`check_future_plan()` throws an error because the `workers` name/slot is no longer provided

[njtierney]

https://github.com/greta-dev/greta/blob/master/R/utils.R#L899-L904

This code expects there to be a workers name/slot in the future object, but there is not.

This then breaks the tests at https://github.com/greta-dev/greta/blob/master/tests/testthat/test_inference.R#L477-L551

f_thing <- future::future(NULL, lazy = TRUE)

names(f_thing)
#>  [1] "label"         "local"         "owner"         "envir"        
#>  [5] "packages"      "gc"            "conditions"    "expr"         
#>  [9] "uuid"          "seed"          "version"       "result"       
#> [13] "asynchronous"  "calls"         "globals"       "stdout"       
#> [17] "earlySignal"   "lazy"          "state"         ".defaultLocal"

f_thing$workers
#> NULL

^{Created on 2021-11-02 by the reprex package (v2.0.1)}

Session info

sessioninfo::session_info()
#> ─ Session info ───────────────────────────────────────────────────────────────
#>  setting  value                       
#>  version  R version 4.1.1 (2021-08-10)
#>  os       macOS Big Sur 10.16         
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#>  ui       X11                         
#>  language (EN)                        
#>  collate  en_AU.UTF-8                 
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#>  date     2021-11-02                  
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#>  yaml          2.2.1   2020-02-01 [1] CRAN (R 4.1.0)
#> 
#> [1] /Library/Frameworks/R.framework/Versions/4.1/Resources/library

[HenrikBengtsson]

The reason why this "hack" no longer works is that in future (>= 1.22.0), a lazy future has not yet been "metamorphosed" into its final form (according to the current plan(). The quick fix, still a hack though, is to use lazy = FALSE.

[HenrikBengtsson]

This then breaks the tests at https://github.com/greta-dev/greta/blob/master/tests/testthat/test_inference.R#L477-L551

Is that a new test? I ran reverse package dependency checks with future 1.23.0 on 2021-10-27 and, except from a teeny NOTE, nothing came up, cf. https://github.com/HenrikBengtsson/future/blob/develop/revdep/problems.md#greta.

[njtierney]

Thanks, @HenrikBengtsson !

I'll need to check in with @goldingn on this part of the future check code, but so far that fix has resolved that test file, thank you!

Yes, that wouldn't have been picked up on reverse pkg dependency checks with future as we end up needing to skip a lot of tests as they either take too long to run on CRAN, or we cannot get CRAN to have a tensorflow setup.

We had the tests running successfully on GH actions but they are a bit fragile. I will try and rig up a cron job for this so we can stay on top of things!

Note to self: make greta import version 1.22.0 of future.

[HenrikBengtsson]

Yes, that wouldn't have been picked up on reverse pkg dependency checks with future as we end up needing to skip a lot of tests as they either take too long to run on CRAN, or we cannot get CRAN to have a tensorflow setup.

Would it be possible to pull that piece of code into a help function that you can run tests on?

BTW, I you still want to condition the code on a workers element actually existing, e.g. if (!is.null(workers)) .... As I mentioned in futureverse/future#556, not all types of futures have an internal workers element (regardless of lazy = TRUE/FALSE).

The actual objective, of being able to fall back to sequential process when workers are forked processes, will be address in future via a new "resource" specification framework, e.g.

f <- future({ ... }, resources = !fork)

It's on the near roadmap but it'll probably be another 6-12 months before there is a working prototype.

[goldingn]

Thanks @HenrikBengtsson!
Yes, makes sense. That 'workers' code was fragile and only meant as a temporary fix.
Testing the functions that interface with future separately is a good idea.

[HenrikBengtsson]

As an alternative to assert that against forked processing upfront, could you just assert this at the first step of your future expression? If so, you could use parallelly::isForkedChild(). Proof of concept:

my_fcn <- function() {
  ## WORKAROUND: https://github.com/HenrikBengtsson/parallelly/issues/71
  dummy <- parallelly::isForkedChild()

  f <- future({ 
    if (parallelly::isForkedChild()) {
      stop("Does not support forked parallelization")
    }
  
    42 
  })
  value(f)
}

For example,

library(future)

plan(sequential)
my_fcn()
#> [1] 42

plan(multisession)
my_fcn()
#> [1] 42

cl <- parallel::makeCluster(2)
plan(cluster, workers = cl)
my_fcn()
#> [1] 42

cl <- parallel::makeCluster(2, type = "FORK")
plan(cluster, workers = cl)
my_fcn()
#> Error in withCallingHandlers({ : Does not support forked parallelization

plan(multicore)
my_fcn()
#> Error in withCallingHandlers({ : Does not support forked parallelization

[HenrikBengtsson]

I discovered a bug in parallelly::isForkedChild() requiring a small workaround until resolved (code edited in above comment).

[njtierney]

Thanks again for this, @HenrikBengtsson !

I don't get the same error at plan(multicore) as you have, not sure if I'm missing something:

library(future)

my_fcn <- function() {
  ## WORKAROUND: https://github.com/HenrikBengtsson/parallelly/issues/71
  dummy <- parallelly::isForkedChild()
  
  f <- future({ 
    if (parallelly::isForkedChild()) {
      stop("Does not support forked parallelization")
    }
    
    42 
  })
  value(f)
}

plan(sequential)
my_fcn()
#> [1] 42

plan(multisession)
my_fcn()
#> [1] 42

cl <- parallel::makeCluster(2)
plan(cluster, workers = cl)
my_fcn()
#> [1] 42

cl <- parallel::makeCluster(2, type = "FORK")
plan(cluster, workers = cl)
my_fcn()
#> Error in withCallingHandlers({: Does not support forked parallelization

plan(multicore)
my_fcn()
#> [1] 42


cl <- parallel::makeForkCluster(2L)
plan(cluster, workers = cl)
my_fcn()
#> Error in withCallingHandlers({: Does not support forked parallelization

^{Created on 2021-11-04 by the reprex package (v2.0.1)}

Session info

sessioninfo::session_info()
#> ─ Session info ───────────────────────────────────────────────────────────────
#>  setting  value                       
#>  version  R version 4.1.1 (2021-08-10)
#>  os       macOS Big Sur 10.16         
#>  system   x86_64, darwin17.0          
#>  ui       X11                         
#>  language (EN)                        
#>  collate  en_AU.UTF-8                 
#>  ctype    en_AU.UTF-8                 
#>  tz       Australia/Perth             
#>  date     2021-11-04                  
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