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diff --git a/third_party/abseil-cpp/absl/synchronization/internal/waiter.h b/third_party/abseil-cpp/absl/synchronization/internal/waiter.h
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+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
+
+#include "absl/base/config.h"
+
+#ifdef _WIN32
+#include <sdkddkver.h>
+#else
+#include <pthread.h>
+#endif
+
+#ifdef __linux__
+#include <linux/futex.h>
+#endif
+
+#ifdef ABSL_HAVE_SEMAPHORE_H
+#include <semaphore.h>
+#endif
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/internal/thread_identity.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+// May be chosen at compile time via -DABSL_FORCE_WAITER_MODE=<index>
+#define ABSL_WAITER_MODE_FUTEX 0
+#define ABSL_WAITER_MODE_SEM 1
+#define ABSL_WAITER_MODE_CONDVAR 2
+#define ABSL_WAITER_MODE_WIN32 3
+
+#if defined(ABSL_FORCE_WAITER_MODE)
+#define ABSL_WAITER_MODE ABSL_FORCE_WAITER_MODE
+#elif defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
+#define ABSL_WAITER_MODE ABSL_WAITER_MODE_WIN32
+#elif defined(__BIONIC__)
+// Bionic supports all the futex operations we need even when some of the futex
+// definitions are missing.
+#define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
+#elif defined(__linux__) && defined(FUTEX_CLOCK_REALTIME)
+// FUTEX_CLOCK_REALTIME requires Linux >= 2.6.28.
+#define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
+#elif defined(ABSL_HAVE_SEMAPHORE_H)
+#define ABSL_WAITER_MODE ABSL_WAITER_MODE_SEM
+#else
+#define ABSL_WAITER_MODE ABSL_WAITER_MODE_CONDVAR
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+// Waiter is an OS-specific semaphore.
+class Waiter {
+ public:
+  // Prepare any data to track waits.
+  Waiter();
+
+  // Not copyable or movable
+  Waiter(const Waiter&) = delete;
+  Waiter& operator=(const Waiter&) = delete;
+
+  // Destroy any data to track waits.
+  ~Waiter();
+
+  // Blocks the calling thread until a matching call to `Post()` or
+  // `t` has passed. Returns `true` if woken (`Post()` called),
+  // `false` on timeout.
+  bool Wait(KernelTimeout t);
+
+  // Restart the caller of `Wait()` as with a normal semaphore.
+  void Post();
+
+  // If anyone is waiting, wake them up temporarily and cause them to
+  // call `MaybeBecomeIdle()`. They will then return to waiting for a
+  // `Post()` or timeout.
+  void Poke();
+
+  // Returns the Waiter associated with the identity.
+  static Waiter* GetWaiter(base_internal::ThreadIdentity* identity) {
+    static_assert(
+        sizeof(Waiter) <= sizeof(base_internal::ThreadIdentity::WaiterState),
+        "Insufficient space for Waiter");
+    return reinterpret_cast<Waiter*>(identity->waiter_state.data);
+  }
+
+  // How many periods to remain idle before releasing resources
+#ifndef THREAD_SANITIZER
+  static const int kIdlePeriods = 60;
+#else
+  // Memory consumption under ThreadSanitizer is a serious concern,
+  // so we release resources sooner. The value of 1 leads to 1 to 2 second
+  // delay before marking a thread as idle.
+  static const int kIdlePeriods = 1;
+#endif
+
+ private:
+#if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
+  // Futexes are defined by specification to be 32-bits.
+  // Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
+  std::atomic<int32_t> futex_;
+  static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");
+
+#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
+  // REQUIRES: mu_ must be held.
+  void InternalCondVarPoke();
+
+  pthread_mutex_t mu_;
+  pthread_cond_t cv_;
+  int waiter_count_;
+  int wakeup_count_;  // Unclaimed wakeups.
+
+#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
+  sem_t sem_;
+  // This seems superfluous, but for Poke() we need to cause spurious
+  // wakeups on the semaphore. Hence we can't actually use the
+  // semaphore's count.
+  std::atomic<int> wakeups_;
+
+#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
+  // WinHelper - Used to define utilities for accessing the lock and
+  // condition variable storage once the types are complete.
+  class WinHelper;
+
+  // REQUIRES: WinHelper::GetLock(this) must be held.
+  void InternalCondVarPoke();
+
+  // We can't include Windows.h in our headers, so we use aligned charachter
+  // buffers to define the storage of SRWLOCK and CONDITION_VARIABLE.
+  alignas(void*) unsigned char mu_storage_[sizeof(void*)];
+  alignas(void*) unsigned char cv_storage_[sizeof(void*)];
+  int waiter_count_;
+  int wakeup_count_;
+
+#else
+  #error Unknown ABSL_WAITER_MODE
+#endif
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_