Scoping of parallel modelling

MRE:

library(fable)
#> Loading required package: fabletools
library(future)
plan(multiprocess)
#> Warning: [ONE-TIME WARNING] Forked processing ('multicore') is disabled
#> in future (>= 1.13.0) when running R from RStudio, because it is
#> considered unstable. Because of this, plan("multicore") will fall
#> back to plan("sequential"), and plan("multiprocess") will fall back to
#> plan("multisession") - not plan("multicore") as in the past. For more details,
#> how to control forked processing or not, and how to silence this warning in
#> future R sessions, see ?future::supportsMulticore
x <- 1:80
tsibble::tourism %>% 
  model(TSLM(Trips ~ I(x)))
#> Warning: 304 errors (1 unique) encountered for TSLM(Trips ~ I(x))
#> [304] object 'x' not found
#> # A mable: 304 x 4
#> # Key:     Region, State, Purpose [304]
#>    Region         State              Purpose  `TSLM(Trips ~ I(x))`
#>    <chr>          <chr>              <chr>    <model>             
#>  1 Adelaide       South Australia    Business <NULL model>        
#>  2 Adelaide       South Australia    Holiday  <NULL model>        
#>  3 Adelaide       South Australia    Other    <NULL model>        
#>  4 Adelaide       South Australia    Visiting <NULL model>        
#>  5 Adelaide Hills South Australia    Business <NULL model>        
#>  6 Adelaide Hills South Australia    Holiday  <NULL model>        
#>  7 Adelaide Hills South Australia    Other    <NULL model>        
#>  8 Adelaide Hills South Australia    Visiting <NULL model>        
#>  9 Alice Springs  Northern Territory Business <NULL model>        
#> 10 Alice Springs  Northern Territory Holiday  <NULL model>        
#> # … with 294 more rows

^{Created on 2020-01-08 by the reprex package (v0.3.0)}

Start by running the provided R reproducible example in fabletools with future::plan(multiprocess), then inspect how the model call is evaluated across parallel workers. Done means the external x object is available during parallel modelling and the tourism models fit without the reported object-not-found errors.