[Rd] Rebuilding and re-checking of downstream dependencies on CRAN Mac build machines

[Winston Chang]

I have two questions about the CRAN machines that build binary
packages for Mac. When a new version of a package is released,
  (A) Do the downstream dependencies get re-checked?
  (B) Do the downstream dependencies get re-built?

I have heard (but do not know for sure) that the answer to (A) is no,
the downstream dependencies do not get rechecked.

>From publicly available information on the CRAN web server, it looks
like the answer to (B) is also no, the downstream dependencies do not
get rebuilt. Looking at
https://www.r-project.org/nosvn/R.check/r-release-osx-x86_64/, I see
the following dates for these binary packages:

- Rcpp_1.0.4.tgz: 2020-03-18
- httpuv_1.5.2.tgz: 2019-09-12
- dplyr_0.8.5.tgz: 2020-03-08

Rcpp was released recently, and httpuv and dplyr (which are downstream
dependencies of Rcpp) have older dates, which indicates that these
binary packages were not rebuilt when Rcpp was released.

In my particular case, I'm interested in the httpuv package (which I
maintain). I and several others have not been able to get the CRAN
version of httpuv to compile using the CRAN version of Rcpp on Mac.
(It seems to compile fine on other platforms.) I have heard from
maintainers of other Rcpp-dependent packages that they also can't get
their packages to compile on Mac, using both the default Mac compiler
toolchain and the CRAN-recommended toolchain, which uses clang 7.

For more technical details about the cause of breakage, see:
https://github.com/RcppCore/Rcpp/issues/1060
https://github.com/rstudio/httpuv/issues/260

If the CRAN Mac build machine is indeed able to build httpuv against
the current version of Rcpp, it would be really helpful to have more
information about the system configuration. If it is not able to
rebuild httpuv and other packages against Rcpp, then this is a
problem. Among other things, it prevents people from building their
packages from source using CRAN versions of packages, and it also
means that none of these packages can release a new version, because
the binaries can't be built on Mac.

-Winston