[R-SIG-Mac] Forking issues in Mavericks

[Jeroen Ooms]

The following works in Linux and OSX 10.8 but in OSX 10.9 it results
in a "__THE_PROCESS_HAS_FORKED_AND_YOU_CANNOT_USE_THIS_COREFOUNDATION_FUNCTIONALITY___YOU_MUST_EXEC__()":

library(parallel)
library(httr)
myfork <- mcparallel(GET("https://api.github.com/users/hadley/repos"))
out <- mccollect(myfork)

I've seen this error before when trying to access X11 from inside a
forked process. In this case it seems to be related to SSL, but I'm
not quite sure how to further debug this in 10.9 (lldb anyone?). What
exactly might be triggering this error?