synchronization/internal/mutex_nonprod.inc

// Do not include.  This is an implementation detail of base/mutex.h.
//
// Declares three classes:
//
// base::internal::MutexImpl - implementation helper for Mutex
// base::internal::CondVarImpl - implementation helper for CondVar
// base::internal::SynchronizationStorage<T> - implementation helper for
//                                             Mutex, CondVar

#include <type_traits>

#if defined(_WIN32)
#include <condition_variable>
#include <mutex>
#else
#include <pthread.h>
#endif

#include "absl/base/call_once.h"
#include "absl/time/time.h"

// Declare that Mutex::ReaderLock is actually Lock().  Intended primarily
// for tests, and even then as a last resort.
#ifdef ABSL_MUTEX_READER_LOCK_IS_EXCLUSIVE
#error ABSL_MUTEX_READER_LOCK_IS_EXCLUSIVE cannot be directly set
#else
#define ABSL_MUTEX_READER_LOCK_IS_EXCLUSIVE 1
#endif

// Declare that Mutex::EnableInvariantDebugging is not implemented.
// Intended primarily for tests, and even then as a last resort.
#ifdef ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED
#error ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED cannot be directly set
#else
#define ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED 1
#endif

namespace absl {
ABSL_NAMESPACE_BEGIN
class Condition;

namespace synchronization_internal {

class MutexImpl;

// Do not use this implementation detail of CondVar. Provides most of the
// implementation, but should not be placed directly in static storage
// because it will not linker initialize properly. See
// SynchronizationStorage<T> below for what we mean by linker
// initialization.
class CondVarImpl {
 public:
  CondVarImpl();
  CondVarImpl(const CondVarImpl&) = delete;
  CondVarImpl& operator=(const CondVarImpl&) = delete;
  ~CondVarImpl();

  void Signal();
  void SignalAll();
  void Wait(MutexImpl* mutex);
  bool WaitWithDeadline(MutexImpl* mutex, absl::Time deadline);

 private:
#if defined(_WIN32)
  std::condition_variable_any std_cv_;
#else
  pthread_cond_t pthread_cv_;
#endif
};

// Do not use this implementation detail of Mutex. Provides most of the
// implementation, but should not be placed directly in static storage
// because it will not linker initialize properly. See
// SynchronizationStorage<T> below for what we mean by linker
// initialization.
class MutexImpl {
 public:
  MutexImpl();
  MutexImpl(const MutexImpl&) = delete;
  MutexImpl& operator=(const MutexImpl&) = delete;
  ~MutexImpl();

  void Lock();
  bool TryLock();
  void Unlock();
  void Await(const Condition& cond);
  bool AwaitWithDeadline(const Condition& cond, absl::Time deadline);

 private:
  friend class CondVarImpl;

#if defined(_WIN32)
  std::mutex std_mutex_;
#else
  pthread_mutex_t pthread_mutex_;
#endif

  // True if the underlying mutex is locked.  If the destructor is entered
  // while locked_, the underlying mutex is unlocked.  Mutex supports
  // destruction while locked, but the same is undefined behavior for both
  // pthread_mutex_t and std::mutex.
  bool locked_ = false;

  // Signaled before releasing the lock, in support of Await.
  CondVarImpl released_;
};

