[R-package] Use R standard routines to access character data in C++

[jameslamb]

Another step towards #3016.

Similar to the changes for numeric data in #4247, this proposes replacing moost uses of the LightGBM-custom R_CHAR_PTR with R's standard interface for accessing character data. Unlike that PR, this requires two steps...extracting a CHARSXP from the object passed in from R, then getting a pointer to its data (char *). That results in a pattern like this:

CHAR(Rf_asChar(filename))

As a result of these changes, the utility function lgb.c_str() is no longer necessary.

Notes for Reviewers

1. [R-package] predict() breaks when using a Dataset stored in a file #4034 is not fixed by this change, but it is partially fixed. As of this PR, the reproducible examples in that issue yield [LightGBM] [Fatal] Unknown format of training data. instead of [LightGBM] [Fatal] Data file ��?��V doesn't exist.. That can be addressed in a followup PR.

2. R_CHAR_PTR cannot be completely removed in this PR, because it is still used in EncodeChar. That can be addressed in a future PR.

LightGBM/R-package/src/lightgbm_R.cpp

Lines 46 to 57 in bb88d92

	LGBM_SE EncodeChar(LGBM_SE dest, const char* src, SEXP buf_len, SEXP actual_len, size_t str_len) {
	if (str_len > INT32_MAX) {
	Log::Fatal("Don't support large string in R-package");
	}
	INTEGER(actual_len)[0] = static_cast<int>(str_len);
	if (Rf_asInteger(buf_len) < static_cast<int>(str_len)) {
	return dest;
	}
	auto ptr = R_CHAR_PTR(dest);
	std::memcpy(ptr, src, str_len);
	return dest;
	}

References

• https://cran.r-project.org/doc/manuals/r-release/R-exts.html#Handling-character-data
• https://github.com/dmlc/xgboost/blob/b35dd76dca6dfa9151394da847567ce99fa76085/R-package/src/xgboost_R.cc#L72

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/808927490

Status: success ✔️.

[jameslamb]

/gha run r-solaris

Workflow Solaris CRAN check has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/808928092

solaris-x86-patched: https://builder.r-hub.io/status/lightgbm_3.2.1.99.tar.gz-794f797bf8d943f1b2c7f39086497200
solaris-x86-patched-ods: https://builder.r-hub.io/status/lightgbm_3.2.1.99.tar.gz-347306e12acd4464b21e7121abcb01dd
Reports also have been sent to LightGBM public e-mail: http://www.yopmail.com/lightgbm_rhub_checks
Status: success ✔️.

[jameslamb]

Getting closer! Hopefully just a few more PRs and we can finally be done with #3016.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.