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A general description is here.
New function ‘utils::findCRANmirror()’ to find out if a CRAN mirror has been selected, otherwise fallback to the main site. This behaves in the same way as ‘tools::CRAN_package_db()’ and is intended for packages wishing to access CRAN for purposes other than installing packages.
The need for this was shown by a day when the main CRAN website was offline and a dozen or so packages which had its URL hardcoded failed their checks.