Make retrieve_tty use std::fs::read_link

As it turns out the Rust standard library already includes functionality
for reading the destination of a symbolic with the std::fs::read_link
function.
With this change we replace our low-level implementation with this
function, simplifying the logic significantly and reducing two instances
of unsafe code.

src/tty/linux.rs

@@ -3,9 +3,8 @@
 // Copyright (C) 2022 The Nitrocli Developers
 // SPDX-License-Identifier: GPL-3.0-or-later
 
-use std::ffi;
+use std::fs;
 use std::io;
-use std::os::unix::ffi::OsStringExt as _;
 use std::os::unix::io::AsRawFd as _;
 use std::path;
 
@@ -15,36 +14,8 @@ use std::path;
 /// error reporting, knowing that callers do not care.
 pub(crate) fn retrieve_tty() -> Result<path::PathBuf, ()> {
  let fd = io::stdin().as_raw_fd();
- let fd_path = format!("/proc/self/fd/{}\0", fd);
- let fd_path = ffi::CStr::from_bytes_with_nul(fd_path.as_bytes()).unwrap();
-
- let mut buffer = Vec::<u8>::with_capacity(56);
- // SAFETY: We provide valid pointers, `fd_path` is NUL terminated, and
- // the provided capacity reflects the actual length of the
- // buffer.
- let rc = unsafe {
- libc::readlink(
- fd_path.as_ptr(),
- buffer.as_mut_ptr() as *mut libc::c_char,
- buffer.capacity(),
- )
- };
- if rc <= 0 {
- return Err(());
- }
-
- let rc = rc as usize;
- // If `readlink` filled the entire buffer we could have experienced
- // silent truncation. So we just bail out out of precaution.
- if rc == buffer.capacity() {
- return Err(());
- }
-
- // SAFETY: At this point we know that `readlink` has written `rc`
- // bytes to `buffer`.
- unsafe { buffer.set_len(rc) };
-
- Ok(ffi::OsString::from_vec(buffer).into())
+ let fd_path = format!("/proc/self/fd/{}", fd);
+ fs::read_link(fd_path).map_err(|_| ())
 }
 
 #[cfg(test)]