Fix is_directory check used in Bos.OS.Path.fold

[melwyn95]

esy uses Bos.OS.Path.fold as a helper to copy files

Recently we face an issue mjambon/dune-deps#23
where esy goes into infinite loops while copying files, the reason for this is,

The project contained a symlink which points to its parent directory (https://github.com/mjambon/dune-deps/blob/master/test/proj/foo/link-to-parent)

The root cause for this infinite loop was Sys.is_directory returned true for the symlink which caused fold to traverse the parent directory repeatedly

The fix for this is to use Unix.lstat to identify the kind of file

[dbuenzli]

The fix for this is to use Unix.lstat to identify the kind of file

That's doesn't look like a fix to me. Do you realize it completely breaks the semantics of the function as it exists ?

The standard behaviour of these functions is not to make any distinction between symlinks and what they point to (ideally a follow_symlink argument should be added like here). With that in mind:

1. It's likely better you do not try to create loops in your file system. You are off for other suprises.
2. I suspect you can get the behaviour you want by using the traverse callback appropriately.

[melwyn95]

Thanks @dbuenzli, I completely missed that this case can be handled via the traverse callback, sorry for the bother