use std::ffi::{OsStr, OsString};
use std::fs::File;
use std::io::{self, Read, Write};
use std::os::fd::{AsFd, AsRawFd, BorrowedFd, OwnedFd};
use std::process::Stdio;
use std::sync::{Mutex, OnceLock};

use lazaret_core::identity::{
    BootId, CoordinatorId, DomainId, GenerationId, HostPidLocator, PidfsDeviceId, PidfsInodeId,
    PidfsProcessAuthorityV1, PidfsProcessIdentityV1, ProcessStartTimeTicks,
};
use lazaret_core::records::{
    HostProcessOwnershipSlot, ProcessOwnershipCommitAck, StagedProcessOwnership,
};
use rustix::event::{PollFd, PollFlags, Timespec, poll};
use rustix::fs::{OFlags, fcntl_getfl, fcntl_setfl, fstat, fstatfs};
use rustix::io::{FdFlags, fcntl_setfd};
use rustix::pipe::{PipeFlags, pipe_with};
use rustix::process::{Pid, PidfdFlags, Signal, WaitId, WaitIdOptions};

pub const PID_FS_MAGIC: i64 = 0x5049_4446;
const BARRIER_RELEASE: &[u8; 32] = b"LAZARET-CAUSAL-BARRIER-RELEASE!!";
const PRIVATE_ROLE_ARGUMENT: &str = "--lazaret-private-process-role";
const WRAPPER_ROLE: &str = "launch-wrapper";
const SIGCHLD_PROBE_ROLE: &str = "sigchld-probe";
static CHILD_BROKER: OnceLock<ChildBroker> = OnceLock::new();

#[derive(Debug)]
pub struct TargetCommand {
    program: OsString,
    arguments: Vec<OsString>,
    parent_edge: ParentEdge,
    executable: Option<OwnedFd>,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum ParentEdge {
    ParentDeathKill,
    LifetimePipe,
}

impl TargetCommand {
    #[must_use]
    pub fn new(program: impl Into<OsString>) -> Self {
        Self {
            program: program.into(),
            arguments: Vec::new(),
            parent_edge: ParentEdge::ParentDeathKill,
            executable: None,
        }
    }

    /// Constructs an argv-only target bound to the exact inherited executable descriptor.
    ///
    /// # Errors
    /// Returns an error when the descriptor cannot be made inheritable by the launch wrapper.
    pub fn from_executable(
        executable: OwnedFd,
        arguments: impl IntoIterator<Item = OsString>,
    ) -> io::Result<Self> {
        fcntl_setfd(executable.as_fd(), FdFlags::empty())?;
        Ok(Self {
            program: OsString::from(format!("/proc/self/fd/{}", executable.as_raw_fd())),
            arguments: arguments.into_iter().collect(),
            parent_edge: ParentEdge::ParentDeathKill,
            executable: Some(executable),
        })
    }

    #[must_use]
    pub fn argument(mut self, argument: impl Into<OsString>) -> Self {
        self.arguments.push(argument.into());
        self
    }

    #[must_use]
    pub fn with_lifetime_pipe_parent_edge(mut self) -> Self {
        self.parent_edge = ParentEdge::LifetimePipe;
        self
    }

    #[must_use]
    pub fn with_parent_death_kill_edge(mut self) -> Self {
        self.parent_edge = ParentEdge::ParentDeathKill;
        self
    }

    #[must_use]
    pub const fn parent_edge(&self) -> ParentEdge {
        self.parent_edge
    }

    #[allow(clippy::disallowed_types)]
    fn append_to(&self, command: &mut std::process::Command) {
        let _ = &self.executable;
        command
            .arg("--parent-action")
            .arg(match self.parent_edge {
                ParentEdge::ParentDeathKill => "kill",
                ParentEdge::LifetimePipe => "lifetime-pipe",
            })
            .arg("--")
            .arg(&self.program)
            .args(&self.arguments);
    }
}

pub struct ChildStandardIo {
    stdout: Option<Stdio>,
    stderr: Option<Stdio>,
    policy: ChildStandardIoPolicy,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum ChildStandardIoPolicy {
    Inherit,
    PrivateControlWithLifetimePipe,
    Silenced,
}

impl ChildStandardIo {
    #[must_use]
    pub const fn inherit() -> Self {
        Self {
            stdout: None,
            stderr: None,
            policy: ChildStandardIoPolicy::Inherit,
        }
    }

    #[must_use]
    pub fn private_control_with_lifetime_pipe(
        control: impl Into<Stdio>,
        lifetime_pipe: impl Into<Stdio>,
    ) -> Self {
        Self {
            stdout: Some(control.into()),
            stderr: Some(lifetime_pipe.into()),
            policy: ChildStandardIoPolicy::PrivateControlWithLifetimePipe,
        }
    }

    #[must_use]
    pub fn silenced() -> Self {
        Self {
            stdout: Some(Stdio::null()),
            stderr: Some(Stdio::null()),
            policy: ChildStandardIoPolicy::Silenced,
        }
    }

