text: // Copyright (c) 2009-2010 Satoshi Nakamoto
  // Copyright (c) 2009-2020 The Bitcoin Core developers
  // Distributed under the MIT software license, see the accompanying
  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
  
  #include <shutdown.h>
  
  #include <config/spiffcoin-config.h>
  
  #include <assert.h>
  #include <atomic>
  #ifdef WIN32
  #include <condition_variable>
  #else
  #include <errno.h>
  #include <fcntl.h>
  #include <unistd.h>
  #endif
  
  static std::atomic<bool> fRequestShutdown(false);
  #ifdef WIN32
  /** On windows it is possible to simply use a condition variable. */
  std::mutex g_shutdown_mutex;
  std::condition_variable g_shutdown_cv;
  #else
  /** On UNIX-like operating systems use the self-pipe trick.
   * Index 0 will be the read end of the pipe, index 1 the write end.
   */
  static int g_shutdown_pipe[2] = {-1, -1};
  #endif
  
  bool InitShutdownState()
  {
  #ifndef WIN32
  #if HAVE_O_CLOEXEC
      // If we can, make sure that the file descriptors are closed on exec()
      // to prevent interference.
      if (pipe2(g_shutdown_pipe, O_CLOEXEC) != 0) {
          return false;
      }
  #else
      if (pipe(g_shutdown_pipe) != 0) {
          return false;
      }
  #endif
  #endif
      return true;
  }
  
  void StartShutdown()
  {
  #ifdef WIN32
      std::unique_lock<std::mutex> lk(g_shutdown_mutex);
      fRequestShutdown = true;
      g_shutdown_cv.notify_one();
  #else
      // This must be reentrant and safe for calling in a signal handler, so using a condition variable is not safe.
      // Make sure that the token is only written once even if multiple threads call this concurrently or in
      // case of a reentrant signal.
      if (!fRequestShutdown.exchange(true)) {
          // Write an arbitrary byte to the write end of the shutdown pipe.
          const char token = 'x';
          while (true) {
              int result = write(g_shutdown_pipe[1], &token, 1);
              if (result < 0) {
                  // Failure. It's possible that the write was interrupted by another signal.
                  // Other errors are unexpected here.
                  assert(errno == EINTR);
              } else {
                  assert(result == 1);
                  break;
              }
          }
      }
  #endif
  }
  
  void AbortShutdown()
  {
      if (fRequestShutdown) {
          // Cancel existing shutdown by waiting for it, this will reset condition flags and remove
          // the shutdown token from the pipe.
          WaitForShutdown();
      }
      fRequestShutdown = false;
  }
  
  bool ShutdownRequested()
  {
      return fRequestShutdown;
  }
  
  void WaitForShutdown()
  {
  #ifdef WIN32
      std::unique_lock<std::mutex> lk(g_shutdown_mutex);
      g_shutdown_cv.wait(lk, [] { return fRequestShutdown.load(); });
  #else
      char token;
      while (true) {
          int result = read(g_shutdown_pipe[0], &token, 1);
          if (result < 0) {
              // Failure. Check if the read was interrupted by a signal.
              // Other errors are unexpected here.
              assert(errno == EINTR);
          } else {
              assert(result == 1);
              break;
          }
      }
  #endif
  }
  