Baton::ready, a const variant of try_wait

Summary: [Folly] `Baton::ready`, a `const` variant of `try_wait`.

Reviewed By: djwatson

Differential Revision: D6508064

fbshipit-source-id: ba458577574ba58165408a93238da7eb09adf1e6

folly/fibers/Baton.cpp

@@ -153,8 +153,7 @@ void Baton::postHelper(intptr_t new_value) {
 }
 
 bool Baton::try_wait() {
-  auto state = waitingFiber_.load();
-  return state == POSTED;
+  return ready();
 }
 
 void Baton::postThread() {

folly/fibers/Baton.h

@@ -40,6 +40,11 @@ class Baton {
 
   ~Baton() {}
 
+  bool ready() const {
+    auto state = waitingFiber_.load();
+    return state == POSTED;
+  }
+
   /**
    * Puts active fiber to sleep. Returns when post is called.
    */

folly/synchronization/Baton.h

@@ -79,6 +79,12 @@ struct Baton {
     assert(state_.load(std::memory_order_relaxed) != WAITING);
   }
 
+  FOLLY_ALWAYS_INLINE bool ready() const noexcept {
+    auto s = state_.load(std::memory_order_acquire);
+    assert(s == INIT || s == EARLY_DELIVERY);
+    return LIKELY(s == EARLY_DELIVERY);
+  }
+
   /// Equivalent to destroying the Baton and creating a new one.  It is
   /// a bug to call this while there is a waiting thread, so in practice
   /// the waiter will be the one that resets the baton.
@@ -176,9 +182,7 @@ struct Baton {
   ///
   /// @return       true if baton has been posted, false othewise
   FOLLY_ALWAYS_INLINE bool try_wait() const noexcept {
-    auto s = state_.load(std::memory_order_acquire);
-    assert(s == INIT || s == EARLY_DELIVERY);
-    return LIKELY(s == EARLY_DELIVERY);
+    return ready();
   }
 
   /// Similar to wait, but with a timeout. The thread is unblocked if the

folly/synchronization/test/BatonTestHelpers.h

@@ -107,8 +107,10 @@ void run_timed_wait_regular_test() {
 template <template <typename> class Atom, bool Blocking>
 void run_try_wait_tests() {
   Baton<Atom, Blocking> b;
+  EXPECT_FALSE(b.ready());
   EXPECT_FALSE(b.try_wait());
   b.post();
+  EXPECT_TRUE(b.ready());
   EXPECT_TRUE(b.try_wait());
 }