    #[must_use]
    pub const fn policy(&self) -> ChildStandardIoPolicy {
        self.policy
    }
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct ProcessOwnershipContext {
    pub domain_id: DomainId,
    pub generation_id: GenerationId,
    pub coordinator_id: CoordinatorId,
    pub slot: HostProcessOwnershipSlot,
}

#[derive(Debug)]
pub struct RetainedProcess {
    pidfd: OwnedFd,
    identity: PidfsProcessIdentityV1,
}

#[derive(Debug)]
pub struct CapturedProcess {
    process: RetainedProcess,
    stdout: File,
    stderr: File,
}

impl CapturedProcess {
    #[must_use]
    pub const fn process(&self) -> &RetainedProcess {
        &self.process
    }

    pub fn streams(&mut self) -> (&mut File, &mut File) {
        (&mut self.stdout, &mut self.stderr)
    }

    #[must_use]
    pub fn into_process(self) -> RetainedProcess {
        self.process
    }
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct ProcessExit {
    pub exit_code: Option<i32>,
    pub terminating_signal: Option<i32>,
}

impl RetainedProcess {
    #[must_use]
    pub const fn identity(&self) -> PidfsProcessIdentityV1 {
        self.identity
    }

    /// # Errors
    /// Returns an error when the exact retained-pidfd action fails.
    #[allow(clippy::disallowed_methods)]
    pub fn send_signal(&self, signal: Signal) -> io::Result<()> {
        rustix::process::pidfd_send_signal(self.pidfd.as_fd(), signal).map_err(Into::into)
    }

    /// # Errors
    /// Returns an error when polling the retained pidfd fails.
    pub fn has_exited(&self) -> io::Result<bool> {
        let mut descriptors = [PollFd::new(&self.pidfd, PollFlags::IN)];
        poll(&mut descriptors, Some(&Timespec::default()))?;
        Ok(descriptors[0]
            .revents()
            .intersects(PollFlags::IN | PollFlags::HUP))
    }
}

impl AsFd for RetainedProcess {
    fn as_fd(&self) -> BorrowedFd<'_> {
        self.pidfd.as_fd()
    }
}

#[derive(Debug)]
pub struct ChildBroker {
    exclusion: Mutex<()>,
}

#[must_use]
pub fn child_broker() -> &'static ChildBroker {
    CHILD_BROKER.get_or_init(|| ChildBroker {
        exclusion: Mutex::new(()),
    })
}

fn require_parent_edge_standard_io(
    parent_edge: ParentEdge,
    standard_io: ChildStandardIoPolicy,
) -> io::Result<()> {
    match (parent_edge, standard_io) {
        (ParentEdge::LifetimePipe, ChildStandardIoPolicy::PrivateControlWithLifetimePipe)
        | (
            ParentEdge::ParentDeathKill,
            ChildStandardIoPolicy::Inherit | ChildStandardIoPolicy::Silenced,
        ) => Ok(()),
        (ParentEdge::LifetimePipe, _) => Err(io::Error::other(
            "lifetime-pipe parent edge requires private control stdout and lifetime-pipe stderr",
        )),
        (ParentEdge::ParentDeathKill, ChildStandardIoPolicy::PrivateControlWithLifetimePipe) => {
            Err(io::Error::other(
                "parent-death edge cannot substitute a lifetime-pipe stdio policy",
            ))
        }
    }
}

fn require_captured_parent_edge(parent_edge: ParentEdge) -> io::Result<()> {
    if parent_edge == ParentEdge::ParentDeathKill {
        Ok(())
    } else {
        Err(io::Error::other(
            "captured output requires the parent-death edge",
        ))
    }
}

impl ChildBroker {
    /// Runs a non-durable direct child under the same spawn/reap exclusion as owned children.
    ///
    /// # Errors
    /// Returns an error when causal launch, exact waiting, or the command exit status fails.
    #[allow(clippy::disallowed_methods, clippy::disallowed_types)]
    pub fn run_transient(&self, target: &TargetCommand) -> io::Result<ProcessExit> {
        let _guard = self
            .exclusion
            .lock()
            .map_err(|_| io::Error::other("child broker lock poisoned"))?;
        require_parent_edge_standard_io(target.parent_edge(), ChildStandardIoPolicy::Silenced)?;
        require_default_sigchld()?;
        require_reapable_sigchld()?;
        let parent = current_process_identity()?;
        let (release, wait) = launch_barrier()?;
        let mut command = private_role_command(WRAPPER_ROLE);
        append_parent_identity(&mut command, parent);
        target.append_to(&mut command);
        command
            .stdin(wait.into_stdio())
            .stdout(Stdio::null())
            .stderr(Stdio::null());
        let mut child = command.spawn()?;
        let process = match acquire_spawned_child(child.id()) {
            Ok(process) => process,
            Err(error) => return fail_before_pidfd(release, &mut child, error),
        };
        if let Err(error) = release.release() {
            return fail_after_pidfd_closed(process, error);
        }
        drop(child);
        let status =
            rustix::process::waitid(WaitId::PidFd(process.pidfd.as_fd()), WaitIdOptions::EXITED)?
                .ok_or_else(|| io::Error::other("transient child exited without wait status"))?;
        let exit = ProcessExit {
            exit_code: status.exit_status(),
            terminating_signal: status.terminating_signal(),
        };
        drop(process);
        if exit.exit_code == Some(0) && exit.terminating_signal.is_none() {
            Ok(exit)
        } else {
            Err(io::Error::other(format!(
                "transient child failed with exit {:?} and signal {:?}",
                exit.exit_code, exit.terminating_signal
            )))
        }
    }

