Descent3/ddio/file.cpp

/*
 * Descent 3
 * Copyright (C) 2024 Descent Developers
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <array>
#include <filesystem>
#include <fstream>
#include <iterator>

#include "IOOps.h"
#include "ddio.h"
#include "pserror.h"

const std::array<char, 4> LOCK_TAG = {'L', 'O', 'C', 'K'};

struct LockFileContent {
  std::array<char, 4> tag = LOCK_TAG;
  int32_t pid = -1;
};

std::ostream &operator<<(std::ostream &output, const LockFileContent &header) {
  std::copy(header.tag.begin(), header.tag.end(), std::ostream_iterator<char>(output));
  D3::bin_write(output, header.pid);
  return output;
}

std::istream &operator>>(std::istream &input, LockFileContent &header) {
  input.read(reinterpret_cast<char*>(header.tag.data()), 4);
  D3::bin_read(input, header.pid);
  return input;
}

bool ddio_CreateLockFile(const std::filesystem::path& dir) {
  std::filesystem::path lock_filename = dir / ".lock";

  if(!std::filesystem::is_directory(lock_filename.parent_path())) {
    return false;
  }

  LockFileContent lock_content;
  int32_t curr_pid = ddio_GetPID();

  if (std::filesystem::exists(lock_filename)) {
    try {
      std::ifstream lockfile(lock_filename, std::ios::binary);
      lockfile >> lock_content;
      lockfile.close();
    } catch (std::exception &e) {
      return false;
    }

    if (lock_content.tag != LOCK_TAG) {
      return false;
    }

    // it appears to be a lock file, check the pid
    ASSERT(lock_content.pid != -1);

    // check the file id in the file, compared to our pid
    if (lock_content.pid == curr_pid) {
      // lock file already exists for the current process
      return true;
    }

    if (ddio_CheckProcess(lock_content.pid)) {
      // Process exists, lock still valid
      return false;
    }

    // the process no longer exists, we can create a lock file if needed
    // we'll delete the useless one now
    // the lock file in the directory belongs to us!
    std::filesystem::remove(lock_filename);
  }

  lock_content.pid = curr_pid;

  try {
    std::ofstream lockfile(lock_filename, std::ios::binary);
    lockfile << lock_content;
    lockfile.close();
  } catch (std::exception &e) {
    return false;
  }

  // at this point result will either be 1 or 2 from checking the lock file
  // either way, a lock file has been created
  return true;
}

bool ddio_DeleteLockFile(const std::filesystem::path& dir) {
  int32_t curr_pid = ddio_GetPID();

  std::filesystem::path lock_filename = dir / ".lock";
  if (!std::filesystem::exists(lock_filename)) {
    return true;
  }

  LockFileContent lock_content;
  try {
    std::ifstream lockfile(lock_filename, std::ios::binary);
    lockfile >> lock_content;
    lockfile.close();
  } catch (std::exception &e) {
    return false;
  }

  if (lock_content.tag != LOCK_TAG) {
    return false;
  }

  ASSERT(lock_content.pid != -1);

  // check the file id in the file, compared to our pid
  if (lock_content.pid != curr_pid) {
    // it doesn't belong to
    return false;
  }

  std::error_code ec;
  // the lock file in the directory belongs to us!
  if (!std::filesystem::remove(lock_filename, ec)) {
    return false;
  }

  return true;
}