/// # Errors
/// Returns an error when the fail-closed pipe cannot be created.
pub fn launch_barrier() -> io::Result<(BarrierRelease, BarrierWait)> {
    let (reader, writer) = pipe_with(PipeFlags::empty())?;
    fcntl_setfd(writer.as_fd(), FdFlags::CLOEXEC)?;
    Ok((
        BarrierRelease(File::from(writer)),
        BarrierWait(File::from(reader)),
    ))
}

#[derive(Debug)]
pub struct BarrierRelease(File);

impl BarrierRelease {
    /// # Errors
    /// Returns an error unless the complete release token is written.
    pub fn release(mut self) -> io::Result<()> {
        self.0.write_all(BARRIER_RELEASE)?;
        self.0.flush()
    }
}

#[derive(Debug)]
pub struct BarrierWait(File);

impl BarrierWait {
    #[must_use]
    pub fn into_stdio(self) -> Stdio {
        Stdio::from(self.0)
    }

    /// # Errors
    /// Returns an error when the inherited standard-input descriptor cannot be duplicated.
    pub fn from_stdin() -> io::Result<Self> {
        Ok(Self(File::open("/proc/self/fd/0")?))
    }

    /// # Errors
    /// Refuses EOF, partial, altered, or overlong releases.
    pub fn wait(mut self) -> io::Result<()> {
        let mut release = [0_u8; BARRIER_RELEASE.len()];
        self.0.read_exact(&mut release)?;
        let mut excess = [0_u8; 1];
        if release != *BARRIER_RELEASE || self.0.read(&mut excess)? != 0 {
            return Err(io::Error::other("malformed launch-barrier release"));
        }
        Ok(())
    }
}

#[allow(clippy::disallowed_types)]
pub fn append_parent_identity(
    command: &mut std::process::Command,
    identity: PidfsProcessIdentityV1,
) {
    let authority = identity.authority();
    command
        .arg("--parent-boot")
        .arg(hex(authority.boot_id.as_bytes()))
        .arg("--parent-device")
        .arg(authority.pidfs_st_dev.get().to_string())
        .arg("--parent-inode")
        .arg(authority.pidfs_st_ino.get().to_string())
        .arg("--parent-pid")
        .arg(identity.host_pid().get().to_string())
        .arg("--parent-start")
        .arg(identity.start_time_ticks().get().to_string());
}

/// # Errors
/// Refuses missing, reordered, malformed, or incomplete wrapper arguments.
pub fn parse_wrapper_arguments(
    arguments: impl IntoIterator<Item = OsString>,
) -> io::Result<(PidfsProcessIdentityV1, TargetCommand)> {
    let mut arguments = arguments.into_iter();
    let boot = named_argument(&mut arguments, "--parent-boot")?;
    let device = number_argument(&mut arguments, "--parent-device")?;
    let inode = number_argument(&mut arguments, "--parent-inode")?;
    let pid = number_argument(&mut arguments, "--parent-pid")?;
    let start = number_argument(&mut arguments, "--parent-start")?;
    let parent_edge = match named_argument(&mut arguments, "--parent-action")?.to_str() {
        Some("kill") => ParentEdge::ParentDeathKill,
        Some("lifetime-pipe") => ParentEdge::LifetimePipe,
        _ => return Err(io::Error::other("wrapper parent action is invalid")),
    };
    let next = arguments.next();
    let stdin = if next.as_deref() == Some(OsStr::new("--target-stdin-fd")) {
        let descriptor = arguments
            .next()
            .and_then(|value| value.to_str().and_then(|value| value.parse::<i32>().ok()))
            .filter(|descriptor| *descriptor > 2)
            .ok_or_else(|| io::Error::other("wrapper target stdin descriptor is invalid"))?;
        let access = TargetStdinAccess::from_descriptor(descriptor)?;
        if arguments.next().as_deref() != Some(OsStr::new("--")) {
            return Err(io::Error::other("wrapper target separator is missing"));
        }
        Some(access.reopen(descriptor)?)
    } else if next.as_deref() == Some(OsStr::new("--")) {
        None
    } else {
        return Err(io::Error::other("wrapper target separator is missing"));
    };
    let program = arguments
        .next()
        .ok_or_else(|| io::Error::other("wrapper target is missing"))?;
    let target = TargetCommand {
        program,
        arguments: arguments.collect(),
        parent_edge,
        executable: None,
        stdin,
    };
    let boot = parse_boot_hex(&boot)?;
    Ok((
        PidfsProcessIdentityV1::new(
            PidfsProcessAuthorityV1 {
                boot_id: boot,
                pidfs_st_dev: PidfsDeviceId::new(device),
                pidfs_st_ino: PidfsInodeId::new(inode),
            },
            HostPidLocator::new(
                u32::try_from(pid).map_err(|_| io::Error::other("parent PID is out of range"))?,
            ),
            ProcessStartTimeTicks::new(start),
        ),
        target,
    ))
}

fn named_argument(
    arguments: &mut impl Iterator<Item = OsString>,
    name: &str,
) -> io::Result<OsString> {
    if arguments.next().as_deref() != Some(OsStr::new(name)) {
        return Err(io::Error::other(format!(
            "wrapper argument {name} is missing"
        )));
    }
    arguments
        .next()
        .ok_or_else(|| io::Error::other(format!("wrapper argument {name} has no value")))
}

fn number_argument(arguments: &mut impl Iterator<Item = OsString>, name: &str) -> io::Result<u64> {
    named_argument(arguments, name)?
        .to_str()
        .and_then(|value| value.parse().ok())
        .ok_or_else(|| io::Error::other(format!("wrapper argument {name} is invalid")))
}

fn parse_boot_hex(value: &OsStr) -> io::Result<BootId> {
    let value = value
        .to_str()
        .ok_or_else(|| io::Error::other("parent boot ID is not UTF-8"))?;
    if value.len() != 32 || !value.bytes().all(|byte| byte.is_ascii_hexdigit()) {
        return Err(io::Error::other("parent boot ID is invalid"));
    }
    let mut bytes = [0_u8; 16];
    for (index, byte) in bytes.iter_mut().enumerate() {
        *byte = u8::from_str_radix(&value[index * 2..index * 2 + 2], 16)
            .map_err(|_| io::Error::other("parent boot ID is invalid"))?;
    }
    Ok(BootId::new(bytes))
}

fn hex(bytes: &[u8]) -> String {
    use std::fmt::Write as _;

    bytes.iter().fold(String::new(), |mut output, byte| {
        write!(output, "{byte:02x}").expect("writing to a String cannot fail");
        output
    })
}

#[allow(clippy::disallowed_methods, clippy::disallowed_types)]
#[must_use]
pub fn exec_target(target: TargetCommand) -> io::Error {
    use std::os::unix::process::CommandExt;

    std::process::Command::new(target.program)
        .args(target.arguments)
        .exec()
}

/// Stops this process until the host resumes it.
///
/// # Errors
/// Returns an error when the kernel refuses the stop signal.
#[allow(clippy::disallowed_methods)]
pub fn suspend_current_process() -> io::Result<()> {
    Ok(rustix::process::kill_process(
        rustix::process::getpid(),
        Signal::STOP,
    )?)
}