Predicate.h

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//===-- Predicate.h ---------------------------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
 
#ifndef LLDB_UTILITY_PREDICATE_H
#define LLDB_UTILITY_PREDICATE_H
 
#include <cstdint>
#include <ctime>
 
#include <condition_variable>
#include <mutex>
#include <optional>
 
#include "lldb/Utility/Timeout.h"
#include "lldb/lldb-defines.h"
 
//#define DB_PTHREAD_LOG_EVENTS
 
/// Enumerations for broadcasting.
namespace lldb_private {
 
enum PredicateBroadcastType {
  eBroadcastNever,   ///< No broadcast will be sent when the value is modified.
  eBroadcastAlways,  ///< Always send a broadcast when the value is modified.
  eBroadcastOnChange ///< Only broadcast if the value changes when the value is
                     /// modified.
};
 
/// \class Predicate Predicate.h "lldb/Utility/Predicate.h"
/// A C++ wrapper class for providing threaded access to a value of
/// type T.
///
/// A templatized class that provides multi-threaded access to a value
/// of type T. Threads can efficiently wait for bits within T to be set
/// or reset, or wait for T to be set to be equal/not equal to a
/// specified values.
template <class T> class Predicate {
public:
  /// Default constructor.
  ///
  /// Initializes the mutex, condition and value with their default
  /// constructors.
  Predicate() : m_value() {}
 
  /// Construct with initial T value \a initial_value.
  ///
  /// Initializes the mutex and condition with their default
  /// constructors, and initializes the value with \a initial_value.
  ///
  /// \param[in] initial_value
  ///     The initial value for our T object.
  Predicate(T initial_value) : m_value(initial_value) {}
 
  /// Destructor.
  ///
  /// Destroy the condition, mutex, and T objects.
  ~Predicate() = default;
 
  /// Value get accessor.
  ///
  /// Copies the current \a m_value in a thread safe manor and returns
  /// the copied value.
  ///
  /// \return
  ///     A copy of the current value.
  T GetValue() const {
    std::lock_guard<std::mutex> guard(m_mutex);
    T value = m_value;
    return value;
  }
 
  /// Value set accessor.
  ///
  /// Set the contained \a m_value to \a new_value in a thread safe
  /// way and broadcast if needed.
  ///
  /// \param[in] value
  ///     The new value to set.
  ///
  /// \param[in] broadcast_type
  ///     A value indicating when and if to broadcast. See the
  ///     PredicateBroadcastType enumeration for details.
  ///
  /// \see Predicate::Broadcast()
  void SetValue(T value, PredicateBroadcastType broadcast_type) {
    std::lock_guard<std::mutex> guard(m_mutex);
#ifdef DB_PTHREAD_LOG_EVENTS
    printf("%s (value = 0x%8.8x, broadcast_type = %i)\n", __FUNCTION__, value,
           broadcast_type);
#endif
    const T old_value = m_value;
    m_value = value;
 