    /// Spawns the repository wrapper, stages exact ownership, accepts its acknowledgment,
    /// and only then releases target execution.
    ///
    /// # Errors
    /// Refuses and reaps the blocked wrapper on every failed acquisition or callback.
    #[allow(clippy::disallowed_methods, clippy::disallowed_types)]
    pub fn acquire<Stage, Acknowledge>(
        &self,
        expected_parent: PidfsProcessIdentityV1,
        ownership: ProcessOwnershipContext,
        target: &TargetCommand,
        stage: Stage,
        acknowledge: Acknowledge,
    ) -> io::Result<RetainedProcess>
    where
        Stage: FnOnce(StagedProcessOwnership) -> io::Result<()>,
        Acknowledge: FnOnce(StagedProcessOwnership) -> io::Result<ProcessOwnershipCommitAck>,
    {
        self.acquire_with_stdio(
            expected_parent,
            ownership,
            target,
            ChildStandardIo::inherit(),
            stage,
            acknowledge,
        )
    }

    /// # Errors
    /// Refuses and reaps the blocked wrapper on every failed acquisition or callback.
    #[allow(clippy::disallowed_methods, clippy::disallowed_types)]
    pub fn acquire_with_stdio<Stage, Acknowledge>(
        &self,
        expected_parent: PidfsProcessIdentityV1,
        ownership: ProcessOwnershipContext,
        target: &TargetCommand,
        standard_io: ChildStandardIo,
        stage: Stage,
        acknowledge: Acknowledge,
    ) -> io::Result<RetainedProcess>
    where
        Stage: FnOnce(StagedProcessOwnership) -> io::Result<()>,
        Acknowledge: FnOnce(StagedProcessOwnership) -> io::Result<ProcessOwnershipCommitAck>,
    {
        self.acquire_with_stdio_after_ack(
            expected_parent,
            ownership,
            target,
            standard_io,
            stage,
            acknowledge,
            || Ok(()),
        )
    }

    /// Acquires an exact child while retaining its blocked wrapper across one acknowledged
    /// parent transition.
    ///
    /// # Errors
    /// Refuses and reaps the blocked wrapper when staging, acknowledgment, transition, or release
    /// fails.
    #[allow(clippy::disallowed_methods, clippy::disallowed_types)]
    pub fn acquire_with_stdio_after_ack<Stage, Acknowledge, Transition>(
        &self,
        expected_parent: PidfsProcessIdentityV1,
        ownership: ProcessOwnershipContext,
        target: &TargetCommand,
        mut standard_io: ChildStandardIo,
        stage: Stage,
        acknowledge: Acknowledge,
        transition: Transition,
    ) -> io::Result<RetainedProcess>
    where
        Stage: FnOnce(StagedProcessOwnership) -> io::Result<()>,
        Acknowledge: FnOnce(StagedProcessOwnership) -> io::Result<ProcessOwnershipCommitAck>,
        Transition: FnOnce() -> io::Result<()>,
    {
        let _guard = self
            .exclusion
            .lock()
            .map_err(|_| io::Error::other("child broker lock poisoned"))?;
        require_parent_edge_standard_io(target.parent_edge(), standard_io.policy())?;
        require_default_sigchld()?;
        require_reapable_sigchld()?;
        let (release, wait) = launch_barrier()?;
        let mut command = private_role_command(WRAPPER_ROLE);
        append_parent_identity(&mut command, expected_parent);
        target.append_to(&mut command);
        command.stdin(wait.into_stdio());
        if let Some(stdout) = standard_io.stdout.take() {
            command.stdout(stdout);
        }
        if let Some(stderr) = standard_io.stderr.take() {
            command.stderr(stderr);
        }
        let mut child = command.spawn()?;
        let host_pid = child.id();
        let acquired = acquire_spawned_child(host_pid);
        let process = match acquired {
            Ok(process) => process,
            Err(error) => return fail_before_pidfd(release, &mut child, error),
        };
        let staged = StagedProcessOwnership::v1(
            ownership.domain_id,
            ownership.generation_id,
            ownership.coordinator_id,
            ownership.slot,
            process.identity,
        );
        if let Err(error) = stage(staged) {
            return fail_after_pidfd(release, process, error);
        }
        let acknowledgment = match acknowledge(staged) {
            Ok(acknowledgment) => acknowledgment,
            Err(error) => return fail_after_pidfd(release, process, error),
        };
        if !acknowledgment.acknowledges(&staged) {
            return fail_after_pidfd(
                release,
                process,
                io::Error::other(
                    "process ownership acknowledgment does not match staged ownership",
                ),
            );
        }
        if let Err(error) = transition() {
            return fail_after_pidfd(release, process, error);
        }
        if let Err(error) = release.release() {
            return fail_after_pidfd_closed(process, error);
        }
        drop(child);
        Ok(process)
    }

