be moved accordingly.
+  ASSERT_TRUE(
+      dynamic_cast<HeapTimekeeper*>(detail::getTimekeeperSingleton().get()) !=
+      nullptr);
+}
+
 } // namespace folly

folly/futures/test/ThreadWheelTimekeeperTest.cpp

@@ -22,13 +22,4 @@ namespace folly {
 INSTANTIATE_TYPED_TEST_SUITE_P(
     ThreadWheelTimekeeperTest, TimekeeperTest, ThreadWheelTimekeeper);
 
-TEST(TimekeeperSingletonTest, ExpectedType) {
-  // This is just to check that the un-mocked default timekeeper singleton
-  // implementation is covered by some instantiation of the test suite. If the
-  // default implementation is changed this test should be moved accordingly.
-  ASSERT_TRUE(
-      dynamic_cast<ThreadWheelTimekeeper*>(
-          detail::getTimekeeperSingleton().get()) != nullptr);
-}
-
 } // namespace folly