[Rcpp-devel] RCPP_USE_UNWIND_PROTECT by default

uptest::uptest()

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.