[Rcpp-devel] `checkUserInterrupt()` might not be safe if Rcout is used.

[Wush Wu]

Hi Kevin,

I tried the latest master branch ("522e66d9ea04502b4a98bb28642fb4724ff0a479")
in R 3.5.1, but the `checkUserInterrupt` still does not throw the exception.

The R is under a docker container (library/r-base:3.5.1), so there should
be no GUI.

Hope these info helps.

Best,
Wush

On Fri, Jul 27, 2018 at 12:06 AM, Kevin Ushey <kevinushey at gmail.com> wrote:

> What environment are you executing this on? (R in terminal, or with a GUI;
> which OS?)
>
> One thing to be cautious of is that many R APIs in graphical environments
> will also call for processing of events, and this in turn can also imply a
> check for, and handling of, interrupts. Rprintf() is in fact one such API
> (and this is called behind the scenes by Rcout).
>
> This implies something quite unfortunate: attempts to use any R APIs which
> call for processing of events within a C++ context can cause a longjmp that
> bypass C++ destructors and leave you in a bad state.
>
> Fortunately, things will be better with the newer Rcpp evaluation system
> from R 3.5 and above, thanks for R_UnwindProtect() and work from Lionel to
> integrate that with the Rcpp evaluation model. See
> https://github.com/RcppCore/Rcpp/pull/789 for some of the details on the
> initial PR. We're still settling out some final details on the API but once
> that's ready we'll have some documentation + hopefully an Rcpp gallery
> example describing its use.
>
> (We might also consider in Rcpp wrapping our calls to Rprintf() in
> R_ToplevelExec() and 'catching' and 'rethrowing' interrupts seen, but this
> might have some unintended side-effects)
>
> Best,
> Kevin
>
> On Thu, Jul 26, 2018 at 8:18 AM Wush Wu <wush978 at gmail.com> wrote:
>
>> Hi all,
>>
>> I just learned the function `checkUserInterrupt` and played with it in my
>> package today. At first, everything was good. However, I sensed something
>> wrong when I interrupted my function and relaunched it. In my case, the
>> thread number of OpenMP decreased to 1 after an user interruption.
>>
>> According to the documentation, the `checkUserInterrupt` will throw an
>> exception to trigger the C++ destructors on the stack. However, if I use
>> `Rcout` several times (verbose mode of my function), then it will not throw
>> the exception but leave the function directly.
>>
>> Here is a toy example to demonstrate: https://gist.github.com/wush97
>> 8/36c4e5d8324dd14040eecb4b1dd1c631
>>
>> It uses `Rcout` / `std::cout` as a progressbar and check the user
>> interruption. If the exception is thrown correctly, then a catch clause
>> will handle the exception and write something to the `std::cerr`.
>>
>> If I turn off the verbose mode, then the `checkUserInterrupt` always
>> throws the exception.
>>
>> If I turn on the verbose mode and use `jsize` to select the number of dot
>> on the screen, then the `checkUserInterrupt` will exit the function
>> immediately if `jsize` is large.  For example, `checkUserInterrupt` throws
>> exception if `jsize = 5` but does not throw if `jsize = 100`.
>>
>> If I use `std::cout` instead of `Rcpp::Rcout`, then everything goes right
>> again. So, there might be something wrong even if `R_TopLevelExec` is used.
>>
>> Well, I guess this bug might be hard to debug, but we should let the
>> others aware about this at least, right?
>>
>> Best,
>> Wush
>> _______________________________________________
>> 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
>
>
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