[R-package] use R standard routine to access read-only ints passed to C++

[jameslamb]

This is another step towards #3016. It proposed using R's standard interface to handle integers passed into the C++ side (for things like array size, buffer size, number of iterations, etc.).

Changes in this PR:

• use R builtin SEXP type for any arguments passed from R to C++ that should be read-only integers
• use R builtin Rf_asInteger() to convert those SEXP objects to ints (replacing LightGBM's R_AS_INT)
• fix minor inconsistencies between lightgbm_R.h and lightgbm_R.cpp (e.g. parameter named nrow in the header file and num_row in the implementation)

Background

There is not a scalar integer type in R. When you run some code like x <- 1L, R creates a length-one array (referred to in R as a "vector").

To treat such data as an integer, instead of an integer array, in C/C++, it's necessary to extract the first element of that array into a new int. R provides a convenience function, Rf_asInteger(), for exactly that purpose.

See https://cran.r-project.org/doc/manuals/r-release/R-exts.html#Some-convenience-functions for more details.

Notes for Reviewers

This does not rely on #4242

[jameslamb]

/gha run r-solaris

Workflow Solaris CRAN check has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/803383736

solaris-x86-patched: https://builder.r-hub.io/status/lightgbm_3.2.1.99.tar.gz-d3e989e4aa394a59bb7795b6b2bb6718
solaris-x86-patched-ods: https://builder.r-hub.io/status/lightgbm_3.2.1.99.tar.gz-df78c8f2290a4a0d983aed8df559ea56
Reports also have been sent to LightGBM public e-mail: http://www.yopmail.com/lightgbm_rhub_checks
Status: success ✔️.

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/803383883

Status: success ✔️.

[StrikerRUS]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/805263054

Status: success ✔️.

[StrikerRUS]

I think we can cast buffer_len only once in two places below.

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/807172257

Status: success ✔️.

[jameslamb]

I think we can cast buffer_len only once in two places below.

nice! valgrind seems happy with that change (#4246 (comment)) and so does the rest of CI, so I'll merge this.

[github-actions]

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