    /// Acquires an exact child with bounded-output pipe ownership retained by the caller.
    ///
    /// # Errors
    /// Refuses and reaps the blocked wrapper on every failed acquisition or callback.
    #[allow(clippy::disallowed_methods, clippy::disallowed_types)]
    pub fn acquire_captured<Stage, Acknowledge>(
        &self,
        expected_parent: PidfsProcessIdentityV1,
        ownership: ProcessOwnershipContext,
        target: &TargetCommand,
        stage: Stage,
        acknowledge: Acknowledge,
    ) -> io::Result<CapturedProcess>
    where
        Stage: FnOnce(StagedProcessOwnership) -> io::Result<()>,
        Acknowledge: FnOnce(StagedProcessOwnership) -> io::Result<ProcessOwnershipCommitAck>,
    {
        let _guard = self
            .exclusion
            .lock()
            .map_err(|_| io::Error::other("child broker lock poisoned"))?;
        require_captured_parent_edge(target.parent_edge())?;
        require_default_sigchld()?;
        require_reapable_sigchld()?;
        let (release, wait) = launch_barrier()?;
        let mut command = private_role_command(WRAPPER_ROLE);
        append_parent_identity(&mut command, expected_parent);
        target.append_to(&mut command);
        command
            .stdin(wait.into_stdio())
            .stdout(Stdio::piped())
            .stderr(Stdio::piped());
        let mut child = command.spawn()?;
        let host_pid = child.id();
        let process = match acquire_spawned_child(host_pid) {
            Ok(process) => process,
            Err(error) => return fail_before_pidfd(release, &mut child, error),
        };
        let staged = StagedProcessOwnership::v1(
            ownership.domain_id,
            ownership.generation_id,
            ownership.coordinator_id,
            ownership.slot,
            process.identity,
        );
        if let Err(error) = stage(staged) {
            return fail_after_pidfd(release, process, error);
        }
        let acknowledgment = match acknowledge(staged) {
            Ok(acknowledgment) => acknowledgment,
            Err(error) => return fail_after_pidfd(release, process, error),
        };
        if !acknowledgment.acknowledges(&staged) {
            return fail_after_pidfd(
                release,
                process,
                io::Error::other(
                    "process ownership acknowledgment does not match staged ownership",
                ),
            );
        }
        if let Err(error) = release.release() {
            return fail_after_pidfd_closed(process, error);
        }
        let Some(stdout) = child.stdout.take() else {
            return fail_running_captured(
                process,
                child,
                io::Error::other("captured child stdout is unavailable"),
            );
        };
        let Some(stderr) = child.stderr.take() else {
            return fail_running_captured(
                process,
                child,
                io::Error::other("captured child stderr is unavailable"),
            );
        };
        let stdout = match child_pipe_file(stdout.into()) {
            Ok(stdout) => stdout,
            Err(error) => return fail_running_captured(process, child, error),
        };
        let stderr = match child_pipe_file(stderr.into()) {
            Ok(stderr) => stderr,
            Err(error) => return fail_running_captured(process, child, error),
        };
        drop(child);
        Ok(CapturedProcess {
            process,
            stdout,
            stderr,
        })
    }

    /// # Errors
    /// Returns an error when serialization or exact pidfd reaping fails.
    #[allow(clippy::disallowed_methods)]
    pub fn reap(&self, process: RetainedProcess) -> io::Result<()> {
        self.reap_status(process).map(|_| ())
    }

    /// # Errors
    /// Returns an error when serialization or exact pidfd reaping fails.
    #[allow(clippy::disallowed_methods)]
    pub fn reap_status(&self, process: RetainedProcess) -> io::Result<ProcessExit> {
        let _guard = self
            .exclusion
            .lock()
            .map_err(|_| io::Error::other("child broker lock poisoned"))?;
        let status =
            rustix::process::waitid(WaitId::PidFd(process.pidfd.as_fd()), WaitIdOptions::EXITED)?
                .ok_or_else(|| io::Error::other("exact child exited without wait status"))?;
        let exit = ProcessExit {
            exit_code: status.exit_status(),
            terminating_signal: status.terminating_signal(),
        };
        drop(process);
        Ok(exit)
    }
}

fn child_pipe_file(fd: OwnedFd) -> io::Result<File> {
    let flags = fcntl_getfl(fd.as_fd())?;
    fcntl_setfl(fd.as_fd(), flags | OFlags::NONBLOCK)?;
    Ok(File::from(fd))
}

#[allow(clippy::disallowed_methods, clippy::disallowed_types)]
fn fail_running_captured<T>(
    process: RetainedProcess,
    child: std::process::Child,
    error: io::Error,
) -> io::Result<T> {
    let _ = process.send_signal(Signal::KILL);
    rustix::process::waitid(WaitId::PidFd(process.pidfd.as_fd()), WaitIdOptions::EXITED)?;
    drop(child);
    drop(process);
    Err(error)
}

#[allow(clippy::disallowed_types)]
fn private_role_command(role: &str) -> std::process::Command {
    let mut command = std::process::Command::new("/proc/self/exe");
    command.arg(PRIVATE_ROLE_ARGUMENT).arg(role);
    command
}

