[Rd] help with gotoExitingHandler(R_NilValue, call, entry); . Implementation of error handling internally

[Romain François]

Never mind. For my initial question, I can just use: R_curErrorBuf

My current implementation now only depend on R_FunTab and R_GlobalContext which R does not conceal behind attribute_hidden. 
https://github.com/romainfrancois/Rcpp11/blob/master/inst/include/Rcpp/Context.h

Romain

Le 27 févr. 2014 à 08:13, Romain François <romain at r-enthusiasts.com> a écrit :

> Of course one problem is that R hides things that I need for this, such as R_HandlerStack and R_ReturnedValue. 
> 
> I understand they have to be hidden, in which case can we have an exposed mechanism similar to what R_ToplevelExec does, but with the added functionality of giving access to the condition where R_ToplevelExec would return TRUE. 
> 
> Would someone be willing to review a patch to R with this added functionality. Please. 
> 
> In any case, I still would like some comments about my initial question about R_NilValue use in gotoExitingHandler(R_NilValue, call, entry);
> 
> Romain
> 
> Le 26 févr. 2014 à 09:41, Romain François <romain at r-enthusiasts.com> a écrit :
> 
>> Hello, 
>> 
>> I’m trying to leverage R_ToplevelExec to implement C level try/catch. 
>> 
>> The way it is implemented allows for running a function in a top level context. This context gets an empty handler stack, so either the function runs correctly or it jumps. When it jumps, it does not find a handler, so it jumps to the top level context. 
>> 
>> R does not allow me to call begin context and end context directly, so instead what I do is call R_ToplevelExec, grab the global context inside the function, install my own handler that I don’t set to be a calling one, pretend this context is a CTXT_FUNCTION. 
>> 
>> Eventually I get to jump fun, so that I can later on grab the condition from R_ReturnedValue. 
>> 
>> This works well in the « not simple error » case, i.e. if I call stop( simpleError(...) ), but with simple errors, i.e. calls to Rf_error internally or bare calls to stop( "vvzvz" ) I can’t access the error. 
>> 
>> And this boils down to this call: 
>> 
>> gotoExitingHandler(R_NilValue, call, entry);
>> 
>> inside vsignalError : 
>> 
>> static void vsignalError(SEXP call, const char *format, va_list ap)
>> {
>>   char localbuf[BUFSIZE];
>>   SEXP list, oldstack;
>> 
>>   oldstack = R_HandlerStack;
>>   Rvsnprintf(localbuf, BUFSIZE - 1, format, ap);
>>   while ((list = findSimpleErrorHandler()) != R_NilValue) {
>> 	char *buf = errbuf;
>> 	SEXP entry = CAR(list);
>> 	R_HandlerStack = CDR(list);
>> 	strncpy(buf, localbuf, BUFSIZE - 1);
>> 	/*	Rvsnprintf(buf, BUFSIZE - 1, format, ap);*/
>> 	buf[BUFSIZE - 1] = 0;
>> 	if (IS_CALLING_ENTRY(entry)) {
>> 	    if (ENTRY_HANDLER(entry) == R_RestartToken)
>> 		return; /* go to default error handling; do not reset stack */
>> 	    else {
>> 		SEXP hooksym, hcall, qcall;
>> 		/* protect oldstack here, not outside loop, so handler
>> 		   stack gets unwound in case error is protect stack
>> 		   overflow */
>> 		PROTECT(oldstack);
>> 		hooksym = install(".handleSimpleError");
>> 		PROTECT(qcall = LCONS(R_QuoteSymbol,
>> 				      LCONS(call, R_NilValue)));
>> 		PROTECT(hcall = LCONS(qcall, R_NilValue));
>> 		hcall = LCONS(mkString(buf), hcall);
>> 		hcall = LCONS(ENTRY_HANDLER(entry), hcall);
>> 		PROTECT(hcall = LCONS(hooksym, hcall));
>> 		eval(hcall, R_GlobalEnv);
>> 		UNPROTECT(4);
>> 	    }
>> 	}
>> 	else gotoExitingHandler(R_NilValue, call, entry);   // <<<  HERE
>>   }
>>   R_HandlerStack = oldstack;
>> }
>> 
>> Would it be possible to construct a simple condition instead of passing down R_NilValue so that I can grab this error and deal with it. 
>> 
>> The alternative is to set the handler to be a calling one, but I’d like to avoid that as much as possible as this means going back to the R side of things just to get access to the condition. 
>> 
>> My code is here: https://gist.github.com/romainfrancois/9225811
>> 
>> I have only tested this on OSX with R-devel. The code only uses R internal api (not Rcpp*). 
>> 
>> So down to what I’d like you to consider please if you are still reading here. Can we feed gotoExitingHandler with something more interesting than NULL. please. 
>> 
>> 
>> 
>> The end game is to add one layer of abstraction, e.g. pass to R_ToplevelExec a function that first deals with contexts, then calls an actual function. Combining this with lambda functions in C++ will make quite a nice and elegant way to handle error handling at the C++ level. 
>> 
>> I can provide the code that would create the simpleError, this is just making a simple VECSXP with names and classes, no big trouble here. 
>> 
>> Romain
>> 
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> 
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