Why latexmk does ignore my (index) custom dependency and subroutine?

Question

Consider the following latexmkrc configuration file (say latexmkrc.tex) containing a custom dependency which is supposed to run the texindy subroutine whenever a .ind file is out-of-date with respect to the corresponding source file:

# $makeindex = 'texindy'; #$

add_cus_dep('idx', 'ind', 0, 'texindy');
sub texindy{
    system("texindy \"$_[0].idx\""); #$
}

$pdf_mode = 1; #$

Now, consider a .tex file (say test.tex) processed with:

latexmk -g -norc -r latexmkrc.tex test

where:

• -g option to force latexmk to process the document fully (just for the example),
• -norc option just to be sure no other latexmk configuration file than latexmkrc.tex is taken in account.

Then, the following happens:

1. Running pdflatex -recorder "test.tex".
2. Running makeindex -o "test.ind" "test.idx".
3. Running pdflatex -recorder "test.tex".

The trouble is with step #2, latexmk creating a new rule:

=== Creating rule for 'cusdep idx ind test'
Latexmk: applying rule 'makeindex test.idx'...
Rule 'makeindex test.idx': File changes, etc:
   Non-existent destination files:
      'test.ind'
------------
Run number 1 of rule 'makeindex test.idx'
------------
------------
Running 'makeindex  -o "test.ind" "test.idx"'

ignoring the custom dependency and subroutine in latexmkrc.tex that is supposed to lead to: texindy -o "test.ind" "test.idx" instead.

As a workaround, one could comment out the line

# $makeindex = 'texindy'; #$

in latexmkrc.tex but I'm still puzzled: why latexmk does ignore my (index) custom dependency and subroutine?

Minimal complete example:

\documentclass{article}
\usepackage[xindy]{imakeidx}
\usepackage{filecontents}

\makeindex

\begin{filecontents*}{latexmkrc}
# $makeindex = 'texindy'; #$

add_cus_dep('idx', 'ind', 0, 'texindy');
sub texindy{
    system("texindy \"$_[0].idx\""); #$
}

$pdf_mode = 1; #$
\end{filecontents*}

\begin{document}
Foo\index{Foo}
\printindex
\end{document}

Answer 1

You've got two rules for making an .ind file from an .idx file: the built-in makeindex rule and your custom dependency. Latexmk has to choose which. The way it's programmed, it finds the built-in rule first, so that's what get used.

As you've noticed, to use texindy you need to redefine $makeindex, either by

$makeindex = 'texindy';

or (slightly better) by

$makeindex = 'texindy %O -o %D %S';