/// Runs a private self-exec role before normal argument handling.
///
/// # Errors
/// Returns the role's fail-closed error when a private invocation is malformed.
pub fn dispatch_private_process_role(arguments: &[OsString]) -> Option<io::Result<()>> {
    if arguments
        .first()
        .is_none_or(|argument| argument != PRIVATE_ROLE_ARGUMENT)
    {
        return None;
    }
    let role = arguments.get(1).and_then(|argument| argument.to_str());
    match role {
        Some(SIGCHLD_PROBE_ROLE) if arguments.len() == 2 => {
            Some(BarrierWait::from_stdin().and_then(BarrierWait::wait))
        }
        Some(WRAPPER_ROLE) => Some(run_wrapper_role(arguments[2..].to_vec())),
        _ => Some(Err(io::Error::other(
            "unknown or malformed private process role",
        ))),
    }
}

fn run_wrapper_role(arguments: Vec<OsString>) -> io::Result<()> {
    let (expected_parent, target) = parse_wrapper_arguments(arguments)?;
    BarrierWait::from_stdin()?.wait()?;
    rustix::io::fcntl_setfd(std::io::stdin(), rustix::io::FdFlags::CLOEXEC)?;
    match target.parent_edge {
        ParentEdge::ParentDeathKill => {
            arm_parent_death_and_recheck(Signal::KILL, expected_parent)?;
        }
        ParentEdge::LifetimePipe => {
            require_lifetime_pipe_and_recheck(expected_parent)?;
        }
    }
    Err(exec_target(target))
}

fn require_lifetime_pipe_and_recheck(
    expected_parent: PidfsProcessIdentityV1,
) -> io::Result<RetainedProcess> {
    let stderr = std::io::stderr();
    let mut descriptors = [PollFd::new(&stderr, PollFlags::IN)];
    poll(&mut descriptors, Some(&Timespec::default()))?;
    if descriptors[0]
        .revents()
        .intersects(PollFlags::IN | PollFlags::HUP | PollFlags::ERR)
    {
        return Err(io::Error::other("parent lifetime pipe is not held"));
    }
    let parent = recover_process(expected_parent)?;
    let observed_parent = rustix::process::getppid()
        .map(Pid::as_raw_nonzero)
        .map(i32::from)
        .and_then(|pid| u32::try_from(pid).ok());
    if observed_parent != Some(expected_parent.host_pid().get()) {
        return Err(io::Error::other(
            "parent changed while arming lifetime pipe",
        ));
    }
    Ok(parent)
}

#[allow(clippy::disallowed_methods)]
fn acquire_spawned_child(host_pid: u32) -> io::Result<RetainedProcess> {
    let raw_pid = i32::try_from(host_pid)
        .map_err(|_| io::Error::other("spawn returned out-of-range child PID"))?;
    let pid = Pid::from_raw(raw_pid)
        .ok_or_else(|| io::Error::other("spawn returned invalid child PID"))?;
    require_host_pid_namespace(host_pid)?;
    let pidfd = rustix::process::pidfd_open(pid, PidfdFlags::empty())?;
    verify_direct_child_pidfd(pidfd.as_fd())?;
    let authority = authority_from_pidfd(pidfd.as_fd(), read_boot_id()?)?;
    let start_time = read_start_time(host_pid)?;
    Ok(RetainedProcess {
        pidfd,
        identity: PidfsProcessIdentityV1::new(
            authority,
            HostPidLocator::new(host_pid),
            ProcessStartTimeTicks::new(start_time),
        ),
    })
}

/// # Errors
/// Refuses descriptors that do not name a waitable direct child of this broker process.
#[allow(clippy::disallowed_methods)]
pub fn verify_direct_child_pidfd(pidfd: impl AsFd) -> io::Result<()> {
    rustix::process::waitid(
        WaitId::PidFd(pidfd.as_fd()),
        WaitIdOptions::EXITED | WaitIdOptions::NOHANG | WaitIdOptions::NOWAIT,
    )?;
    Ok(())
}

#[allow(clippy::disallowed_methods, clippy::disallowed_types)]
fn fail_before_pidfd<T>(
    release: BarrierRelease,
    child: &mut std::process::Child,
    error: io::Error,
) -> io::Result<T> {
    drop(release);
    child.wait()?;
    Err(error)
}

fn fail_after_pidfd<T>(
    release: BarrierRelease,
    process: RetainedProcess,
    error: io::Error,
) -> io::Result<T> {
    drop(release);
    fail_after_pidfd_closed(process, error)
}

#[allow(clippy::disallowed_methods)]
fn fail_after_pidfd_closed<T>(process: RetainedProcess, error: io::Error) -> io::Result<T> {
    rustix::process::waitid(WaitId::PidFd(process.pidfd.as_fd()), WaitIdOptions::EXITED)?;
    drop(process);
    Err(error)
}