    Broadcast(old_value, broadcast_type);
  }
 
  /// Wait for Cond(m_value) to be true.
  ///
  /// Waits in a thread safe way for Cond(m_value) to be true. If Cond(m_value)
  /// is already true, this function will return without waiting.
  ///
  /// It is possible for the value to be changed between the time the value is
  /// set and the time the waiting thread wakes up. If the value no longer
  /// satisfies the condition when the waiting thread wakes up, it will go back
  /// into a wait state. It may be necessary for the calling code to use
  /// additional thread synchronization methods to detect transitory states.
  ///
  /// \param[in] Cond
  ///     The condition we want \a m_value satisfy.
  ///
  /// \param[in] timeout
  ///     How long to wait for the condition to hold.
  ///
  /// \return
  ///     m_value if Cond(m_value) is true, std::nullopt otherwise (timeout
  ///     occurred).
  template <typename C>
  std::optional<T> WaitFor(C Cond, const Timeout<std::micro> &timeout) {
    std::unique_lock<std::mutex> lock(m_mutex);
    auto RealCond = [&] { return Cond(m_value); };
    if (!timeout) {
      m_condition.wait(lock, RealCond);
      return m_value;
    }
    if (m_condition.wait_for(lock, *timeout, RealCond))
      return m_value;
    return std::nullopt;
  }
  /// Wait for \a m_value to be equal to \a value.
  ///
  /// Waits in a thread safe way for \a m_value to be equal to \a
  /// value. If \a m_value is already equal to \a value, this
  /// function will return without waiting.
  ///
  /// It is possible for the value to be changed between the time
  /// the value is set and the time the waiting thread wakes up.
  /// If the value no longer matches the requested value when the
  /// waiting thread wakes up, it will go back into a wait state.  It
  /// may be necessary for the calling code to use additional thread
  /// synchronization methods to detect transitory states.
  ///
  /// \param[in] value
  ///     The value we want \a m_value to be equal to.
  ///
  /// \param[in] timeout
  ///     How long to wait for the condition to hold.
  ///
  /// \return
  ///     true if the \a m_value is equal to \a value, false otherwise (timeout
  ///     occurred).
  bool WaitForValueEqualTo(T value,
                           const Timeout<std::micro> &timeout = std::nullopt) {
    return WaitFor([&value](T current) { return value == current; }, timeout) !=
           std::nullopt;
  }
 
  /// Wait for \a m_value to not be equal to \a value.
  ///
  /// Waits in a thread safe way for \a m_value to not be equal to \a
  /// value. If \a m_value is already not equal to \a value, this
  /// function will return without waiting.
  ///
  /// It is possible for the value to be changed between the time
  /// the value is set and the time the waiting thread wakes up.
  /// If the value is equal to the test value when the waiting thread
  /// wakes up, it will go back into a wait state.  It may be
  /// necessary for the calling code to use additional thread
  /// synchronization methods to detect transitory states.
  ///
  /// \param[in] value
  ///     The value we want \a m_value to not be equal to.
  ///
  /// \param[in] timeout
  ///     How long to wait for the condition to hold.
  ///
  /// \return
  ///     m_value if m_value != value, std::nullopt otherwise (timeout
  ///     occurred).
  std::optional<T>
  WaitForValueNotEqualTo(T value,
                         const Timeout<std::micro> &timeout = std::nullopt) {
    return WaitFor([&value](T current) { return value != current; }, timeout);
  }
 
protected:
  // pthread condition and mutex variable to control access and allow blocking
  // between the main thread and the spotlight index thread.
  T m_value; ///< The templatized value T that we are protecting access to
  mutable std::mutex m_mutex; ///< The mutex to use when accessing the data
  std::condition_variable m_condition; ///< The pthread condition variable to
                                       /// use for signaling that data available
                                       /// or changed.
 
private:
  /// Broadcast if needed.
  ///
  /// Check to see if we need to broadcast to our condition variable
  /// depending on the \a old_value and on the \a broadcast_type.
  ///
  /// If \a broadcast_type is eBroadcastNever, no broadcast will be
  /// sent.
  ///
  /// If \a broadcast_type is eBroadcastAlways, the condition variable
  /// will always be broadcast.
  ///
  /// If \a broadcast_type is eBroadcastOnChange, the condition
  /// variable be broadcast if the owned value changes.
  void Broadcast(T old_value, PredicateBroadcastType broadcast_type) {
    bool broadcast =
        (broadcast_type == eBroadcastAlways) ||
        ((broadcast_type == eBroadcastOnChange) && old_value != m_value);
#ifdef DB_PTHREAD_LOG_EVENTS
    printf("%s (old_value = 0x%8.8x, broadcast_type = %i) m_value = 0x%8.8x, "
           "broadcast = %u\n",
           __FUNCTION__, old_value, broadcast_type, m_value, broadcast);
#endif
    if (broadcast)
      m_condition.notify_all();
  }
 
  Predicate(const Predicate &) = delete;
  const Predicate &operator=(const Predicate &) = delete;
};
 
} // namespace lldb_private
 
#endif // LLDB_UTILITY_PREDICATE_H