g13_mount_container (ctrl_t ctrl, const char *filename, const char *mountpoint)
{
  gpg_error_t err;
  dotlock_t lock;
  void *enckeyblob = NULL;
  size_t enckeybloblen;
  void *keyblob = NULL;
  size_t keybloblen;
  tupledesc_t tuples = NULL;
  size_t n;
  const unsigned char *value;
  int conttype;
  unsigned int rid;
  char *mountpoint_buffer = NULL;

  /* A quick check to see whether the container exists.  */
  if (access (filename, R_OK))
    return gpg_error_from_syserror ();

  if (!mountpoint)
    {
      mountpoint_buffer = xtrystrdup ("/tmp/g13-XXXXXX");
      if (!mountpoint_buffer)
        return gpg_error_from_syserror ();
      if (!gnupg_mkdtemp (mountpoint_buffer))
        {
          err = gpg_error_from_syserror ();
          log_error (_("can't create directory '%s': %s\n"),
                     "/tmp/g13-XXXXXX", gpg_strerror (err));
          xfree (mountpoint_buffer);
          return err;
        }
      mountpoint = mountpoint_buffer;
    }

  /* Try to take a lock.  */
  lock = dotlock_create (filename, 0);
  if (!lock)
    {
      xfree (mountpoint_buffer);
      return gpg_error_from_syserror ();
    }

  if (dotlock_take (lock, 0))
    {
      err = gpg_error_from_syserror ();
      goto leave;
    }
  else
    err = 0;

  /* Check again that the file exists.  */
  {
    struct stat sb;

    if (stat (filename, &sb))
      {
        err = gpg_error_from_syserror ();
        goto leave;
      }
  }

  /* Read the encrypted keyblob.  */
  err = read_keyblob (filename, &enckeyblob, &enckeybloblen);
  if (err)
    goto leave;

  /* Decrypt that keyblob and store it in a tuple descriptor.  */
  err = decrypt_keyblob (ctrl, enckeyblob, enckeybloblen,
                         &keyblob, &keybloblen);
  if (err)
    goto leave;
  xfree (enckeyblob);
  enckeyblob = NULL;

  err = create_tupledesc (&tuples, keyblob, keybloblen);
  if (!err)
    keyblob = NULL;
  else
    {
      if (gpg_err_code (err) == GPG_ERR_NOT_SUPPORTED)
        log_error ("unknown keyblob version\n");
      goto leave;
    }
  if (opt.verbose)
    dump_keyblob (tuples);

  value = find_tuple (tuples, KEYBLOB_TAG_CONTTYPE, &n);
  if (!value || n != 2)
    conttype = 0;
  else
    conttype = (value[0] << 8 | value[1]);
  if (!be_is_supported_conttype (conttype))
    {
      log_error ("content type %d is not supported\n", conttype);
      err = gpg_error (GPG_ERR_NOT_SUPPORTED);
      goto leave;
    }
  err = be_mount_container (ctrl, conttype, filename, mountpoint, tuples, &rid);
  if (!err)
    {
      err = mountinfo_add_mount (filename, mountpoint, conttype, rid,
                                 !!mountpoint_buffer);
      /* Fixme: What shall we do if this fails?  Add a provisional
         mountinfo entry first and remove it on error? */
      if (!err)
        {
          char *tmp = percent_plus_escape (mountpoint);
          if (!tmp)
            err = gpg_error_from_syserror ();
          else
            {
              g13_status (ctrl, STATUS_MOUNTPOINT, tmp, NULL);
              xfree (tmp);
            }
        }
    }

 leave:
  destroy_tupledesc (tuples);
  xfree (keyblob);
  xfree (enckeyblob);
  dotlock_destroy (lock);
  xfree (mountpoint_buffer);
  return err;
}