[R-package] [ci] Cover 100% of R-to-C++ calls with unit tests

[jameslamb]

LightGBM's R package calls its C++ library using R's .Call() interface. That C++ is one codebase that is reused as a standalone C++ library, with Java, with Spark (via the Java interface), with Python, and from a command-line interface.

Because that library is shared, it's regularly updated by developers who have no knowledge of the lightgbm R package. The R package's unit tests are the best defense against such updates having unintended consequences and breaking the R package.

The following calls to the C++ side from within the R package are not covered by this project's unit tests (this will be updated as tests are added):

• LGBM_BoosterFree_R ([R-package] Added unit tests on creating Booster from file or string #2945)
• LGBM_BoosterCreateFromModelfile_R ([R-package] Added unit tests on creating Booster from file or string #2945)
• LGBM_BoosterGetCurrentIteration_R ([R-package] Added unit tests on creating Booster from file or string #2945)
• LGBM_BoosterMerge_R ([R-package] Added tests on creating a Booster from a Dataset #3007)
• LGBM_BoosterLoadModelFromString_R ([R-package] Added unit tests on creating Booster from file or string #2945)
• LGBM_BoosterResetTrainingData_R ([R-package] added tests on LGBM_BoosterResetTrainingData_R #3020)
• LGBM_BoosterRollbackOneIter_R ([R-package] added tests on LGBM_BoosterRollbackOneIter_R #3006)
• LGBM_BoosterSaveModelToString_R ([R-package] Added unit tests on creating Booster from file or string #2945)
• LGBM_GetLastError_R ([R-package] Added tests on LGBM_GetLastError_R #3005)

I figured that out by following the coverage instructions in the R-package README.

Covering those calls is important because it would improve our confidence in releases and allow non-R developers to work on the C++ library without worrying that their changes are negatively impacting the R package.

This issue will likely not be closed by a single PR. Any PR that addresses some of those functions above would be a welcome addition.

[jameslamb]

Closing this issue in favor of being in #2302 . If you'd like to address this issue, leave a comment here and it will be re-opened.

[jameslamb]

I'm going to open this since we have an open PR for it and I'm working on the other PRs in the next few days

[jameslamb]

All functions have been covered now that we've merged #3020, so this can be closed

[github-actions]

This issue 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.