packrat hangs if CRAN mirror unavailable

Last night (and at time of writing) the CRAN mirror http://cran.ms.unimelb.edu.au/ was not accessible. This was the mirror I was using in my packrat/packrat.lock file:

PackratFormat: 1.4
PackratVersion: 0.4.0.12
RVersion: 3.1.1
Repos: CRAN= http://cran.ms.unimelb.edu.au/,
    BioCsoft=http://bioconductor.org/packages/3.0/bioc,
    BioCann=http://bioconductor.org/packages/3.0/data/annotation,
    BioCexp=http://bioconductor.org/packages/3.0/data/experiment,
    BioCextra=http://bioconductor.org/packages/3.0/extra
# <snip>

As a result of this (I suspect) all packrat operations (e.g. packrat::status()) were taking a very long time and initially I thought my RStudio installation was borked. However, after some fiddling around I realised it seemed to be due to packrat since I got similar behaviour from the R command line.

The reason I suspect it was due to my choice of (an inaccessible) CRAN mirror is that the problem went away once I switched mirrors, i.e.:

PackratFormat: 1.4
PackratVersion: 0.4.0.12
RVersion: 3.1.1
Repos: CRAN=http://cran.rstudio.com,
    BioCsoft=http://bioconductor.org/packages/3.0/bioc,
    BioCann=http://bioconductor.org/packages/3.0/data/annotation,
    BioCexp=http://bioconductor.org/packages/3.0/data/experiment,
    BioCextra=http://bioconductor.org/packages/3.0/extra
# <snip>

I'm sorry this is a little light on details and lacks a reproducible example, but I thought you might like to know since it was rather mysterious what was going wrong. Please let me know if you'd like further details and keep up the great work with packrat.

Start by reproducing the delay with packrat::status() using the CRAN URL in packrat/packrat.lock, then compare behavior after switching to cran.rstudio.com. Trace the repository-access path and define done as packrat operations responding promptly or failing clearly when a configured mirror is unreachable.