[Rcpp-devel] RCPP_USE_UNWIND_PROTECT by default

[Kevin Ushey]

If I tweak your example package code so that all exceptions are caught via
a catch (...) {} block, I can see something like:

> uptest::uptest()
Error in (function ()  : Ouch from R
Caught some other exception.

The fact that we're still printing the R error message seems undesirable,
but it looks like the exception can be caught -- just not via
std::exception.

On Sun, Feb 6, 2022 at 9:21 AM Kevin Ushey <kevinushey at gmail.com> wrote:

> 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.
>> _______________________________________________
>> Rcpp-devel mailing list
>> Rcpp-devel at lists.r-forge.r-project.org
>> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.r-forge.r-project.org/pipermail/rcpp-devel/attachments/20220206/18787f2a/attachment.html>