Standardize fiber baton behavior across threads and fibers

Summary:
The behavior of fiber Baton is currently inconsistent around posting of a timed out baton, and subsequent calls to try_wait().  As stated in the post, only when the waiter is a thread will a post overwrite a timeout internally, and this is observable externally because try_wait returns true only if a baton is in the POSTED state, but not the timed out state.  Example code:

```
folly::fibers::Baton b;

b.try_wait_for(std::chrono::milliseconds(1));
b.try_wait(); // returns false

b.post();
b.try_wait(); // Returns true on threads, false on fibers
```

Other options we could consider for the fix:
 * Update postThread to leave baton in a timeout state, and have waitThread move the baton into the timeout state on a failed wait.
 * Update try_wait() to return true if the state is either POSTED or TIMEOUT.

Reviewed By: andriigrynenko

Differential Revision: D14324532

fbshipit-source-id: 51b993a6153cba0fc3cf5701f765fff8ad1c8e3c

folly/fibers/Baton.cpp

@@ -160,12 +160,12 @@ void Baton::postHelper(intptr_t new_value) {
       return postThread();
     }
 
-    if (waiter == POSTED || waiter == TIMEOUT) {
+    if (waiter == POSTED) {
       return;
     }
   } while (!waiter_.compare_exchange_weak(waiter, new_value));
 
-  if (waiter != NO_WAITER) {
+  if (waiter != NO_WAITER && waiter != TIMEOUT) {
     reinterpret_cast<Waiter*>(waiter)->post();
   }
 }

folly/fibers/test/FibersTest.cpp

@@ -217,6 +217,35 @@ TEST(FiberManager, batonTryWait) {
   manager.loopUntilNoReady();
 }
 
+TEST(FiberManager, batonTryWaitConsistent) {
+  folly::EventBase evb;
+  struct ExpectedException {};
+  auto& fm = getFiberManager(evb);
+
+  // Check if try_wait and post have a consistent behavior on a timed out
+  // baton
+  Baton b;
+
+  b.try_wait_for(std::chrono::milliseconds(1));
+  b.try_wait(); // returns false
+
+  b.post();
+  EXPECT_TRUE(b.try_wait());
+  b.reset();
+
+  fm.addTask([&]() {
+    b.try_wait_for(std::chrono::milliseconds(1));
+    b.try_wait(); // returns false
+
+    b.post();
+    EXPECT_TRUE(b.try_wait());
+  });
+
+  while (fm.hasTasks()) {
+    evb.loopOnce();
+  }
+}
+
 TEST(FiberManager, genericBatonFiberWait) {
   FiberManager manager(std::make_unique<SimpleLoopController>());