src/threading/ProtectedData.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef threading_ProtectedData_h
#define threading_ProtectedData_h

#include "threading/Thread.h"

namespace js {

// This file provides classes for encapsulating pieces of data with a check
// that ensures the data is only accessed if certain conditions are met.
// Checking is only done in debug builds; in release builds these classes
// have no space or time overhead. These classes are mainly used for ensuring
// that data is used in threadsafe ways.
//
// ProtectedData does not by itself ensure that data is threadsafe: it only
// documents and checks synchronization constraints that need to be established
// by the code using the data. If a mutex can be created and directly
// associated with the data, consider using the ExclusiveData class instead.
// Otherwise, ProtectedData should be used to document whatever synchronization
// method is used.

// Protected data checks are enabled in debug builds, except on android where
// they cause some permatimeouts in automation.
#if defined(DEBUG) && !defined(ANDROID)
#define JS_HAS_PROTECTED_DATA_CHECKS
#endif

#define DECLARE_ONE_BOOL_OPERATOR(OP, T)   \
  template <typename U>                    \
  bool operator OP(const U& other) const { \
    return ref() OP static_cast<T>(other); \
  }

#define DECLARE_BOOL_OPERATORS(T)  \
  DECLARE_ONE_BOOL_OPERATOR(==, T) \
  DECLARE_ONE_BOOL_OPERATOR(!=, T) \
  DECLARE_ONE_BOOL_OPERATOR(<=, T) \
  DECLARE_ONE_BOOL_OPERATOR(>=, T) \
  DECLARE_ONE_BOOL_OPERATOR(<, T)  \
  DECLARE_ONE_BOOL_OPERATOR(>, T)

// Mark a region of code that should be treated as single threaded and suppress
// any ProtectedData checks.
//
// Note that in practice there may be multiple threads running when this class
// is used, due to the presence of multiple runtimes in the process. When each
// process has only a single runtime this will no longer be a concern.
class MOZ_RAII AutoNoteSingleThreadedRegion {
 public:
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
  static mozilla::Atomic<size_t> count;
  AutoNoteSingleThreadedRegion() { count++; }
  ~AutoNoteSingleThreadedRegion() { count--; }
#else
  AutoNoteSingleThreadedRegion() {}
#endif
};

// Class for protected data that may be written to any number of times. Checks
// occur when the data is both read from and written to.
template <typename Check, typename T>
class ProtectedData {
  typedef ProtectedData<Check, T> ThisType;

 public:
  template <typename... Args>
  explicit ProtectedData(const Check& check, Args&&... args)
      : value(mozilla::Forward<Args>(args)...)
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
        ,
        check(check)
#endif
  {
  }

  DECLARE_BOOL_OPERATORS(T)

  operator const T&() const { return ref(); }
  const T& operator->() const { return ref(); }

  template <typename U>
  ThisType& operator=(const U& p) {
    this->ref() = p;
    return *this;
  }

  template <typename U>
  T& operator+=(const U& rhs) {
    return ref() += rhs;
  }
  template <typename U>
  T& operator-=(const U& rhs) {
    return ref() -= rhs;
  }
  template <typename U>
  T& operator*=(const U& rhs) {
    return ref() *= rhs;
  }
  template <typename U>
  T& operator/=(const U& rhs) {
    return ref() /= rhs;
  }
  template <typename U>
  T& operator&=(const U& rhs) {
    return ref() &= rhs;
  }
  template <typename U>
  T& operator|=(const U& rhs) {
    return ref() |= rhs;
  }
  T& operator++() { return ++ref(); }
  T& operator--() { return --ref(); }
  T operator++(int) { return ref()++; }
  T operator--(int) { return ref()--; }

  T& ref() {
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
    if (!AutoNoteSingleThreadedRegion::count) check.check();
#endif
    return value;
  }

  const T& ref() const {
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
    if (!AutoNoteSingleThreadedRegion::count) check.check();
#endif
    return value;
  }

  T& refNoCheck() { return value; }
  const T& refNoCheck() const { return value; }

 private:
  T value;
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
  Check check;
#endif
};

