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A general description is here.
Finalizers are marked to be run at garbage collection, but run only at a somewhat safer later time (when interrupts are next checked). This circumvents some problems with finalizers running arbitrary code during garbage collection (the known instance being running ‘options()’ re-entrantly).
‘available.packages()’ is better protected against corrupt metadata files (a recurring problem with Debian package ‘shogun-r’: PR#14713).