Coverage Report

Created: 2024-09-23 06:29

/src/abseil-cpp/absl/synchronization/internal/kernel_timeout.h
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// Copyright 2017 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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//      https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_
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#define ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_
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#ifndef _WIN32
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#include <sys/types.h>
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#endif
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#include <algorithm>
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#include <chrono>  // NOLINT(build/c++11)
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#include <cstdint>
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#include <ctime>
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#include <limits>
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#include "absl/base/config.h"
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#include "absl/base/internal/raw_logging.h"
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#include "absl/time/clock.h"
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#include "absl/time/time.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace synchronization_internal {
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// An optional timeout, with nanosecond granularity.
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//
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// This is a private low-level API for use by a handful of low-level
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// components. Higher-level components should build APIs based on
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// absl::Time and absl::Duration.
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class KernelTimeout {
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 public:
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  // Construct an absolute timeout that should expire at `t`.
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  explicit KernelTimeout(absl::Time t);
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  // Construct a relative timeout that should expire after `d`.
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  explicit KernelTimeout(absl::Duration d);
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  // Infinite timeout.
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  constexpr KernelTimeout() : rep_(kNoTimeout) {}
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  // A more explicit factory for those who prefer it.
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  // Equivalent to `KernelTimeout()`.
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  static constexpr KernelTimeout Never() { return KernelTimeout(); }
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  // Returns true if there is a timeout that will eventually expire.
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  // Returns false if the timeout is infinite.
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  bool has_timeout() const { return rep_ != kNoTimeout; }
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  // If `has_timeout()` is true, returns true if the timeout was provided as an
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  // `absl::Time`. The return value is undefined if `has_timeout()` is false
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  // because all indefinite timeouts are equivalent.
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  bool is_absolute_timeout() const { return (rep_ & 1) == 0; }
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  // If `has_timeout()` is true, returns true if the timeout was provided as an
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  // `absl::Duration`. The return value is undefined if `has_timeout()` is false
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  // because all indefinite timeouts are equivalent.
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  bool is_relative_timeout() const { return (rep_ & 1) == 1; }
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  // Convert to `struct timespec` for interfaces that expect an absolute
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  // timeout. If !has_timeout() or is_relative_timeout(), attempts to convert to
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  // a reasonable absolute timeout, but callers should to test has_timeout() and
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  // is_relative_timeout() and prefer to use a more appropriate interface.
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  struct timespec MakeAbsTimespec() const;
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  // Convert to `struct timespec` for interfaces that expect a relative
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  // timeout. If !has_timeout() or is_absolute_timeout(), attempts to convert to
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  // a reasonable relative timeout, but callers should to test has_timeout() and
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  // is_absolute_timeout() and prefer to use a more appropriate interface. Since
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  // the return value is a relative duration, it should be recomputed by calling
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  // this method in the case of a spurious wakeup.
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  struct timespec MakeRelativeTimespec() const;
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#ifndef _WIN32
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  // Convert to `struct timespec` for interfaces that expect an absolute timeout
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  // on a specific clock `c`. This is similar to `MakeAbsTimespec()`, but
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  // callers usually want to use this method with `CLOCK_MONOTONIC` when
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  // relative timeouts are requested, and when the appropriate interface expects
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  // an absolute timeout relative to a specific clock (for example,
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  // pthread_cond_clockwait() or sem_clockwait()). If !has_timeout(), attempts
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  // to convert to a reasonable absolute timeout, but callers should to test
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  // has_timeout() prefer to use a more appropriate interface.
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  struct timespec MakeClockAbsoluteTimespec(clockid_t c) const;
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#endif
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  // Convert to unix epoch nanos for interfaces that expect an absolute timeout
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  // in nanoseconds. If !has_timeout() or is_relative_timeout(), attempts to
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  // convert to a reasonable absolute timeout, but callers should to test
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  // has_timeout() and is_relative_timeout() and prefer to use a more
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  // appropriate interface.
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  int64_t MakeAbsNanos() const;
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  // Converts to milliseconds from now, or INFINITE when
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  // !has_timeout(). For use by SleepConditionVariableSRW on
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  // Windows. Callers should recognize that the return value is a
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  // relative duration (it should be recomputed by calling this method
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  // in the case of a spurious wakeup).
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  // This header file may be included transitively by public header files,
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  // so we define our own DWORD and INFINITE instead of getting them from
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  // <intsafe.h> and <WinBase.h>.
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  typedef unsigned long DWord;  // NOLINT
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  DWord InMillisecondsFromNow() const;
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  // Convert to std::chrono::time_point for interfaces that expect an absolute
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  // timeout, like std::condition_variable::wait_until(). If !has_timeout() or
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  // is_relative_timeout(), attempts to convert to a reasonable absolute
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  // timeout, but callers should test has_timeout() and is_relative_timeout()
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  // and prefer to use a more appropriate interface.
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  std::chrono::time_point<std::chrono::system_clock> ToChronoTimePoint() const;
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  // Convert to std::chrono::time_point for interfaces that expect a relative
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  // timeout, like std::condition_variable::wait_for(). If !has_timeout() or
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  // is_absolute_timeout(), attempts to convert to a reasonable relative
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  // timeout, but callers should test has_timeout() and is_absolute_timeout()
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  // and prefer to use a more appropriate interface. Since the return value is a
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  // relative duration, it should be recomputed by calling this method in the
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  // case of a spurious wakeup.
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  std::chrono::nanoseconds ToChronoDuration() const;
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  // Returns true if steady (aka monotonic) clocks are supported by the system.
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  // This method exists because go/btm requires synchronized clocks, and
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  // thus requires we use the system (aka walltime) clock.
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  static constexpr bool SupportsSteadyClock() { return true; }
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 private:
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  // Returns the current time, expressed as a count of nanoseconds since the
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  // epoch used by an arbitrary clock. The implementation tries to use a steady
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  // (monotonic) clock if one is available.
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  static int64_t SteadyClockNow();
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  // Internal representation.
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  //   - If the value is kNoTimeout, then the timeout is infinite, and
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  //     has_timeout() will return true.
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  //   - If the low bit is 0, then the high 63 bits is the number of nanoseconds
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  //     after the unix epoch.
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  //   - If the low bit is 1, then the high 63 bits is the number of nanoseconds
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  //     after the epoch used by SteadyClockNow().
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  //
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  // In all cases the time is stored as an absolute time, the only difference is
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  // the clock epoch. The use of absolute times is important since in the case
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  // of a relative timeout with a spurious wakeup, the program would have to
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  // restart the wait, and thus needs a way of recomputing the remaining time.
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  uint64_t rep_;
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  // Returns the number of nanoseconds stored in the internal representation.
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  // When combined with the clock epoch indicated by the low bit (which is
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  // accessed through is_absolute_timeout() and is_relative_timeout()), the
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  // return value is used to compute when the timeout should occur.
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  int64_t RawAbsNanos() const { return static_cast<int64_t>(rep_ >> 1); }
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  // Converts to nanoseconds from now. Since the return value is a relative
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  // duration, it should be recomputed by calling this method in the case of a
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  // spurious wakeup.
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  int64_t InNanosecondsFromNow() const;
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  // A value that represents no timeout (or an infinite timeout).
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  static constexpr uint64_t kNoTimeout = (std::numeric_limits<uint64_t>::max)();
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  // The maximum value that can be stored in the high 63 bits.
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  static constexpr int64_t kMaxNanos = (std::numeric_limits<int64_t>::max)();
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};
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}  // namespace synchronization_internal
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ABSL_NAMESPACE_END
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}  // namespace absl
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#endif  // ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_