[Rcpp-devel] Rcpp::exception + threads = disaster

[Dirk Eddelbuettel]

On 7 February 2020 at 08:23, Joshua N Pritikin wrote:
| On Thu, Feb 06, 2020 at 08:40:02PM -0600, Dirk Eddelbuettel wrote:
| > On 6 February 2020 at 20:47, Joshua N Pritikin wrote:
| > | The Rcpp::exception constructor does,
| > | 
| > |   rcpp_set_stack_trace(Shield<SEXP>(stack_trace()))
| > | 
| > | This can corrupt R if called within an OpenMP block.
| > 
| > ... here. In general, we can _never_ call back into R for anything,
| > exceptions or other things.
| > 
| > The RcppParallel package documentation is quite good and clear about 
| > this; it even has extra data types RMatrix and RVector to stay away 
| > from R's memory (which Rcpp is close to for performance and zero 
| > copy reasons). The Writing R Extensions manual also as a little, but 
| > maybe less clearly.  In short, there is simply "so much going with 
| > R" that it stands little chance of every being threadsafe.
| > 
| > Which means that your OpenMP (or pthreads or TBB or ..) code has to 
| > stay away from R.
| 
| Yeah, so I replaced Rcpp::stop with,
| 
| template <typename... Args>
| inline void NORET mxThrow(const char* fmt, Args&&... args) {
|     throw std::runtime_error( tfm::format(fmt, std::forward<Args>(args)... ).c_str() );
| }
| 
| And now things work great. 

Good to know!

| But why does Rcpp::stop need to get the stack_trace? R's stack trace isn't
| going to change until the control flow returns back to R. So why can't you
| just set a flag to indicate that "some C++ exception happened" and grab the
| stack_trace immediately before returning control to R?

See several (extended) discussions at GitHub around the issues that changed
and improved exception handling and stack traces. A fair amount of very nice
work made Rcpp more robust here.

In short, it not a trivial issue.

Dirk

| > | It would be nice if there were some warnings about this (mis)usage OR,
| > | better, if Rcpp::exception was thread-safe.
| > 
| > Maybe some clever folks will come up with a sanitizer variant; otherwise I
| > just don't see how.

-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org