// Intermediate class for protected data whose checks take no constructor
// arguments.
template <typename Check, typename T>
class ProtectedDataNoCheckArgs : public ProtectedData<Check, T> {
  typedef ProtectedDataNoCheckArgs<Check, T> ThisType;

 public:
  template <typename... Args>
  explicit ProtectedDataNoCheckArgs(Args&&... args)
      : ProtectedData<Check, T>(Check(), mozilla::Forward<Args>(args)...) {}

  template <typename U>
  ThisType& operator=(const U& p) {
    this->ref() = p;
    return *this;
  }
};

class ZoneGroup;

// Intermediate class for protected data whose checks take a ZoneGroup
// constructor argument.
template <typename Check, typename T>
class ProtectedDataZoneGroupArg : public ProtectedData<Check, T> {
  typedef ProtectedDataZoneGroupArg<Check, T> ThisType;

 public:
  template <typename... Args>
  explicit ProtectedDataZoneGroupArg(ZoneGroup* group, Args&&... args)
      : ProtectedData<Check, T>(Check(group), mozilla::Forward<Args>(args)...) {
  }

  template <typename U>
  ThisType& operator=(const U& p) {
    this->ref() = p;
    return *this;
  }
};

class CheckUnprotected {
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
 public:
  inline void check() const {}
#endif
};

// Data with a no-op check that permits all accesses. This is tantamount to not
// using ProtectedData at all, but is in place to document points which need
// to be fixed in order for runtimes to be multithreaded (see bug 1323066).
template <typename T>
using UnprotectedData = ProtectedDataNoCheckArgs<CheckUnprotected, T>;

class CheckThreadLocal {
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
  Thread::Id id;

 public:
  CheckThreadLocal() : id(ThisThread::GetId()) {}

  void check() const;
#endif
};

// Data which may only be accessed by the thread on which it is created.
template <typename T>
using ThreadLocalData = ProtectedDataNoCheckArgs<CheckThreadLocal, T>;

// Enum describing which helper threads (GC tasks or Ion compilations) may
// access data even though they do not have exclusive access to any zone group.
enum class AllowedHelperThread { None, GCTask, IonCompile, GCTaskOrIonCompile };

template <AllowedHelperThread Helper>
class CheckActiveThread {
 public:
  void check() const;
};

// Data which may only be accessed by the runtime's cooperatively scheduled
// active thread.
template <typename T>
using ActiveThreadData =
    ProtectedDataNoCheckArgs<CheckActiveThread<AllowedHelperThread::None>, T>;

// Data which may only be accessed by the runtime's cooperatively scheduled
// active thread, or by various helper thread tasks.
template <typename T>
using ActiveThreadOrGCTaskData =
    ProtectedDataNoCheckArgs<CheckActiveThread<AllowedHelperThread::GCTask>, T>;
template <typename T>
using ActiveThreadOrIonCompileData =
    ProtectedDataNoCheckArgs<CheckActiveThread<AllowedHelperThread::IonCompile>,
                             T>;

template <AllowedHelperThread Helper>
class CheckZoneGroup {
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
  ZoneGroup* group;

 public:
  explicit CheckZoneGroup(ZoneGroup* group) : group(group) {}
  void check() const;
#else
 public:
  explicit CheckZoneGroup(ZoneGroup* group) {}
#endif
};

// Data which may only be accessed by threads with exclusive access to the
// associated zone group, or by the runtime's cooperatively scheduled
// active thread for zone groups which are not in use by a helper thread.
template <typename T>
using ZoneGroupData =
    ProtectedDataZoneGroupArg<CheckZoneGroup<AllowedHelperThread::None>, T>;

// Data which may only be accessed by threads with exclusive access to the
// associated zone group, or by various helper thread tasks.
template <typename T>
using ZoneGroupOrGCTaskData =
    ProtectedDataZoneGroupArg<CheckZoneGroup<AllowedHelperThread::GCTask>, T>;
template <typename T>
using ZoneGroupOrIonCompileData =
    ProtectedDataZoneGroupArg<CheckZoneGroup<AllowedHelperThread::IonCompile>,
                              T>;
template <typename T>
using ZoneGroupOrGCTaskOrIonCompileData = ProtectedDataZoneGroupArg<
    CheckZoneGroup<AllowedHelperThread::GCTaskOrIonCompile>, T>;

