rmarkdown::render crashes in parallel with parallel::makeForkCluster on macOS and Apple Silicon

[huguesmercier]

Problem description

I am knitting multiple HTML documents from RMD files in parallel. Here is a small sample reproducing the bug:

library(foreach)
library(doParallel)
library(rmarkdown)

cluster <- parallel::makeForkCluster(bigstatsr::nb_cores())
# cluster <- parallel::makePSOCKcluster(bigstatsr::nb_cores())

doParallel::registerDoParallel(cluster)

foreach(temp_counter = seq(1, 10)) %dopar% {

  html_file <- (paste0(temp_counter, ".html"))
  Rmd_file <- (paste0(temp_counter, ".Rmd"))
  
  text_string <- paste0("---\n", "title: 'TEST'\n", "---\n", "```{r setup, echo=FALSE}\n","print(temp_counter)\n","```")
  write(text_string, file = Rmd_file, append = FALSE)
  
  rmarkdown::render(Rmd_file, output_format = "html_document", output_file = html_file)
}

parallel::stopCluster(cluster)

I receive error messages like this:

The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec().
Break on __THE_PROCESS_HAS_FORKED_AND_YOU_CANNOT_USE_THIS_COREFOUNDATION_FUNCTIONALITY___YOU_MUST_EXEC__() to debug.
objc[19006]: +[__NSPlaceholderDate initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.

• parallel::makePSOCKcluster works fine.
• It is only rmarkdown::render that does not work with parallel::makeForkCluster. The .Rmd files are generated properly with parallel::makeForkCluster.

System information

The bug appears on multiple versions of RStudio / rmarkdown / pandoc, including this one:

R version 4.3.2 (2023-10-31) Platform: aarch64-apple-darwin20 (64-bit) Running under: macOS Sonoma 14.5, RStudio 2024.4.2.764

Locale: en_US.UTF-8 / en_US.UTF-8 / en_US.UTF-8 / C / en_US.UTF-8 / en_US.UTF-8

Package version: base64enc_0.1.3 bslib_0.6.0 cachem_1.0.8 cli_3.6.2 digest_0.6.33 ellipsis_0.3.2 evaluate_0.23
fastmap_1.1.1 fontawesome_0.5.2 fs_1.6.3 glue_1.7.0 graphics_4.3.2 grDevices_4.3.2 highr_0.10
htmltools_0.5.7 jquerylib_0.1.4 jsonlite_1.8.7 knitr_1.43 lifecycle_1.0.4 magrittr_2.0.3 memoise_2.0.1
methods_4.3.2 mime_0.12 R6_2.5.1 rappdirs_0.3.3 rlang_1.1.4 rmarkdown_2.22 sass_0.4.7
stats_4.3.2 stringi_1.8.2 stringr_1.5.1 tinytex_0.49 tools_4.3.2 utils_4.3.2 vctrs_0.6.5
xfun_0.41 yaml_2.3.7

Pandoc version: 3.1.11

Checklist

When filing a bug report, please check the boxes below to confirm that you have provided us with the information we need. Have you:

• [x] formatted your issue so it is easier for us to read?

• [x] included a minimal, self-contained, and reproducible example?

• [x] pasted the output from xfun::session_info('rmarkdown') in your issue?

• [x] upgraded all your packages to their latest versions (including your versions of R, the RStudio IDE, and relevant R packages)?

• [ ] installed and tested your bug with the development version of the rmarkdown package using remotes::install_github("rstudio/rmarkdown") ?