Use safe code for calling readlinkat

[sunfishcode]

On Linux, readlinkat returns EINVAL if given a zero-length buffer. WASI's wasi_path_readlink should follow the POSIX semantics for readlinkat instead. wasi-common implements this, however it uses an unsafe call to the libc readlinkat rather than calling through a safe nix wrapper.

See the discussion here for more details.

[sendilkumarn]

This looks like a bug in nix.

IIUC, the following line is the problem

https://github.com/nix-rust/nix/blob/414cc86c0af09fd44454b93b6dc738316b16c43c/src/fcntl.rs#L184

fn wrap_readlink_result(buffer: &mut[u8], res: ssize_t) -> Result<&OsStr> {
    match Errno::result(res) {
        Err(err) => Err(err),
        Ok(len) => {
            if (len as usize) >= buffer.len() { // *** This check ***
                Err(Error::Sys(Errno::ENAMETOOLONG))
            } else {
                Ok(OsStr::from_bytes(&buffer[..(len as usize)]))
            }
        }
    }
}

In nix testing, we are manually adding an 1 to increase the buffer size at here. I think this makes it successful.

int main(int argc, const char * argv[]) {
    
    std::string filename = "/tmp/test-dir/target";
    
    size_t bufferSize = 9;
    char* buffer = new char[bufferSize];
    size_t rc = readlink (filename.c_str(), buffer, bufferSize); 
   // Since `readlink's` output depends on the `bufferSize`. If the`bufferSize` is 9 or greater than 
   // 9 then it will return 9 or else it will return `bufferSize` as the return value. 

    return 0;
}

[sendilkumarn]

Created nix-rust/nix#1109 (hopefully that will fix the error).

[kubkon]

Nice one, thanks!

[kubkon]

Similarly to the story in #521, given that we've decided to replace nix dependency with our own thin, low-level in-house wrapper crate yanix (see #649), I'm closing this issue as it's no longer relevant.