POSIX leaves it implementation-defined whether `link` follows symlinks.
In practice, for example, on Linux it does not and on FreeBSD it does.
So, switch to `linkat`, so that we can pick a behavior rather than
depending on OS defaults.
Pick the option to not follow symlinks. This is somewhat arbitrary, but
seems the less surprising choice because hard linking is a very
low-level feature which requires the source and destination to be on
the same mounted filesystem, and following a symbolic link could end
up in a different mounted filesystem.
`DirEntry` contains a `ReadDir` handle, which used to just be a wrapper
on `Arc<InnerReadDir>`. Commit af75314ecd added `end_of_stream: bool`
which is not needed by `DirEntry`, but adds 8 bytes after padding. We
can let `DirEntry` have an `Arc<InnerReadDir>` directly to avoid that.
Function to convert OpenOptions to c_int
Fixes: #74943
The creation_mode and access_mode function were already available in the OpenOptions struct, but currently private. I've added a new free functions to unix/fs.rs which takes the OpenOptions, and returns the c_int to be used as parameter for the `open` call.
This solves several problems
- race conditions where a file is truncated while copying from it. if we blindly trusted
the file size this would lead to an infinite loop
- proc files appearing empty to copy_file_range but not to read/write
https://github.com/coreutils/coreutils/commit/4b04a0c
- copy_file_range returning 0 for some filesystems (overlay? bind mounts?)
inside docker, again leading to an infinite loop
