[Rcpp-devel] RCPP_USE_UNWIND_PROTECT by default

[Kevin Ushey]

Is it possible the issue here is ultimately just that our
LongjumpException class doesn't inherit from std::exception?

On Sun, Feb 6, 2022 at 9:18 AM Jeroen Ooms <jeroenooms at gmail.com> wrote:

> On Sun, Feb 6, 2022 at 5:56 PM Dirk Eddelbuettel <edd at debian.org> wrote:
> >
> >
> > On 6 February 2022 at 17:40, Jeroen Ooms wrote:
> > | We can try to take V8 out of the equation, and see what actually
> > | causes the change. V8 uses (and tests!) the Rcpp feature to call an R
> > | function from C++. This behaves quite differently when using
> > | RCPP_UNWIND_PROTECT.
> > |
> > | Here is a dummy package to demonstrate this:
> https://github.com/jeroen/uptest
> > |
> > | The use case is simple: we want to call some R function from C++, and
> > | if it fails, catch the error message and deal with it in C++. I
> > | suspect there are more packages doing this, but perhaps they are not
> > | testing this case.
> > |
> > | The example from uptest above show that when we compile with
> > | RCPP_UNWIND_PROTECT, any R error is always printed directly to the
> > | console. Even if we catch the Rcpp::LongjumpException in C++, the R
> > | error still seems to bubble up (as Iñaki also noted). I think this is
> > | at least surprising.
> > |
> > | Then I am not sure what we do in C++ now with this
> > | Rcpp::LongjumpException if we catch it. We certainly don't want to
> > | unwind all the way in the middle of running the javascript code. The
> > | JavaScript code should be able to catch the R error, and continue
> > | running the script.
> > |
> > | Anyway if you want to make RCPP_UNWIND_PROTECT the default, I can
> > | obviously opt-out in V8, but as an Rcpp user I am not convinced this
> > | is an improvement.
> >
> > Thanks for the feedback. I think there is an easy-to-understand,
> easy-to-make
> > error in a sample package (but kudos for creating one!).  Because Rcpp
> will
> > always wrap try/catch around the function you create with its mechanism,
> the
> > layer inside is redundant.  So I suggest the C++ function file becomes
> this
> > simpler / shorter variant:
> >
> >   #include <Rcpp.h>
> >   using namespace Rcpp;
> >
> >   // [[Rcpp::export]]
> >   Rcpp::NumericVector call_r_from_rcpp() {
> >     Rcpp::Function callback =
> Rcpp::Environment::namespace_env("uptest")["callback"];
> >     callback();
> >     return Rcpp::NumericVector::create(42);
> >   }
> >
> > which behaves fine for me in both cases.
>
> Well not really, this kind of misses my point that it the
> unwind-protect makes it impossible to meaningfully catch the R error
> in C++, handle it, and continue running the C++ code, without aborting
> the entire mission and throwing the user back in the console.
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