reset_once

Summary: Add `reset_once`, a mechanism to reset a `once_flag`.

Reviewed By: Mizuchi

Differential Revision: D27071246

fbshipit-source-id: 4824782fb0ce6380b72e3e657e6f5073029eaf14

folly/synchronization/CallOnce.h

@@ -103,6 +103,16 @@ FOLLY_ALWAYS_INLINE bool test_once(OnceFlag const& flag) noexcept {
   return flag.test_once();
 }
 
+//  reset_once
+//
+//  Resets a flag.
+//
+//  Warning: unsafe to call concurrently with any other flag operations.
+template <typename OnceFlag>
+FOLLY_ALWAYS_INLINE void reset_once(OnceFlag& flag) noexcept {
+  return flag.reset_once();
+}
+
 template <typename Mutex, template <typename> class Atom>
 class basic_once_flag {
  public:
@@ -117,6 +127,9 @@ class basic_once_flag {
   template <typename OnceFlag>
   friend bool test_once(OnceFlag const& flag) noexcept;
 
+  template <typename OnceFlag>
+  friend void reset_once(OnceFlag&) noexcept;
+
   template <typename F, typename... Args>
   FOLLY_NOINLINE void call_once_slow(F&& f, Args&&... args) {
     std::lock_guard<Mutex> lock(mutex_);
@@ -145,6 +158,10 @@ class basic_once_flag {
     return called_.load(std::memory_order_acquire);
   }
 
+  FOLLY_ALWAYS_INLINE void reset_once() noexcept {
+    called_.store(false, std::memory_order_relaxed);
+  }
+
   Atom<bool> called_{false};
   Mutex mutex_;
 };
@@ -162,6 +179,9 @@ class compact_once_flag {
   template <typename OnceFlag>
   friend bool test_once(OnceFlag const& flag) noexcept;
 
+  template <typename OnceFlag>
+  friend void reset_once(OnceFlag&) noexcept;
+
   template <typename F, typename... Args>
   FOLLY_NOINLINE void call_once_slow(F&& f, Args&&... args) {
     folly::MicroLock::LockGuardWithData guard(mutex_);
@@ -191,6 +211,11 @@ class compact_once_flag {
     return mutex_.load(std::memory_order_acquire) != 0;
   }
 
+  FOLLY_ALWAYS_INLINE void reset_once() noexcept {
+    folly::MicroLock::LockGuardWithData guard(mutex_);
+    guard.storeValue(0);
+  }
+
   folly::MicroLock mutex_;
 };
 

folly/synchronization/test/CallOnceTest.cpp

@@ -135,3 +135,12 @@ TYPED_TEST(FollyCallOnce, TryCallOnce) {
   EXPECT_TRUE(folly::try_call_once(once, []() noexcept { return true; }));
   EXPECT_TRUE(folly::test_once(once));
 }
+
+TYPED_TEST(FollyCallOnce, ResetOnce) {
+  typename TestFixture::OnceFlag once;
+  EXPECT_FALSE(folly::test_once(once));
+  folly::call_once(once, [] {});
+  EXPECT_TRUE(folly::test_once(once));
+  folly::reset_once(once);
+  EXPECT_FALSE(folly::test_once(once));
+}