Normally, operations scheduled with later() will not execute unless/until
no other R code is on the stack (i.e. at the top-level). If you need to run
blocking R code for a long time and want to allow scheduled operations to run
at well-defined points of your own operation, you can call run_now() at
those points and any operations that are due to run will do so.
run_now()A logical indicating whether any callbacks were actually run.
If one of the callbacks throws an error, the error will not be caught, and
subsequent callbacks will not be executed (until run_now() is called again,
or control returns to the R prompt). You must use your own base::tryCatch()
if you want to handle errors.