pthreadcond.hpp

//    OpenVPN -- An application to securely tunnel IP networks
//               over a single port, with support for SSL/TLS-based
//               session authentication and key exchange,
//               packet encryption, packet authentication, and
//               packet compression.
//
//    Copyright (C) 2012-2017 OpenVPN Inc.
//
//    This program is free software: you can redistribute it and/or modify
//    it under the terms of the GNU Affero General Public License Version 3
//    as published by the Free Software Foundation.
//
//    This program is distributed in the hope that it will be useful,
//    but WITHOUT ANY WARRANTY; without even the implied warranty of
//    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//    GNU Affero General Public License for more details.
//
//    You should have received a copy of the GNU Affero General Public License
//    along with this program in the COPYING file.
//    If not, see <http://www.gnu.org/licenses/>.

#ifndef OPENVPN_COMMON_PTHREADCOND_H
#define OPENVPN_COMMON_PTHREADCOND_H

#include <mutex>
#include <condition_variable>
#include <chrono>

#include <openvpn/common/stop.hpp>

namespace openvpn {

  // Barrier class that is useful in cases where all threads
  // need to reach a known point before executing some action.
  // Note that this barrier implementation is
  // constructed using C++11 condition variables.
  class PThreadBarrier
  {
    enum State {
      UNSIGNALED=0,  // initial state
      SIGNALED,      // signal() was called
      ERROR_THROWN,  // error() was called
    };

  public:
    // status return from wait()
    enum Status {
      SUCCESS=0,  // successful
      CHOSEN_ONE, // successful and chosen (only one thread is chosen)
      TIMEOUT,    // timeout
      ERROR,      // at least one thread called error()
    };

    PThreadBarrier(const int initial_limit = -1)
      : stop(nullptr),
	limit(initial_limit)
    {
    }

    PThreadBarrier(Stop* stop_arg, const int initial_limit = -1)
      : stop(stop_arg),
	limit(initial_limit)
    {
    }

    // All callers will increment count and block until
    // count == limit.  CHOSEN_ONE will be returned to
    // the first caller to reach limit.  This caller can
    // then release all the other callers by calling
    // signal().
    int wait(const unsigned int seconds)
    {
      // allow asynchronous stop
      Stop::Scope stop_scope(stop, [this]() {
	  error();
	});

      bool timeout = false;
      int ret;
      std::unique_lock<std::mutex> lock(mutex);
      const unsigned int c = ++count;
      while (state == UNSIGNALED
	     && (limit < 0 || c < limit)
	     && !timeout)
	timeout = (cv.wait_for(lock, std::chrono::seconds(seconds)) == std::cv_status::timeout);
      if (timeout)
	ret = TIMEOUT;
      else if (state == ERROR_THROWN)
	ret = ERROR;
      else if (state == UNSIGNALED && !chosen)
	{
	  ret = CHOSEN_ONE;
	  chosen = true;
	}
      else
	ret = SUCCESS;
      return ret;
    }

    void set_limit(const int new_limit)
    {
      std::unique_lock<std::mutex> lock(mutex);
      limit = new_limit;
      cv.notify_all();
    }

    // Generally, only the CHOSEN_ONE calls signal() after its work
    // is complete, to allow the other threads to pass the barrier.
    void signal()
    {
      signal_(SIGNALED);
    }

    // Causes all threads waiting on wait() (and those which call wait()
    // in the future) to exit with ERROR status.
    void error()
    {
      signal_(ERROR_THROWN);
    }

  private:
    void signal_(const State newstate)
    {
      std::unique_lock<std::mutex> lock(mutex);
      if (state == UNSIGNALED)
	{
	  state = newstate;
	  cv.notify_all();
	}
    }

    std::mutex mutex;
    std::condition_variable cv;
    Stop* stop;
    State state{UNSIGNALED};
    bool chosen = false;
    unsigned int count = 0;
    int limit;
  };

}

#endif