为什么 latexmk 会为 lualatex 文档调用 PdfLatex？

Question

我准备了一个.latexmkrc配置

$lualatex='lualatex -synctex=1 %O %B';
$pdf_mode = 4;
$aux_dir = "temp/";
$out_dir = "out/";
$success_cmd = "rm %R.pdf && mv out/*.pdf tex/";

add_cus_dep('glo', 'gls', 0, 'run_makeglossaries');
add_cus_dep('acn', 'acr', 0, 'run_makeglossaries');

sub run_makeglossaries {
  if ( $silent ) {
    system "makeglossaries -q '$_[0]'";
  }
  else {
    system "makeglossaries '$_[0]'";
  };
}

push @generated_exts, 'glo', 'gls', 'glg';
push @generated_exts, 'acn', 'acr', 'alg';
$clean_ext .= ' %R.ist %R.xdy';

并使用以下方式编译 lualatex 文档

latexmk -lualatex -gg -interaction=batchmode testx.tex 

我终于收到了消息

Output written on testx.pdf (33 pages, 5058191 bytes).
SyncTeX written on testx.synctex.gz.
Transcript written on testx.log.
Latexmk: Errors, so I did not complete making targets
Collected error summary (may duplicate other messages):
  lualatex: (Pdf)LaTeX failed to generate the expected log file 'temp/testx.log'
Latexmk: Did not finish processing file 'testx.tex':
   (Pdf)LaTeX failed to generate the expected log file 'temp/testx.log'
Latexmk: Use the -f option to force complete processing,
 unless error was exceeding maximum runs of latex/pdflatex.

想知道它为什么会叫pdflatex。我期望

lualatex failed to generate the expected...

系统：

Linux Mint 18.2 (latest at 2017-07-28), TeXLive 2017

$ latexmk --version 
   Latexmk, John Collins, 19 Jan. 2017. Version 4.52c

$ tlmgr --version
   tlmgr revision 44452 (2017-06-05 01:45:39 +0200)
   tlmgr using installation: /usr/local/texlive/2017
   TeX Live (http://tug.org/texlive) version 2017