// Do not use this implementation detail of CondVar and Mutex.  A storage
// space for T that supports a LinkerInitialized constructor. T must
// have a default constructor, which is called by the first call to
// get(). T's destructor is never called if the LinkerInitialized
// constructor is called.
//
// Objects constructed with the default constructor are constructed and
// destructed like any other object, and should never be allocated in
// static storage.
//
// Objects constructed with the LinkerInitialized constructor should
// always be in static storage. For such objects, calls to get() are always
// valid, except from signal handlers.
//
// Note that this implementation relies on undefined language behavior that
// are known to hold for the set of supported compilers. An analysis
// follows.
//
// From the C++11 standard:
//
// [basic.life] says an object has non-trivial initialization if it is of
// class type and it is initialized by a constructor other than a trivial
// default constructor.  (the LinkerInitialized constructor is
// non-trivial)
//
// [basic.life] says the lifetime of an object with a non-trivial
// constructor begins when the call to the constructor is complete.
//
// [basic.life] says the lifetime of an object with non-trivial destructor
// ends when the call to the destructor begins.
//
// [basic.life] p5 specifies undefined behavior when accessing non-static
// members of an instance outside its
// lifetime. (SynchronizationStorage::get() access non-static members)
//
// So, LinkerInitialized object of SynchronizationStorage uses a
// non-trivial constructor, which is called at some point during dynamic
// initialization, and is therefore subject to order of dynamic
// initialization bugs, where get() is called before the object's
// constructor is, resulting in undefined behavior.
//
// Similarly, a LinkerInitialized SynchronizationStorage object has a
// non-trivial destructor, and so its lifetime ends at some point during
// destruction of objects with static storage duration [basic.start.term]
// p4. There is a window where other exit code could call get() after this
// occurs, resulting in undefined behavior.
//
// Combined, these statements imply that LinkerInitialized instances
// of SynchronizationStorage<T> rely on undefined behavior.
//
// However, in practice, the implementation works on all supported
// compilers. Specifically, we rely on:
//
// a) zero-initialization being sufficient to initialize
// LinkerInitialized instances for the purposes of calling
// get(), regardless of when the constructor is called. This is
// because the is_dynamic_ boolean is correctly zero-initialized to
// false.
//
// b) the LinkerInitialized constructor is a NOP, and immaterial to
// even to concurrent calls to get().
//
// c) the destructor being a NOP for LinkerInitialized objects
// (guaranteed by a check for !is_dynamic_), and so any concurrent and
// subsequent calls to get() functioning as if the destructor were not
// called, by virtue of the instances' storage remaining valid after the
// destructor runs.
//
// d) That a-c apply transitively when SynchronizationStorage<T> is the
// only member of a class allocated in static storage.
//
// Nothing in the language standard guarantees that a-d hold.  In practice,
// these hold in all supported compilers.
//
// Future direction:
//
// Ideally, we would simply use std::mutex or a similar class, which when
// allocated statically would support use immediately after static
// initialization up until static storage is reclaimed (i.e. the properties
// we require of all "linker initialized" instances).
//
// Regarding construction in static storage, std::mutex is required to
// provide a constexpr default constructor [thread.mutex.class], which
// ensures the instance's lifetime begins with static initialization
// [basic.start.init], and so is immune to any problems caused by the order
// of dynamic initialization. However, as of this writing Microsoft's
// Visual Studio does not provide a constexpr constructor for std::mutex.
// See
// https://blogs.msdn.microsoft.com/vcblog/2015/06/02/constexpr-complete-for-vs-2015-rtm-c11-compiler-c17-stl/
//
// Regarding destruction of instances in static storage, [basic.life] does
// say an object ends when storage in which the occupies is released, in
// the case of non-trivial destructor. However, std::mutex is not specified
// to have a trivial destructor.
//
// So, we would need a class with a constexpr default constructor and a
// trivial destructor. Today, we can achieve neither desired property using
// std::mutex directly.
template <typename T>
class SynchronizationStorage {
 public:
  // Instances allocated on the heap or on the stack should use the default
  // constructor.
  SynchronizationStorage()
      : is_dynamic_(true), once_() {}

  // Instances allocated in static storage (not on the heap, not on the
  // stack) should use this constructor.
  explicit SynchronizationStorage(base_internal::LinkerInitialized) {}

  constexpr explicit SynchronizationStorage(absl::ConstInitType)
      : is_dynamic_(false), once_(), space_{{0}} {}

  SynchronizationStorage(SynchronizationStorage&) = delete;
  SynchronizationStorage& operator=(SynchronizationStorage&) = delete;

  ~SynchronizationStorage() {
    if (is_dynamic_) {
      get()->~T();
    }
  }

  // Retrieve the object in storage. This is fast and thread safe, but does
  // incur the cost of absl::call_once().
  //
  // For instances in static storage constructed with the
  // LinkerInitialized constructor, may be called at any time without
  // regard for order of dynamic initialization or destruction of objects
  // in static storage. See the class comment for caveats.
  T* get() {
    absl::call_once(once_, SynchronizationStorage::Construct, this);
    return reinterpret_cast<T*>(&space_);
  }

 private:
  static void Construct(SynchronizationStorage<T>* self) {
    new (&self->space_) T();
  }

  // When true, T's destructor is run when this is destructed.
  //
  // The LinkerInitialized constructor assumes this value will be set
  // false by static initialization.
  bool is_dynamic_;

  absl::once_flag once_;

  // An aligned space for the T.
  alignas(T) unsigned char space_[sizeof(T)];
};

}  // namespace synchronization_internal
ABSL_NAMESPACE_END
}  // namespace absl