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, 15 insertions, 113 deletions

diff --git a/absl/synchronization/internal/waiter.h b/absl/synchronization/internal/waiter.h
index b8adfeb5..07bba10a 100644
--- a/absl/synchronization/internal/waiter.h
+++ b/absl/synchronization/internal/waiter.h
@@ -17,27 +17,10 @@
 #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/futex.h"
-#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/futex_waiter.h"
+#include "absl/synchronization/internal/pthread_waiter.h"
+#include "absl/synchronization/internal/sem_waiter.h"
+#include "absl/synchronization/internal/win32_waiter.h"
 
 // May be chosen at compile time via -DABSL_FORCE_WAITER_MODE=<index>
 #define ABSL_WAITER_MODE_FUTEX 0
@@ -47,112 +30,31 @@
 
 #if defined(ABSL_FORCE_WAITER_MODE)
 #define ABSL_WAITER_MODE ABSL_FORCE_WAITER_MODE
-#elif defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
+#elif defined(ABSL_INTERNAL_HAVE_WIN32_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_WIN32
-#elif defined(ABSL_INTERNAL_HAVE_FUTEX)
+#elif defined(ABSL_INTERNAL_HAVE_FUTEX_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
-#elif defined(ABSL_HAVE_SEMAPHORE_H)
+#elif defined(ABSL_INTERNAL_HAVE_SEM_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_SEM
-#else
+#elif defined(ABSL_INTERNAL_HAVE_PTHREAD_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_CONDVAR
+#else
+#error ABSL_WAITER_MODE is undefined
 #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;
-
-  // 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 ABSL_HAVE_THREAD_SANITIZER
-  static constexpr 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:
-  // The destructor must not be called since Mutex/CondVar
-  // can use PerThreadSem/Waiter after the thread exits.
-  // Waiter objects are embedded in ThreadIdentity objects,
-  // which are reused via a freelist and are never destroyed.
-  ~Waiter() = delete;
-
 #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.
-
+using Waiter = FutexWaiter;
 #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_;
-
+using Waiter = SemWaiter;
+#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
+using Waiter = PthreadWaiter;
 #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 character
-  // buffers to define the storage of SRWLOCK and CONDITION_VARIABLE.
-  // SRW locks and condition variables do not need to be explicitly destroyed.
-  // https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
-  // https://stackoverflow.com/questions/28975958/why-does-windows-have-no-deleteconditionvariable-function-to-go-together-with
-  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
+using Waiter = Win32Waiter;
 #endif
-};
 
 }  // namespace synchronization_internal
 ABSL_NAMESPACE_END