Condition handlers not properly respected by `Rcpp::Function` evaluation

[kevinushey]

Example code that exercises different bits of weirdness / brokenness:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
void ouch() { stop("ouch"); }

// [[Rcpp::export]]
void exec(Function f) { f(); }

/*** R
## Fails (no error thrown)
exec(stop)

## Correct (error is thrown)
exec(function() stop("ouch"))

## Correct (error is silenced)
try(silent = TRUE, exec(function() stop("ouch")))

## Correct (error is silenced)
exec(function() try(silent = TRUE, stop("ouch")))

## Fails (error is not silenced)
exec(function() try(silent = TRUE, exec(ouch)))

## Fails (error is not silenced)
exec(function() try(silent = TRUE, exec(function() stop("ouch"))))
*/

cc: @jcheng5, rstudio/shiny#879

[kevinushey]

I think the first bug, re: exec(stop), is related to these lines:

https://github.com/RcppCore/Rcpp/blob/master/inst/include/Rcpp/api/meat/Rcpp_eval.h#L105-L106

It's possible to throw an empty error (as per stop()) and so we need to 'forward' empty errors thrown as well.

[kevinushey]

The first (tiny) part of the issue is fixed; unfortunately the (much uglier) issue remains in that this fails to silence the error:

exec(function() try(silent = TRUE, exec(ouch)))

With some Rf_PrintValue debugging, here's what the expressions look like for this script:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
void ouch() { stop("ouch"); }

// [[Rcpp::export]]
void exec(Function f) { f(); }

/*** R
exec(function() try(silent = TRUE, exec(ouch)))
*/

I get:

> exec(function() try(silent = TRUE, exec(ouch)))
Evaluating call:
withCallingHandlers(tryCatch(evalq((function () 
try(silent = TRUE, exec(ouch)))(), <environment>), error = .rcpp_error_recorder), 
    warning = .rcpp_warning_recorder)
Evaluating call:
withCallingHandlers(tryCatch(evalq((function () 
invisible(.Primitive(".Call")(<pointer: 0x114ad6dc0>)))(), <environment>), 
    error = .rcpp_error_recorder), warning = .rcpp_warning_recorder)
Caught error: ouch
Forwarding error message: ouch
Caught error: ouch
Forwarding error message: ouch
Error: ouch

Note that the error message is captured twice, once for each Rcpp_eval call. Two error messages are then sent back to R. R successfully suppresses one of those messages; it sends along the other one.

[kevinushey]

I think the issue here is that the error message is not reset at the appropriate place. Currently, it's reset before evaluation takes place:

https://github.com/RcppCore/Rcpp/blob/master/inst/include/Rcpp/api/meat/Rcpp_eval.h#L41

Unfortunately, the following Rf_eval call will eventually recurse back into the same place, causing the error to once again be reset:

https://github.com/RcppCore/Rcpp/blob/master/inst/include/Rcpp/api/meat/Rcpp_eval.h#L71

What this means is that when the evaluation stack is unwound, all Rcpp_eval invocations are looking at the same error, and so fill their call information up with that information.

Just to confirm that, with some more debugging output:

> exec(function() try(silent = TRUE, exec(ouch)))
Resetting current error!
Evaluating call:
withCallingHandlers(tryCatch(evalq((function () 
try(silent = TRUE, exec(ouch)))(), <environment>), error = .rcpp_error_recorder), 
    warning = .rcpp_warning_recorder)
Resetting current error!
Evaluating call:
withCallingHandlers(tryCatch(evalq((function () 
invisible(.Primitive(".Call")(<pointer: 0x10d753d60>)))(), <environment>), 
    error = .rcpp_error_recorder), warning = .rcpp_warning_recorder)
Caught error: ouch
Current depth: 1
Forwarding error message: ouch
Caught error: ouch
Current depth: 0
Forwarding error message: ouch
Error: ouch

This means that the fix should simply be to ensure that we reset the current error after the evaluation.