text: // Copyright 2018 The Chromium Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  #ifndef BASE_TIME_TIME_OVERRIDE_H_
  #define BASE_TIME_TIME_OVERRIDE_H_
  
  #include <atomic>
  
  #include "base/base_export.h"
  #include "base/time/time.h"
  #include "build/build_config.h"
  #include "third_party/abseil-cpp/absl/types/optional.h"
  
  namespace base {
  
  using TimeNowFunction = decltype(&Time::Now);
  using TimeTicksNowFunction = decltype(&TimeTicks::Now);
  using ThreadTicksNowFunction = decltype(&ThreadTicks::Now);
  
  // Time overrides should be used with extreme caution. Discuss with //base/time
  // OWNERS before adding a new one.
  namespace subtle {
  
  // Override the return value of Time::Now and Time::NowFromSystemTime /
  // TimeTicks::Now / ThreadTicks::Now to emulate time, e.g. for tests or to
  // modify progression of time. It is recommended that the override be set while
  // single-threaded and before the first call to Now() to avoid threading issues
  // and inconsistencies in returned values. Overriding time while other threads
  // are running is very subtle and should be reserved for developer only use
  // cases (e.g. virtual time in devtools) where any flakiness caused by a racy
  // time update isn't surprising. Instantiating a ScopedTimeClockOverrides while
  // other threads are running might break their expectation that TimeTicks and
  // ThreadTicks increase monotonically. Nested overrides are not allowed.
  class BASE_EXPORT ScopedTimeClockOverrides {
   public:
    // Pass |nullptr| for any override if it shouldn't be overriden.
    ScopedTimeClockOverrides(TimeNowFunction time_override,
                             TimeTicksNowFunction time_ticks_override,
                             ThreadTicksNowFunction thread_ticks_override);
  
    ScopedTimeClockOverrides(const ScopedTimeClockOverrides&) = delete;
    ScopedTimeClockOverrides& operator=(const ScopedTimeClockOverrides&) = delete;
  
    // Restores the platform default Now() functions.
    ~ScopedTimeClockOverrides();
  
    static bool overrides_active() { return overrides_active_; }
  
   private:
    static bool overrides_active_;
  };
  
  // These methods return the platform default Time::Now / TimeTicks::Now /
  // ThreadTicks::Now values even while an override is in place. These methods
  // should only be used in places where emulated time should be disregarded. For
  // example, they can be used to implement test timeouts for tests that may
  // override time.
  BASE_EXPORT Time TimeNowIgnoringOverride();
  BASE_EXPORT Time TimeNowFromSystemTimeIgnoringOverride();
  BASE_EXPORT TimeTicks TimeTicksNowIgnoringOverride();
  BASE_EXPORT ThreadTicks ThreadTicksNowIgnoringOverride();
  
  #if defined(OS_POSIX)
  // Equivalent to TimeTicksNowIgnoringOverride(), but is allowed to fail and
  // return absl::nullopt. This may safely be used in a signal handler.
  BASE_EXPORT absl::optional<TimeTicks> MaybeTimeTicksNowIgnoringOverride();
  #endif
  
  }  // namespace subtle
  
  namespace internal {
  
  // These function pointers are used by platform-independent implementations of
  // the Now() methods and ScopedTimeClockOverrides. They are set to point to the
  // respective NowIgnoringOverride functions by default, but can also be set by
  // platform-specific code to select a default implementation at runtime, thereby
  // avoiding the indirection via the NowIgnoringOverride functions. Note that the
  // pointers can be overridden and later reset to the NowIgnoringOverride
  // functions by ScopedTimeClockOverrides.
  extern std::atomic<TimeNowFunction> g_time_now_function;
  extern std::atomic<TimeNowFunction> g_time_now_from_system_time_function;
  extern std::atomic<TimeTicksNowFunction> g_time_ticks_now_function;
  extern std::atomic<ThreadTicksNowFunction> g_thread_ticks_now_function;
  
  }  // namespace internal
  
  }  // namespace base
  
  #endif  // BASE_TIME_TIME_OVERRIDE_H_
  