add tryReadUntil and make fixes along the way

Summary:
this diff adds tryReadUntil, which is a mirror of tryWriteUntil in both function and implementation.
Two bugs were exposed in the process of implementing and testing tryWriteUntil; they are fixed as well and are as follows:
  1. tryObtainPromisedPopTicket didn't assign to the passed ticket return reference in the failure case
  2. TurnSequencer::tryWaitForTurn() didn't distinguish between past turns and timeouts in the failure case; they need to be
     differentiated because SingleElementQueue::tryWaitFor{De/En}queue() should only fail in the timeout case, not if the turn has passed.

The two added unit tests are admittedly clumsy, but making the obvious simplifications to them keeps them from triggering the premature timeout race caused by bug 2 above, so I kept them as is.

Reviewed By: magedm

Differential Revision: D4050515

fbshipit-source-id: b0a3dd894d502c44be62d362ea347a1837df4c2f

folly/detail/TurnSequencer.h

@@ -17,13 +17,14 @@
 #pragma once
 
 #include <algorithm>
-#include <assert.h>
 #include <limits>
 
 #include <folly/detail/Futex.h>
 #include <folly/portability/Asm.h>
 #include <folly/portability/Unistd.h>
 
+#include <glog/logging.h>
+
 namespace folly {
 
 namespace detail {
@@ -79,14 +80,15 @@ struct TurnSequencer {
     return decodeCurrentSturn(state) == (turn << kTurnShift);
   }
 
+  enum class TryWaitResult { SUCCESS, PAST, TIMEDOUT };
+
   /// See tryWaitForTurn
   /// Requires that `turn` is not a turn in the past.
   void waitForTurn(const uint32_t turn,
                    Atom<uint32_t>& spinCutoff,
                    const bool updateSpinCutoff) noexcept {
-    bool success = tryWaitForTurn(turn, spinCutoff, updateSpinCutoff);
-    (void)success;
-    assert(success);
+    const auto ret = tryWaitForTurn(turn, spinCutoff, updateSpinCutoff);
+    DCHECK(ret == TryWaitResult::SUCCESS);
   }
 
   // Internally we always work with shifted turn values, which makes the
@@ -98,16 +100,18 @@ struct TurnSequencer {
   /// updateSpinCutoff is true then this will spin for up to kMaxSpins tries
   /// before blocking and will adjust spinCutoff based on the results,
   /// otherwise it will spin for at most spinCutoff spins.
-  /// Returns true if the wait succeeded, false if the turn is in the past
-  /// or the absTime time value is not nullptr and is reached before the turn
-  /// arrives
-  template <class Clock = std::chrono::steady_clock,
-            class Duration = typename Clock::duration>
-  bool tryWaitForTurn(const uint32_t turn,
-                      Atom<uint32_t>& spinCutoff,
-                      const bool updateSpinCutoff,
-                      const std::chrono::time_point<Clock, Duration>* absTime =
-                          nullptr) noexcept {
+  /// Returns SUCCESS if the wait succeeded, PAST if the turn is in the past
+  /// or TIMEDOUT if the absTime time value is not nullptr and is reached before
+  /// the turn arrives
+  template <
+      class Clock = std::chrono::steady_clock,
+      class Duration = typename Clock::duration>
+  TryWaitResult tryWaitForTurn(
+      const uint32_t turn,
+      Atom<uint32_t>& spinCutoff,
+      const bool updateSpinCutoff,
+      const std::chrono::time_point<Clock, Duration>* absTime =
+          nullptr) noexcept {
     uint32_t prevThresh = spinCutoff.load(std::memory_order_relaxed);
     const uint32_t effectiveSpinCutoff =
         updateSpinCutoff || prevThresh == 0 ? kMaxSpins : prevThresh;
@@ -124,7 +128,7 @@ struct TurnSequencer {
       // wrap-safe version of (current_sturn >= sturn)
       if(sturn - current_sturn >= std::numeric_limits<uint32_t>::max() / 2) {
         // turn is in the past
-        return false;
+        return TryWaitResult::PAST;
       }
 
       // the first effectSpinCutoff tries are spins, after that we will
@@ -152,7 +156,7 @@ struct TurnSequencer {
         auto futexResult =
             state_.futexWaitUntil(new_state, *absTime, futexChannel(turn));
         if (futexResult == FutexResult::TIMEDOUT) {
-          return false;
+          return TryWaitResult::TIMEDOUT;
         }
       } else {
         state_.futexWait(new_state, futexChannel(turn));
@@ -184,14 +188,14 @@ struct TurnSequencer {
       }
     }
 