/// # Errors
/// Refuses before PID lookup on boot mismatch and before action on every later mismatch.
#[allow(clippy::disallowed_methods)]
pub fn recover_process(identity: PidfsProcessIdentityV1) -> io::Result<RetainedProcess> {
    recover_process_with(identity, read_boot_id, |host_pid| {
        let raw_pid = i32::try_from(host_pid)
            .map_err(|_| io::Error::other("stored PID locator is out of range"))?;
        let pid = Pid::from_raw(raw_pid)
            .ok_or_else(|| io::Error::other("stored PID locator is invalid"))?;
        require_host_pid_namespace(host_pid)?;
        rustix::process::pidfd_open(pid, PidfdFlags::empty()).map_err(Into::into)
    })
}

fn recover_process_with<Boot, Open>(
    identity: PidfsProcessIdentityV1,
    boot: Boot,
    open: Open,
) -> io::Result<RetainedProcess>
where
    Boot: FnOnce() -> io::Result<BootId>,
    Open: FnOnce(u32) -> io::Result<OwnedFd>,
{
    let current_boot = boot()?;
    if current_boot != identity.authority().boot_id {
        return Err(io::Error::other(
            "stored process authority belongs to another boot",
        ));
    }
    let host_pid = identity.host_pid().get();
    let pidfd = open(host_pid)?;
    let observed = authority_from_pidfd(pidfd.as_fd(), current_boot)?;
    if observed != identity.authority() {
        return Err(io::Error::other("pidfs process authority mismatch"));
    }
    if read_start_time(host_pid)? != identity.start_time_ticks().get() {
        return Err(io::Error::other("process start time mismatch"));
    }
    Ok(RetainedProcess { pidfd, identity })
}

/// # Errors
/// Refuses if arming fails or the exact parent changes during the recheck.
pub fn arm_parent_death_and_recheck(
    signal: Signal,
    expected_parent: PidfsProcessIdentityV1,
) -> io::Result<RetainedProcess> {
    rustix::process::set_parent_process_death_signal(Some(signal))?;
    let parent = recover_process(expected_parent)?;
    let observed_parent = rustix::process::getppid()
        .map(Pid::as_raw_nonzero)
        .map(i32::from)
        .and_then(|pid| u32::try_from(pid).ok());
    if observed_parent != Some(expected_parent.host_pid().get()) {
        return Err(io::Error::other(
            "parent changed while arming parent-death action",
        ));
    }
    Ok(parent)
}

/// # Errors
/// Returns an error if the current process cannot form a same-pidfd identity.
#[allow(clippy::disallowed_methods)]
pub fn current_process_identity() -> io::Result<PidfsProcessIdentityV1> {
    let pid = rustix::process::getpid();
    let host_pid = u32::try_from(i32::from(pid.as_raw_nonzero()))
        .map_err(|_| io::Error::other("current PID is out of range"))?;
    require_host_pid_namespace(host_pid)?;
    let pidfd = rustix::process::pidfd_open(pid, PidfdFlags::empty())?;
    let authority = authority_from_pidfd(pidfd.as_fd(), read_boot_id()?)?;
    Ok(PidfsProcessIdentityV1::new(
        authority,
        HostPidLocator::new(host_pid),
        ProcessStartTimeTicks::new(read_start_time(host_pid)?),
    ))
}

fn authority_from_pidfd(pidfd: impl AsFd, boot_id: BootId) -> io::Result<PidfsProcessAuthorityV1> {
    authority_from_pidfd_with(
        pidfd,
        boot_id,
        |fd| Ok(fstatfs(fd)?.f_type as i64),
        |fd| {
            let stat = fstat(fd)?;
            Ok((stat.st_dev, stat.st_ino))
        },
    )
}

fn authority_from_pidfd_with<Fs, Stat>(
    pidfd: impl AsFd,
    boot_id: BootId,
    filesystem: Fs,
    stat: Stat,
) -> io::Result<PidfsProcessAuthorityV1>
where
    Fs: FnOnce(std::os::fd::BorrowedFd<'_>) -> io::Result<i64>,
    Stat: FnOnce(std::os::fd::BorrowedFd<'_>) -> io::Result<(u64, u64)>,
{
    require_pidfs_magic(filesystem(pidfd.as_fd())?)?;
    let (device, inode) = stat(pidfd.as_fd())?;
    Ok(PidfsProcessAuthorityV1 {
        boot_id,
        pidfs_st_dev: PidfsDeviceId::new(device),
        pidfs_st_ino: PidfsInodeId::new(inode),
    })
}

/// # Errors
/// Refuses every filesystem magic other than `PID_FS_MAGIC`.
pub fn require_pidfs_magic(magic: i64) -> io::Result<()> {
    if magic == PID_FS_MAGIC {
        Ok(())
    } else {
        Err(io::Error::other("descriptor is not a pidfs process handle"))
    }
}

/// # Errors
/// Refuses unreadable or non-canonical boot identifiers.
pub fn read_boot_id() -> io::Result<BootId> {
    parse_boot_id(&std::fs::read_to_string("/proc/sys/kernel/random/boot_id")?)
}

