[R] Expand file paths when passing to readers

[asfimport]

All file paths in R are wrapped in fs::path_abs(), which handles relative paths, but it doesn't expand ~, so this fails:

> df <- read_parquet("~/Downloads/demofile.parquet")
 Error in io___MemoryMappedFile__Open(fs::path_abs(path), mode) :
 IOError: Failed to open local file '~/Downloads/demofile.parquet', error: No such file or directory

This is fixed by using fs::path_real() instead.

Should this be properly handled in C++ though? cc @pitrou

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

PRs and other links:

• GitHub Pull Request #5169

_{Note: This issue was originally created as ARROW-6323. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
It sounds fine to handle it in R. If we want we can add some handling for that in C++ too, but that doesn't sound urgent.

[asfimport]

Neal Richardson / @nealrichardson:
Not urgent, just seems like the kind of thing the C++ filesystem API should handle so that every language doesn't have to reimplement it. Plus, if C++ did this, I could drop an R package dependency. 

I just made ARROW-6324 for that.

[asfimport]

Antoine Pitrou / @pitrou:
Well, the C++ filesystem API isn't used currently, so if you want a fix now, you need to do it on the R side ;-)

Though of course we could fix the issue in the C++ file APIs.

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 5169
#5169