signal_handler_guard.h

#pragma once

#include <atomic>
#include <csignal>

namespace dreal {

/// Sets a new signal handler and restores the old one when it goes
/// out of scope. When the flag is set, its destructor clears it out.
///
/// Motivation
/// ----------
///
/// Python's signal handler merely set a flag which is only checked in
/// the next python instruction. As a result, "C/C++ code may run
/// uninterrupted for an arbitrary amount of time" (from :
/// https://docs.python.org/3/library/signal.html#execution-of-python-signal-handlers)
///
/// Our approach
/// ------------
///
/// We provide a custom signal handler for SIGINT, which sets a global
/// flag (g_interrupted) which is monitored inside of dReal code. When
/// the flag is set, the checking code is supposed to throw an
/// exception, which will terminate the C++ call gracefully.  The C++
/// exception will be handled by pybind11, which will translate it to
/// a Python exception.
class SignalHandlerGuard {
 public:
  using handler_t = void (*)(int);

  /// Registers the new handler and saves the current one.
  SignalHandlerGuard(int sig, handler_t handler,
                     volatile std::atomic_bool* flag);
  SignalHandlerGuard(const SignalHandlerGuard&) = delete;
  SignalHandlerGuard(SignalHandlerGuard&&) = delete;
  SignalHandlerGuard& operator=(const SignalHandlerGuard&) = delete;
  SignalHandlerGuard& operator=(SignalHandlerGuard&&) = delete;

  /// Restores the old signal handler. If the flag is set, clear it.
  ~SignalHandlerGuard();

 private:
  const int sig_{0};
  volatile std::atomic_bool* flag_;

  using sigaction_t = struct sigaction;
  sigaction_t old_action_;
};
}  // namespace dreal