Segfault in simple exponential model

Hello everyone,

I want to first thank the developers for an awesome software package that has helped me extensively in several projects.

This bug doesn’t really affect me, but I felt like I should report it anyways. With a specific model/data combination and a specific random seed, I observe a segfault when invoking the optimizing function. The model is rather simple.

Steps to reproduce:

  1. Please find the model attached (exp_model.stan) exp_model.stan (558 Bytes)
  2. Please find an R script attached (breaks_stan.R) breaks_stan.R (917 Bytes)
  3. Modify line 36 of the R file to point to where exp_model.stan lives on disk.
  4. Run the R file in its entirety. This is important to advance PRNG to the point of failure. This involves sampling from the posterior, but it terminates in less than half a second on my machine.

Bug Description:
Posterior sampling from the model works fine. Afterwards, when trying to find the optimum, it will print the initial loglikelihood, then R will freeze for about half a minute before this message appears (the shrug emoji is just my replacement for the standard prompt):

¯\_(ツ)_/¯ optimizing(mod, data = stan_dat)
Initial log joint probability = -8.18984

 *** caught segfault ***
address 0x256b38fff, cause 'memory not mapped'

 1: .External(list(name = "CppMethod__invoke_notvoid", address = <pointer: 0x7fdbf945fd70>,     dll = list(name = "Rcpp", path = "/Library/Frameworks/R.framework/Versions/3.4/Resources/library/Rcpp/libs/",         dynamicLookup = TRUE, handle = <pointer: 0x7fdbf9708d90>,         info = <pointer: 0x7fdbf986aa20>), numParameters = -1L),     <pointer: 0x7fdbf97fb9b0>, <pointer: 0x7fdbf9739640>, .pointer,     ...)
 2: sampler$call_sampler(c(args, dotlist))
 3: .local(object, ...)
 4: optimizing(mod, data = stan_dat)
 5: optimizing(mod, data = stan_dat)

Possible actions:
1: abort (with core dump, if enabled)
2: normal R exit
3: exit R without saving workspace
4: exit R saving workspace

This only seems to occur for a specific PRNG state.

Facts about the platform this was discovered on:

OS: Mac OS High Sierra 10.13.2

R: R version 3.4.3 (2017-11-30) – “Kite-Eating Tree”
Copyright © 2017 The R Foundation for Statistical Computing
Platform: x86_64-apple-darwin15.6.0 (64-bit)

rstan (Version 2.17.3, GitRev: 2e1f913d3ca3)

R was executed directly from a bash terminal.

1 Like

I wasn’t able to reproduce this on Ubuntu 16.04. But if it seems rng dependent then maybe it’s hard to trigger.

Maybe someone with a Mac can try it.

I get tons of error evaluating model log probabilities, then the optimization fails and things quit:

Error evaluating model log probability: Non-finite gradient.
Error evaluating model log probability: Non-finite gradient.
Error evaluating model log probability: Non-finite gradient.

Optimization terminated with error: 
  Line search failed to achieve a sufficient decrease, no more progress can be made
     lambda_1      lambda_2            nu 
 7.322885e-02 8.411680e-309  3.661443e-02 

edit: Not saying the bug doesn’t exist – just adding that I’d tried it.

Actually maybe make an issue in ? It might get lost in the shuffle on Discourse.

On a Mac, I can reproduce the situation as described by @nwycoff. If I don’t run sampling first, then I get the output that @bbbales2 got.

R version 3.5.0 (2018-04-23)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS High Sierra 10.13.5

This seems similar to

The crashing part seems like a bug, as I imagine Stan should just complain about the boundary issue instead of segfault. In fact, if you don’t run sampling first, then Stan does just complain – like bbbales2 got. R’s optim issues a warning about NaNs.

The lack of robustness to this boundary issue matches optim exactly.

I’m at least convinced that this is a boundary issue because optimizing, even after following steps 1-4 to reproduce, produces results similar to optim – within about 1e-4 – for all of the following cases.

  1. Flat prios
  2. Normal priors – N(0, 1)
  3. Gamma priors with boundary avoiding shape – \Gamma(\alpha = 2, 1)

The prior in the reproducible example, \Gamma(0.2, 1), seems to put too much weight near 0.

Should I open a new issue or follow up on issue 1928 with this simpler reproducible example?

Thanks for the replies! Indeed, one could argue that the optimization problem is not well conditioned, and I’m not actually using this prior/likelihood combination for any analyses, but my understanding of segfaults is that they basically shouldn’t happen no matter how much the user is asking for it :)

I’d be happy to either make an issue on github, or let Edward make an issue, or just add a link to this thread in the existing issue.

Yeah, if it reproduces on a mac (thanks for checking @roualdes!) then go ahead and make an issue. I’d say a new one since this is actually segfaulting.

gotcha, I’ve made an issue here.

1 Like