-    return true;
+    return TryWaitResult::SUCCESS;
   }
 
   /// Unblocks a thread running waitForTurn(turn + 1)
   void completeTurn(const uint32_t turn) noexcept {
     uint32_t state = state_.load(std::memory_order_acquire);
     while (true) {
-      assert(state == encode(turn << kTurnShift, decodeMaxWaitersDelta(state)));
+      DCHECK(state == encode(turn << kTurnShift, decodeMaxWaitersDelta(state)));
       uint32_t max_waiter_delta = decodeMaxWaitersDelta(state);
       uint32_t new_state =
           encode((turn + 1) << kTurnShift,

folly/experimental/LockFreeRingBuffer.h

@@ -204,7 +204,8 @@ public:
   bool waitAndTryRead(T& dest, uint32_t turn) noexcept {
     uint32_t desired_turn = (turn + 1) * 2;
     Atom<uint32_t> cutoff(0);
-    if(!sequencer_.tryWaitForTurn(desired_turn, cutoff, false)) {
+    if (sequencer_.tryWaitForTurn(desired_turn, cutoff, false) !=
+        TurnSequencer<Atom>::TryWaitResult::SUCCESS) {
       return false;
     }
     memcpy(&dest, &data, sizeof(T));

folly/stop_watch.h

@@ -271,6 +271,13 @@ struct custom_stop_watch {
     return true;
   }
 
+  /**
+   * Returns the current checkpoint
+   */
+  typename clock_type::time_point getCheckpoint() const {
+    return checkpoint_;
+  }
+
  private:
   typename clock_type::time_point checkpoint_;
 };

folly/test/MPMCQueueTest.cpp

@@ -14,18 +14,20 @@
  * limitations under the License.
  */
 
-#include <folly/MPMCQueue.h>
 #include <folly/Format.h>
+#include <folly/MPMCQueue.h>
 #include <folly/Memory.h>
 #include <folly/portability/GTest.h>
 #include <folly/portability/SysResource.h>
 #include <folly/portability/SysTime.h>
 #include <folly/portability/Unistd.h>
+#include <folly/stop_watch.h>
 #include <folly/test/DeterministicSchedule.h>
 
 #include <boost/intrusive_ptr.hpp>
-#include <memory>
+#include <boost/thread/barrier.hpp>
 #include <functional>
+#include <memory>
 #include <thread>
 #include <utility>
 
@@ -1158,3 +1160,92 @@ TEST(MPMCQueue, explicit_zero_capacity_fail) {
   using DynamicMPMCQueueInt = MPMCQueue<int, std::atomic, true>;
   ASSERT_THROW(DynamicMPMCQueueInt cq(0), std::invalid_argument);
 }
+
+template <bool Dynamic>
+void testTryReadUntil() {
+  MPMCQueue<int, std::atomic, Dynamic> q{1};
+
+  const auto wait = std::chrono::milliseconds(100);
+  stop_watch<> watch;
+  bool rets[2];
+  int vals[2];
+  std::vector<std::thread> threads;
+  boost::barrier b{3};
+  for (int i = 0; i < 2; i++) {
+    threads.emplace_back([&, i] {
+      b.wait();
+      rets[i] = q.tryReadUntil(watch.getCheckpoint() + wait, vals[i]);
+    });
+  }
+
+  b.wait();
+  EXPECT_TRUE(q.write(42));
+
+  for (int i = 0; i < 2; i++) {
+    threads[i].join();
+  }
+
+  for (int i = 0; i < 2; i++) {
+    int other = (i + 1) % 2;
+    if (rets[i]) {
+      EXPECT_EQ(42, vals[i]);
+      EXPECT_FALSE(rets[other]);
+    }
+  }
+
+  EXPECT_TRUE(watch.elapsed(wait));
+}
+
+template <bool Dynamic>
+void testTryWriteUntil() {
+  MPMCQueue<int, std::atomic, Dynamic> q{1};
+  EXPECT_TRUE(q.write(42));
+
+  const auto wait = std::chrono::milliseconds(100);
+  stop_watch<> watch;
+  bool rets[2];
+  std::vector<std::thread> threads;
+  boost::barrier b{3};
+  for (int i = 0; i < 2; i++) {
+    threads.emplace_back([&, i] {
+      b.wait();
+      rets[i] = q.tryWriteUntil(watch.getCheckpoint() + wait, i);
+    });
+  }
+
+  b.wait();
+  int x;
+  EXPECT_TRUE(q.read(x));
+  EXPECT_EQ(42, x);
+
+  for (int i = 0; i < 2; i++) {
+    threads[i].join();
+  }
+  EXPECT_TRUE(q.read(x));
+
+  for (int i = 0; i < 2; i++) {
+    int other = (i + 1) % 2;
+    if (rets[i]) {
+      EXPECT_EQ(i, x);
+      EXPECT_FALSE(rets[other]);
+    }
+  }
+
+  EXPECT_TRUE(watch.elapsed(wait));
+}
+
+TEST(MPMCQueue, try_read_until) {
+  testTryReadUntil<false>();
+}
+
+TEST(MPMCQueue, try_read_until_dynamic) {
+  testTryReadUntil<true>();
+}
+
+TEST(MPMCQueue, try_write_until) {
+  testTryWriteUntil<false>();
+}
+
+TEST(MPMCQueue, try_write_until_dynamic) {
+  testTryWriteUntil<true>();
+}