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, 90 insertions, 0 deletions

diff --git a/absl/synchronization/internal/futex_waiter.cc b/absl/synchronization/internal/futex_waiter.cc
new file mode 100644
index 00000000..0a76bc9a
--- /dev/null
+++ b/absl/synchronization/internal/futex_waiter.cc
@@ -0,0 +1,90 @@
+// 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.
+
+#include "absl/synchronization/internal/futex_waiter.h"
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX_WAITER
+
+#include <atomic>
+#include <cstdint>
+#include <cerrno>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/futex.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr char FutexWaiter::kName[];
+#endif
+
+bool FutexWaiter::Wait(KernelTimeout t) {
+  // Loop until we can atomically decrement futex from a positive
+  // value, waiting on a futex while we believe it is zero.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+
+  while (true) {
+    int32_t x = futex_.load(std::memory_order_relaxed);
+    while (x != 0) {
+      if (!futex_.compare_exchange_weak(x, x - 1,
+                                        std::memory_order_acquire,
+                                        std::memory_order_relaxed)) {
+        continue;  // Raced with someone, retry.
+      }
+      return true;  // Consumed a wakeup, we are done.
+    }
+
+    if (!first_pass) MaybeBecomeIdle();
+    const int err = Futex::WaitUntil(&futex_, 0, t);
+    if (err != 0) {
+      if (err == -EINTR || err == -EWOULDBLOCK) {
+        // Do nothing, the loop will retry.
+      } else if (err == -ETIMEDOUT) {
+        return false;
+      } else {
+        ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
+      }
+    }
+    first_pass = false;
+  }
+}
+
+void FutexWaiter::Post() {
+  if (futex_.fetch_add(1, std::memory_order_release) == 0) {
+    // We incremented from 0, need to wake a potential waiter.
+    Poke();
+  }
+}
+
+void FutexWaiter::Poke() {
+  // Wake one thread waiting on the futex.
+  const int err = Futex::Wake(&futex_, 1);
+  if (ABSL_PREDICT_FALSE(err < 0)) {
+    ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_FUTEX_WAITER