Windows R session crashes when assigning multiple runs to same output

Summary:

Following on from this issue in the brms repo, the R session on Windows will crash when repeatedly assigning to the same output.

This is most easily replicated with brms. Using this list of brms arguments: test_brm.RData, the crash can be replicated via:

options(mc.cores=4)
library(brms)
load("test_brm.RData")

out = do.call(brm,test_brm)
out = do.call(brm,test_brm)
out = do.call(brm,test_brm)

#Crashes on this call:
out = do.call(brm,test_brm)

However, when I extract the Stan code, data and initial values from the brms object into
test_rstan.txt and do the same runs with 'pure' RStan, I don't get the same failures:

options(mc.cores=4)
library(rstan)
source("test_rstan.txt")

out = do.call(stan,test_rstan)
out = do.call(stan,test_rstan)
out = do.call(stan,test_rstan)

#Does not crash here
out = do.call(stan,test_rstan)

This leads me to believe the crash is related to the model compilation in some way, since the brms calls re-compile the model each time, but the rstan calls do not (even if I set rstan_options(auto_write = FALSE))

Contents of my Makevars & .Renviron files:

> readLines("~/.R/Makevars.win")
[1] "CXX14FLAGS += -O3"

> readLines("~/.Renviron")
[1] "PATH=\"${RTOOLS40_HOME}\\usr\\bin;${PATH}\""

Session Info:

> sessionInfo()
R version 4.0.2 (2020-06-22)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 18363)

Matrix products: default

locale:
[1] LC_COLLATE=English_Australia.1252  LC_CTYPE=English_Australia.1252   
[3] LC_MONETARY=English_Australia.1252 LC_NUMERIC=C                      
[5] LC_TIME=English_Australia.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] rstan_2.21.2         ggplot2_3.3.2        StanHeaders_2.21.0-6

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.5         pillar_1.4.6       compiler_4.0.2     prettyunits_1.1.1  tools_4.0.2       
 [6] pkgbuild_1.1.0     jsonlite_1.7.0     lifecycle_0.2.0    tibble_3.0.3       gtable_0.3.0      
[11] pkgconfig_2.0.3    rlang_0.4.7        cli_2.0.2          rstudioapi_0.11    parallel_4.0.2    
[16] curl_4.3           loo_2.3.1          gridExtra_2.3      withr_2.2.0        dplyr_1.0.2       
[21] generics_0.0.2     vctrs_0.3.4        stats4_4.0.2       grid_4.0.2         tidyselect_1.1.0  
[26] glue_1.4.2         inline_0.3.15      R6_2.4.1           processx_3.4.3     fansi_0.4.1       
[31] callr_3.4.3        purrr_0.3.4        magrittr_1.5       codetools_0.2-16   matrixStats_0.56.0
[36] scales_1.1.1       ps_1.3.4           ellipsis_0.3.1     assertthat_0.2.1   colorspace_1.4-1  
[41] V8_3.2.0           RcppParallel_5.0.2 munsell_0.5.0      crayon_1.3.4      

Start by reproducing the Windows crash with the repeated do.call(brm, test_brm) calls and the linked test_brm.RData; compare this with the repeated do.call(stan, test_rstan) calls that do not crash. Investigate the model-compilation path implicated by the report and verify that repeated compilation no longer terminates the R session on Windows.