[R-package] Expand user paths in file names

[david-cortes]

Many R functions take file paths as inputs, but will not expand paths like ~/myfile, which this PR fixes. It comes without any tests because generating files in the user folder in examples and tests would be against CRAN policies.

[jameslamb]

Thanks for doing this! Really nice fix to smooth out a rough edge of the R API.

Noting here the specific errors that users of {lightgbm} versions without this fix might face, so that this can be found from search engines when people encounter those errors, and so that anyone finding this PR from release notes will understand why this change was made.

In all of the cases below, as of latest master (d88b445), {lightgbm} raises an error when using paths with ~ and succeeds when replacing the ~ with an absolute path.

Dataset I/O

library(lightgbm)

data(agaricus.test, package = "lightgbm")
test_data <- agaricus.test$data[1L:100L, ]
test_label <- agaricus.test$label[1L:100L]

dtest1 <- lgb.Dataset(test_data, label = test_label)

ds_file <- "~/lightgbm-data.bin"
lgb.Dataset.save(dtest1, ds_file)

[LightGBM] [Fatal] Cannot write binary data to ~/lightgbm-data.bin
Error in dataset$save_binary(fname = fname) :
Cannot write binary data to ~/lightgbm-data.bin

Booster I/O

library(lightgbm)
data(agaricus.train, package = "lightgbm")
train <- agaricus.train

bst <- lightgbm(
    data = as.matrix(train$data)
    , label = train$label
    , params = list(
        num_leaves = 4L
        , learning_rate = 1.0
        , objective = "binary"
    )
    , nrounds = 2L
    , save_name = "~/lightgbm.model"
)

[LightGBM] [Fatal] Model file ~/lightgbm.model is not available for writes
Error in bst$save_model(filename = save_name) :
Model file ~/lightgbm.model is not available for writes

---

tests because generating files in the user folder in examples and tests would be against CRAN policies

Makes sense, thanks for noting that explicitly. I ran the code samples above after building {lightgbm} from your branch with the following code.

installing lightgbm from this feature branch

git 

Happy to report that as of the changes in this PR, that code succeeds and has the expected behavior.

Thanks again for the help!

[jameslamb]

@StrikerRUS I'm going to merge this one.

I think it's a good candidate for a potential 3.3.1 release (since it seems #4674 will probably require such a release) as it improves the usability of the R package in a way that doesn't introduce breaking changes, large new functionality, new dependencies, or substantial risk of submission issues with CRAN.

[StrikerRUS]

I think it's a good candidate for a potential 3.3.1 release

Absolutely support this idea!

[github-actions]

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