Synchronization: Refactor Waiter to allow us to write tests

Instead of being only able to test the platform Waiter implementation,
this allows us to be able to test all Waiter implementations that
build on a specific platform.

A unittest is added that tests all implementations that work for the
platform, and allows us to check that the expected one is being used
by printing the name of the selected implementation.

PiperOrigin-RevId: 518072415
Change-Id: Ie9e6fcd9d8283b4038e6f4e68a304d2adcc04b19

1 files changed, 58 insertions, 0 deletions

diff --git a/absl/synchronization/internal/futex_waiter.h b/absl/synchronization/internal/futex_waiter.h
new file mode 100644
index 00000000..a6a72095
--- /dev/null
+++ b/absl/synchronization/internal/futex_waiter.h
@@ -0,0 +1,58 @@
+// Copyright 2023 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_FUTEX_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_WAITER_H_
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/futex.h"
+#include "absl/synchronization/internal/waiter_base.h"
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#define ABSL_INTERNAL_HAVE_FUTEX_WAITER 1
+
+class FutexWaiter : public WaiterCrtp<FutexWaiter> {
+ public:
+  FutexWaiter() : futex_(0) {}
+
+  bool Wait(KernelTimeout t);
+  void Post();
+  void Poke();
+
+  static constexpr char kName[] = "FutexWaiter";
+
+ private:
+  // 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");
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_FUTEX
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_WAITER_H_