In r-lib/pkgcache: Cache 'CRAN'-Like Metadata and R Packages

Platforms

[x] get_cran_extension()
[x] get_all_package_dirs()
[x] async_repo_status()
[x] packages_make_target()
[x] packages_make_sources()
[x] cmc__get_cache_files()
[x] current_r_platform()
[x] default_platforms()

Windows

Windows is special, because the same repository is used for multiple architectures. Probably most people use 64 bit R on Windows, because that's what RStudio starts by default. But we also need to make sure that people who need 32 bit R will have a way to install packages, without ruining the 64 bit installation.

Luckily, there are no packages on CRAN or Bioconductor currently that are 32 bit only. (There are a few that are 64 bit only.) In addition, 32 bit R currently installs the package for both architectures, when installing from source. (64 bit R-devel does the same currently, but this might change.)

In light of these, this is what we do:

The default platform is x86_64-w64-mingw32 on 64 bit R from R 4.2. It is i386+x86_64-mingw32 on 64 bit R before R 4.2.
The default platform is i386+x86_64-w64-mingw32 on 32 bit R.
The windows platform name is an alias to i386+x86_64-w64-mingw32.
When compiling a package from source, we'll observe the requested platform name:
For x86_64-w64-mingw32 on 64 bit R, we compile 64 bit only.
For i386-w64-mingw32 on 64 bit R, we compile for both 32 bit and 64 bit. (There is probably no way to avoid using the current arch.)
For i386+x86_64-w64-mingw32 on 64 bit R, we compile for both 32 bit and 64.
For i386-w64-mingw32 on 32 bit R, we compile for both 32 bit and 64 bit. This is to avoid mistakenly messing up a 64 bit library.
For x86_64-w64-mingw32 on 32 bit R, we compile for both 32 bit and 64 bit. (There is probably no way to avoid using the current arch.)
For i386+x86_64-w64-mingw32 on 32 bit R, we compile for both 32 bit and 64 bit.