unconditionally get/put RNG state

[kevinushey]

Resolves #823. With this PR, we ensure that Rcpp functions called by other Rcpp functions synchronize the RNG state.

Although @wch's example showed this through the use of Rcpp::Function, in theory this could occur if an Rcpp function were to e.g. call source() or eval() or another API that happened to call another Rcpp function.

[eddelbuettel]

If we do this, we should probably also remove the RNGScopeCounter variable.

[kevinushey]

I wanted to leave the counter there just because it's part of the API of Rcpp (in particular, they're registered routines) and I want to avoid touching anything around that interface just out of an abundance of caution.

[eddelbuettel]

That seems ... weird. The entire block is https://github.com/RcppCore/Rcpp/blob/master/src/api.cpp#L66-L83 and the counter is not part of the interface. We could simply make it const. I just don't see why you want to increment/decrement it. AFAIK no other code touches it.

And FWIW the silversearcher agrees:

edd@rob:~/git/rcpp(master)$ ag  RNGScopeCounter
src/api.cpp
68:            unsigned long RNGScopeCounter = 0;
73:            if (RNGScopeCounter == 0) GetRNGstate();
74:            RNGScopeCounter++;
75:            return RNGScopeCounter;
80:            RNGScopeCounter--;
81:            if (RNGScopeCounter == 0) PutRNGstate();
82:            return RNGScopeCounter;
edd@rob:~/git/rcpp(master)$ 

[kevinushey]

Sorry; I meant that enterRNGScope() and exitRNGScope() are registered routines (just because we wanted to make sure that all packages using Rcpp were looking at the same counter, I presume).

I'll get rid of the counter.

[eddelbuettel]

And I presume you and @wch tested this and ensured that it addressed the corner case seen with shiny?