[Rd] calling R API functions after engine shutdown

[Lukas Stadler]

If this is not considered to be safe, then I guess my message can be seen as a heads-up:
If R_ReleaseObject ever does something that is not safe after the R engine has shut down, you will start to see crashes from packages like “minqa”.
In FastR, we ended up adding a global flag that disables parts of the native API because of this.

- Lukas

> On 21 Sep 2017, at 17:53, Tomas Kalibera <tomas.kalibera at gmail.com> wrote:
> 
> 
> Calling R_ReleaseObject in a C++ destructor is not reliable - it can be bypassed by a non-local return, such as an error. Generally in R one cannot use C++ destructors reliably for anything that the R runtime wouldn't do on its own in case of a non-local return.
> 
> A destructor that calls just UNPROTECT, in a way that balances out the protection stack (e.g. Rcpp Shield), is safe because R runtime balances the protection stack on non-local return. Destructors used in code that will never call into any R API (such as in a third party library) are safe, because the R runtime could not do non-local return. All other destructors are a problem.
> 
> UNPROTECT will work even during R shutdown.
> 
> In some cases cleanup code that would be put in C++ destructors, if they were safe with R, can instead be put into R finalizers.
> 
> Tomas
> 
> 
> 
> On 09/21/2017 04:41 PM, Lukas Stadler wrote:
>> Hi!
>> 
>> We’ve recently come across an example where a package (minqa) creates an Rcpp Function object in a static variable.
>> This causes R_ReleaseObject to be called by the destructor at a very late point in time - as part of the system exit function:
>> 
>> static Function cf("c");
>> 
>> I’m wondering if that is considered to be “safe”?
>> Is the R engine supposed to stay in a state where calls to API functions are valid, even after it has shut down?
>> It probably only ever happens with the ReleaseObject function, but even that could cause problems, e.g., with more elaborate refcounting schemes.
>> 
>> - Lukas
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