[Rcpp-devel] `checkUserInterrupt()` might not be safe if Rcout is used.
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/wush978/36c4e5d8324dd14040eecb4b1dd1c631 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
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