[XeTeX] Gentium Book / Book Basic + TeX Ligatures = memory leak

[Simon Cozens]

OK, this is an odd one. Here's my test document:

\documentclass{book}
\usepackage{fontspec}
\setromanfont{Gentium:Ligatures=TeX}
\usepackage{blindtext}

\begin{document}
\Blinddocument
\Blinddocument
\Blinddocument

\end{document}

It runs pretty well:

% time xelatex -no-pdf test
...
Output written on test.xdv (48 pages, 920024 bytes).
Transcript written on test.log.
xelatex -no-pdf test  1.96s user 0.10s system 98% cpu 2.083 total

Memory usage peaks at around 185M on this MacBook Pro. (I'm using
-no-pdf to eliminate xdvipdfmx as a source of any problems.)

If I change that font line to

\setromanfont{Gentium Book Basic:Ligatures=TeX}

Then this happens:

% time xelatex -no-pdf test
...
xelatex -no-pdf test  118.87s user 0.91s system 99% cpu 2:00.26 total

60x slowdown! Memory usage rises to 318M. Same happens with Gentium
Basic. Take off the 'Ligatures=TeX' and it's fine.

% xetex --version
XeTeX 3.14159265-2.6-0.99991 (TeX Live 2014)