num_pars_unconstrained SEGV

Summary:

Sometimes I get a crash like this,

 *** caught segfault ***
address 0x7f84b02f27c8, cause 'memory not mapped'

Traceback:
 1: .External(list(name = "CppMethod__invoke_notvoid", address = <pointer: 0x562b43783430>,     dll = list(name = "Rcpp", path = "/opt/R/x86_64-pc-linux-gnu-library/3.5/Rcpp/libs/Rcpp.so",         dynamicLookup = TRUE, handle = <pointer: 0x562b4488ea40>,         info = <pointer: 0x562b4226dd60>), numParameters = -1L),     <pointer: 0x562b4d307df0>, <pointer: 0x562b48d644a0>, .pointer)
 2: object@.MISC$stan_fit_instance$num_pars_unconstrained()
 3: .local(object, ...)
 4: get_num_upars(object)
 5: get_num_upars(object)
 6: throw_sampler_warnings(nfits)
 7: .local(object, ...)
 8: sampling(sm, data, pars, chains, iter, warmup, thin, seed, init,     check_data = TRUE, sample_file = sample_file, diagnostic_file = diagnostic_file,     verbose = verbose, algorithm = match.arg(algorithm), control = control,     check_unknown_args = FALSE, cores = cores, open_progress = open_progress,     include = include, ...)
 9: sampling(sm, data, pars, chains, iter, warmup, thin, seed, init,     check_data = TRUE, sample_file = sample_file, diagnostic_file = diagnostic_file,     verbose = verbose, algorithm = match.arg(algorithm), control = control,     check_unknown_args = FALSE, cores = cores, open_progress = open_progress,     include = include, ...)
10: stan(file = "unidim-adaptScale.stan", chains = numChains, cores = numChains,     iter = curIter, control = list(max_treedepth = 15), data = list(NCMP = N,         NPA = length(palist), pa1 = match(df$pa1, palist), pa2 = match(df$pa2,             palist), diff = df$pick, varCorrection = trial[tx,             "varCorrection"]), open_progress = FALSE)
An irrecoverable exception occurred. R is aborting now ...
Segmentation fault (core dumped)

Description:

It doesn't happen all the time.

Reproducible Steps:

Try running the following for half an hour,

sim2.R.txt
lib.R.txt
unidim-adaptScale.stan.txt

Obviously rename to remove the .txt suffix.

RStan Version: 2.18.2

R Version: R version 3.5.1 (2018-07-02)

Operating System: Ubuntu Cosmic w/ Linux 4.18.0-17-generic

Start by running sim2.R with lib.R and unidim-adaptScale.stan after removing the .txt suffixes, using the reported RStan and R versions. Inspect the num_pars_unconstrained() entry in the stack trace while reproducing the intermittent SEGV. Done means identifying and fixing the crash and confirming the reproducer no longer aborts.