paleolimbot commented on PR #14250:
URL: https://github.com/apache/arrow/pull/14250#issuecomment-1275475932

   I'll investigate more when I'm back from PTO on Monday, but I did a quick 
check and (1) cancellation still works interactively and (2) this does seem to 
break forked process behaviour when using R + arrow:
   
   <details>
   
   ``` r
   library(arrow, warn.conflicts = FALSE)
   #> Some features are not enabled in this build of Arrow. Run `arrow_info()` 
for more information.
   
   tf <- tempfile()
   readr::write_csv(vctrs::vec_rep(mtcars, 5e5), tf)
   
   # try to slow down CSV reading
   set_cpu_count(1)
   set_io_thread_count(2)
   
   # make sure we can cancel
   read_csv_arrow(tf)
   #> # A tibble: 16,000,000 × 11
   #>      mpg   cyl  disp    hp  drat    wt  qsec    vs    am  gear  carb
   #>    <dbl> <int> <dbl> <int> <dbl> <dbl> <dbl> <int> <int> <int> <int>
   #>  1  21       6  160    110  3.9   2.62  16.5     0     1     4     4
   #>  2  21       6  160    110  3.9   2.88  17.0     0     1     4     4
   #>  3  22.8     4  108     93  3.85  2.32  18.6     1     1     4     1
   #>  4  21.4     6  258    110  3.08  3.22  19.4     1     0     3     1
   #>  5  18.7     8  360    175  3.15  3.44  17.0     0     0     3     2
   #>  6  18.1     6  225    105  2.76  3.46  20.2     1     0     3     1
   #>  7  14.3     8  360    245  3.21  3.57  15.8     0     0     3     4
   #>  8  24.4     4  147.    62  3.69  3.19  20       1     0     4     2
   #>  9  22.8     4  141.    95  3.92  3.15  22.9     1     0     4     2
   #> 10  19.2     6  168.   123  3.92  3.44  18.3     1     0     4     4
   #> # … with 15,999,990 more rows
   
   # see if arrow works in a forked R process
   parallel::mclapply(1:2, function(...) read_csv_arrow(tf))
   #> Warning in parallel::mclapply(1:2, function(...) read_csv_arrow(tf)): 
scheduled
   #> cores 1, 2 did not deliver results, all values of the jobs will be 
affected
   #> [[1]]
   #> NULL
   #> 
   #> [[2]]
   #> NULL
   ```
   
   <sup>Created on 2022-10-11 by the [reprex 
package](https://reprex.tidyverse.org) (v2.0.1)</sup>
   
   </details>


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