// Runtime wide locks which might protect some data.
enum class GlobalLock {
  GCLock,
  ExclusiveAccessLock,
  ScriptDataLock,
  HelperThreadLock
};

template <GlobalLock Lock, AllowedHelperThread Helper>
class CheckGlobalLock {
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
 public:
  void check() const;
#endif
};

// Data which may only be accessed while holding the GC lock.
template <typename T>
using GCLockData = ProtectedDataNoCheckArgs<
    CheckGlobalLock<GlobalLock::GCLock, AllowedHelperThread::None>, T>;

// Data which may only be accessed while holding the exclusive access lock.
template <typename T>
using ExclusiveAccessLockData = ProtectedDataNoCheckArgs<
    CheckGlobalLock<GlobalLock::ExclusiveAccessLock, AllowedHelperThread::None>,
    T>;

// Data which may only be accessed while holding the exclusive access lock or
// by GC helper thread tasks (at which point a foreground thread should be
// holding the exclusive access lock, though we do not check this).
template <typename T>
using ExclusiveAccessLockOrGCTaskData =
    ProtectedDataNoCheckArgs<CheckGlobalLock<GlobalLock::ExclusiveAccessLock,
                                             AllowedHelperThread::GCTask>,
                             T>;

// Data which may only be accessed while holding the script data lock.
template <typename T>
using ScriptDataLockData = ProtectedDataNoCheckArgs<
    CheckGlobalLock<GlobalLock::ScriptDataLock, AllowedHelperThread::None>, T>;

// Data which may only be accessed while holding the helper thread lock.
template <typename T>
using HelperThreadLockData = ProtectedDataNoCheckArgs<
    CheckGlobalLock<GlobalLock::HelperThreadLock, AllowedHelperThread::None>,
    T>;

// Class for protected data that is only written to once. 'const' may sometimes
// be usable instead of this class, but in cases where the data cannot be set
// to its final value in its constructor this class is helpful. Protected data
// checking only occurs when writes are performed, not reads. Steps may need to
// be taken to ensure that reads do not occur until the written value is fully
// initialized, as such guarantees are not provided by this class.
template <typename Check, typename T>
class ProtectedDataWriteOnce {
  typedef ProtectedDataWriteOnce<Check, T> ThisType;

 public:
  template <typename... Args>
  explicit ProtectedDataWriteOnce(Args&&... args)
      : value(mozilla::Forward<Args>(args)...)
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
        ,
        nwrites(0)
#endif
  {
  }

  DECLARE_BOOL_OPERATORS(T)

  operator const T&() const { return ref(); }
  const T& operator->() const { return ref(); }

  template <typename U>
  ThisType& operator=(const U& p) {
    if (ref() != p) this->writeRef() = p;
    return *this;
  }

  const T& ref() const { return value; }

  T& writeRef() {
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
    if (!AutoNoteSingleThreadedRegion::count) check.check();
    // Despite the WriteOnce name, actually allow two writes to accommodate
    // data that is cleared during teardown.
    MOZ_ASSERT(++nwrites <= 2);
#endif
    return value;
  }

 private:
  T value;
#ifdef JS_HAS_PROTECTED_DATA_CHECKS
  Check check;
  size_t nwrites;
#endif
};

// Data that is written once with no requirements for exclusive access when
// that write occurs.
template <typename T>
using WriteOnceData = ProtectedDataWriteOnce<CheckUnprotected, T>;

// Data that is written once, and only while holding the exclusive access lock.
template <typename T>
using ExclusiveAccessLockWriteOnceData = ProtectedDataWriteOnce<
    CheckGlobalLock<GlobalLock::ExclusiveAccessLock, AllowedHelperThread::None>,
    T>;

#undef DECLARE_ASSIGNMENT_OPERATOR
#undef DECLARE_ONE_BOOL_OPERATOR
#undef DECLARE_BOOL_OPERATORS

}  // namespace js

#endif  // threading_ProtectedData_h