/// # Errors
/// Refuses every non-canonical UUID spelling.
pub fn parse_boot_id(value: &str) -> io::Result<BootId> {
    let value = value.strip_suffix('\n').unwrap_or(value);
    if value.len() != 36
        || value.as_bytes()[8] != b'-'
        || value.as_bytes()[13] != b'-'
        || value.as_bytes()[18] != b'-'
        || value.as_bytes()[23] != b'-'
        || value
            .bytes()
            .enumerate()
            .any(|(index, byte)| !matches!(index, 8 | 13 | 18 | 23) && !byte.is_ascii_hexdigit())
    {
        return Err(io::Error::other("boot ID is not a canonical UUID"));
    }
    let compact = value.replace('-', "");
    let mut bytes = [0_u8; 16];
    for (index, byte) in bytes.iter_mut().enumerate() {
        *byte = u8::from_str_radix(&compact[index * 2..index * 2 + 2], 16)
            .map_err(|_| io::Error::other("boot ID is not a canonical UUID"))?;
    }
    Ok(BootId::new(bytes))
}

/// # Errors
/// Refuses unreadable or malformed process statistics.
pub fn read_start_time(host_pid: u32) -> io::Result<u64> {
    let stat = std::fs::read_to_string(format!("/proc/{host_pid}/stat"))?;
    let command_end = stat
        .rfind(") ")
        .ok_or_else(|| io::Error::other("process stat has no command terminator"))?;
    stat[command_end + 2..]
        .split_whitespace()
        .nth(19)
        .and_then(|value| value.parse().ok())
        .ok_or_else(|| io::Error::other("process stat has no valid start time"))
}

fn require_host_pid_namespace(host_pid: u32) -> io::Result<()> {
    let status = std::fs::read_to_string(format!("/proc/{host_pid}/status"))?;
    let nspid = status
        .lines()
        .find_map(|line| line.strip_prefix("NSpid:"))
        .ok_or_else(|| io::Error::other("process status has no PID namespace mapping"))?;
    if nspid.trim() == host_pid.to_string() {
        Ok(())
    } else {
        Err(io::Error::other(
            "PID locator is not unambiguous in the host PID namespace",
        ))
    }
}

fn require_default_sigchld() -> io::Result<()> {
    let status = std::fs::read_to_string("/proc/self/status")?;
    let ignored = signal_mask(&status, "SigIgn:")?;
    let caught = signal_mask(&status, "SigCgt:")?;
    let sigchld = 1_u128 << (17 - 1);
    if ignored & sigchld == 0 && caught & sigchld == 0 {
        Ok(())
    } else {
        Err(io::Error::other("SIGCHLD is not at default disposition"))
    }
}

#[allow(clippy::disallowed_methods, clippy::disallowed_types)]
fn require_reapable_sigchld() -> io::Result<()> {
    let (release, wait) = launch_barrier()?;
    let mut command = private_role_command(SIGCHLD_PROBE_ROLE);
    command.stdin(wait.into_stdio());
    let mut child = command.spawn()?;
    let host_pid = child.id();
    let raw_pid = i32::try_from(host_pid)
        .map_err(|_| io::Error::other("SIGCHLD probe returned out-of-range child PID"))?;
    let pid = Pid::from_raw(raw_pid)
        .ok_or_else(|| io::Error::other("SIGCHLD probe returned invalid child PID"))?;
    let pidfd = match rustix::process::pidfd_open(pid, PidfdFlags::empty()) {
        Ok(pidfd) => pidfd,
        Err(error) => return fail_sigchld_probe(release, &mut child, error.into()),
    };
    if let Err(error) = verify_direct_child_pidfd(pidfd.as_fd()) {
        return fail_sigchld_probe(release, &mut child, error);
    }
    if let Err(error) = release.release() {
        return cleanup_sigchld_probe(pidfd.as_fd(), child, error);
    }
    if let Err(error) = rustix::process::waitid(WaitId::PidFd(pidfd.as_fd()), WaitIdOptions::EXITED)
    {
        return cleanup_sigchld_probe(
            pidfd.as_fd(),
            child,
            io::Error::new(
                error.kind(),
                format!("SIGCHLD semantic probe was not reapable: {error}"),
            ),
        );
    }
    drop(child);
    Ok(())
}

#[allow(clippy::disallowed_methods, clippy::disallowed_types)]
fn cleanup_sigchld_probe(
    pidfd: std::os::fd::BorrowedFd<'_>,
    child: std::process::Child,
    error: io::Error,
) -> io::Result<()> {
    let result = cleanup_sigchld_probe_with(
        error,
        || rustix::process::pidfd_send_signal(pidfd, Signal::KILL).map_err(Into::into),
        || {
            rustix::process::waitid(WaitId::PidFd(pidfd), WaitIdOptions::EXITED)
                .map(|_| ())
                .map_err(Into::into)
        },
    );
    drop(child);
    result
}

fn cleanup_sigchld_probe_with<Terminate, Wait>(
    error: io::Error,
    terminate: Terminate,
    mut wait: Wait,
) -> io::Result<()>
where
    Terminate: FnOnce() -> io::Result<()>,
    Wait: FnMut() -> io::Result<()>,
{
    let _ = terminate();
    loop {
        match wait() {
            Ok(()) => return Err(error),
            Err(wait_error) if wait_error.kind() == io::ErrorKind::Interrupted => {}
            Err(wait_error)
                if wait_error.raw_os_error() == Some(rustix::io::Errno::CHILD.raw_os_error()) =>
            {
                return Err(error);
            }
            Err(wait_error) => {
                return Err(io::Error::new(
                    error.kind(),
                    format!("{error}; exact SIGCHLD probe cleanup failed: {wait_error}"),
                ));
            }
        }
    }
}

