Almost degenerate dash patterns (where all dashes are very short), can cause infinite loops in raster backends and invalid output in vector backends. I've been able to confirm that quartz, svg and ps/pdf are affected by this problem, but other backends might have it too. A patch for quartz is available here: http://lists.cairographics.org/archives/cairo/2010-January/018753.html Vector backends (svg, pdf, ps) will produce invalid output for dash-infinite-loop, because they print the dash pattern using the "%f" printf format (which will happily print zeroes for very small numbers), but pdf specification prohibits zero-sum patterns (8.4.3.6 of pdf specification 1.7) and svg says that they are considered as non-dashed. _cairo_stroke_style_dash_approximate could probably be used to partially solve this problem (it would not explicitly guarantee that patterns are not zero-sum when printf'd unless appropriate limits on the tolerance are chosen). It could either be used in the backend or right before passing the dash pattern to the stroking function.
This has been partially fixed by 2e91648bcba06832dfa92bd742c32b2f5c1ad989. No check is done about the real output, but all dash patterns have a period at least as long as the tolerance (tanking the ctm into account). The true output might still be all zeroes in some degenerate cases (for example many sub-output-precision dashes summing up to a dash pattern with a period greater than tolerance), but it should be very infrequent as long as output precision in higher than tolerance and/or patterns are not composed of a lot of dashes.
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