// SPDX-License-Identifier: EUPL-1.2+
// SPDX-FileCopyrightText: 2022 Alyssa Ross <hi@alyssa.is>

use std::ffi::CString;
use std::fs::{read_link, File};
use std::io::{Error, Result};
use std::os::raw::c_char;
use std::os::unix::prelude::*;
use std::path::{Path, PathBuf};

extern "C" {
    fn openat_path_resolve_in_root(dirfd: BorrowedFd, pathname: *const c_char) -> Option<OwnedFd>;
}

// We can't make a const Path yet.
// https://github.com/rust-lang/rust/issues/102619
const PROC_FD_PATH: &str = "/proc/self/fd";

pub trait OwnedFdExt: AsFd + AsRawFd + Sized {
    /// Returns the path in proc(5) that corresponds to this file descriptor.
    fn fd_path(&self) -> PathBuf {
        let mut path = PathBuf::from(PROC_FD_PATH);
        path.push(self.as_raw_fd().to_string());
        path
    }

    /// Returns the path to this file, according to the magic link in proc(5).
    fn path(&self) -> Result<PathBuf> {
        read_link(self.fd_path())
    }
}

impl OwnedFdExt for OwnedFd {}

/// An `O_PATH` file descriptor.
#[derive(Debug)]
#[repr(transparent)]
pub struct Root(OwnedFd);

fn c_string(path: impl Into<PathBuf>) -> CString {
    let bytes = path.into().into_os_string().into_vec();
    CString::new(bytes).unwrap()
}

impl Root {
    /// Panics if `path` contains NUL bytes.
    pub fn open(path: impl AsRef<Path>) -> Result<Self> {
        Ok(Self(OwnedFd::from(File::open(path)?)))
    }

    /// Resolves `path` as if the directory represented by `self` was the root directory
    /// (including for symlink resolution).  The returned file descriptor will not have
    /// the `O_CLOEXEC` flag set.
    ///
    /// Panics if `path` contains NUL bytes.
    pub fn resolve_no_cloexec(&self, path: impl Into<PathBuf>) -> Result<OwnedFd> {
        let c_path = c_string(path);

        unsafe { openat_path_resolve_in_root(self.as_fd(), c_path.as_ptr()) }
            .ok_or_else(Error::last_os_error)
    }
}

impl AsFd for Root {
    fn as_fd(&self) -> BorrowedFd {
        self.0.as_fd()
    }
}