Function __syscall_renameat

system/lib/wasmfs/syscalls.cpp:977–1079

TODO: Test this with non-AT_FDCWD values.

// TODO: Test this with non-AT_FDCWD values.
int __syscall_renameat(int olddirfd,
                       const char* oldpath,
                       int newdirfd,
                       const char* newpath) {
  // Rename is the only syscall that needs to (or is allowed to) acquire locks
  // on two directories at once. It requires locks on both the old and new
  // parent directories to ensure that the moved file can be atomically removed
  // from the old directory and added to the new directory without something
  // changing that would prevent the move.
  //
  // To prevent deadlock in the case of simultaneous renames, serialize renames
  // with an additional global lock.
  static std::mutex renameMutex;
  std::lock_guard<std::mutex> renameLock(renameMutex);

  // Get the old directory.
  auto parsedOld = path::parseParent(oldpath, olddirfd);
  if (auto err = parsedOld.getError()) {
    return err;
  }
  auto& [oldParent, oldFileNameView] = parsedOld.getParentChild();
  std::string oldFileName(oldFileNameView);

  // Get the new directory.
  auto parsedNew = path::parseParent(newpath, newdirfd);
  if (auto err = parsedNew.getError()) {
    return err;
  }
  auto& [newParent, newFileNameView] = parsedNew.getParentChild();
  std::string newFileName(newFileNameView);

  if (newFileNameView.size() > WASMFS_NAME_MAX) {
    return -ENAMETOOLONG;
  }

  // Lock both directories.
  auto lockedOldParent = oldParent->locked();
  auto lockedNewParent = newParent->locked();

  // Get the source and destination files.
  auto oldFile = lockedOldParent.getChild(oldFileName);
  auto newFile = lockedNewParent.getChild(newFileName);

  if (!oldFile) {
    return -ENOENT;
  }

  // If the source and destination are the same, do nothing.
  if (oldFile == newFile) {
    return 0;
  }

  // Never allow renaming or overwriting the root.
  auto root = wasmFS.getRootDirectory();
  if (oldFile == root || newFile == root) {
    return -EBUSY;
  }