#[allow(clippy::disallowed_methods, clippy::disallowed_types)]
fn fail_sigchld_probe(
    release: BarrierRelease,
    child: &mut std::process::Child,
    error: io::Error,
) -> io::Result<()> {
    drop(release);
    let _ = child.wait();
    Err(error)
}

fn signal_mask(status: &str, name: &str) -> io::Result<u128> {
    status
        .lines()
        .find_map(|line| line.strip_prefix(name))
        .and_then(|value| u128::from_str_radix(value.trim(), 16).ok())
        .ok_or_else(|| io::Error::other("process status has no valid signal mask"))
}

/// # 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")),
    };
    if arguments.next().as_deref() != Some(OsStr::new("--")) {
        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,
    };
    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()
}

#[cfg(test)]
mod tests {
    use std::cell::Cell;
    use std::os::fd::{AsFd, AsRawFd};
    use std::rc::Rc;

    use super::*;

    #[test]
    fn broker_is_process_global() {
        assert!(std::ptr::eq(child_broker(), child_broker()));
    }

    #[test]
    fn parent_edges_require_their_exact_stdio_policy() {
        assert!(
            require_parent_edge_standard_io(
                ParentEdge::LifetimePipe,
                ChildStandardIoPolicy::PrivateControlWithLifetimePipe,
            )
            .is_ok()
        );
        assert!(
            require_parent_edge_standard_io(
                ParentEdge::LifetimePipe,
                ChildStandardIoPolicy::Silenced,
            )
            .is_err()
        );
        assert!(
            require_parent_edge_standard_io(
                ParentEdge::ParentDeathKill,
                ChildStandardIoPolicy::Silenced,
            )
            .is_ok()
        );
        assert!(
            require_parent_edge_standard_io(
                ParentEdge::ParentDeathKill,
                ChildStandardIoPolicy::PrivateControlWithLifetimePipe,
            )
            .is_err()
        );
    }

    #[test]
    fn barrier_reader_is_inheritable_and_writer_is_close_on_exec() {
        let (release, wait) = launch_barrier().unwrap();
        let reader_flags = rustix::io::fcntl_getfd(wait.0.as_fd()).unwrap();
        let writer_flags = rustix::io::fcntl_getfd(release.0.as_fd()).unwrap();
        assert!(!reader_flags.contains(FdFlags::CLOEXEC));
        assert!(writer_flags.contains(FdFlags::CLOEXEC));
        assert_ne!(wait.0.as_raw_fd(), release.0.as_raw_fd());
    }

    #[test]
    fn boot_id_requires_exact_canonical_uuid_syntax() {
        parse_boot_id("00112233-4455-6677-8899-aabbccddeeff\n").unwrap();
        for invalid in [
            "00112233445566778899aabbccddeeff",
            "00112233_4455-6677-8899-aabbccddeeff",
            "00112233-4455-6677-8899-aabbccddeeff-extra",
            "00112233-4455-6677-8899-aabbccddeefg",
            "00112233-4455-6677-8899-aabbccddeeff\n\n",
        ] {
            assert!(parse_boot_id(invalid).is_err(), "accepted {invalid}");
        }
    }

    #[test]
    fn recovery_checks_boot_before_pid_lookup() {
        let identity = PidfsProcessIdentityV1::new(
            PidfsProcessAuthorityV1 {
                boot_id: BootId::new([1; 16]),
                pidfs_st_dev: PidfsDeviceId::new(1),
                pidfs_st_ino: PidfsInodeId::new(1),
            },
            HostPidLocator::new(1),
            ProcessStartTimeTicks::new(1),
        );
        let opened = Cell::new(false);
        let result = recover_process_with(
            identity,
            || Ok(BootId::new([2; 16])),
            |_| {
                opened.set(true);
                unreachable!()
            },
        );
        assert!(result.is_err());
        assert!(!opened.get());
    }

    #[test]
    fn pidfs_magic_precedes_same_descriptor_stat_and_errors_refuse() {
        let file = File::open("/dev/null").unwrap();
        let raw = file.as_raw_fd();
        let events = Rc::new(std::cell::RefCell::new(Vec::new()));
        let filesystem_events = Rc::clone(&events);
        let stat_events = Rc::clone(&events);
        let authority = authority_from_pidfd_with(
            file.as_fd(),
            BootId::new([1; 16]),
            move |fd| {
                filesystem_events
                    .borrow_mut()
                    .push(("fstatfs", fd.as_raw_fd()));
                Ok(PID_FS_MAGIC)
            },
            move |fd| {
                stat_events.borrow_mut().push(("fstat", fd.as_raw_fd()));
                Ok((7, 9))
            },
        )
        .unwrap();
        assert_eq!(&*events.borrow(), &[("fstatfs", raw), ("fstat", raw)]);
        assert_eq!(authority.pidfs_st_dev.get(), 7);
        assert_eq!(authority.pidfs_st_ino.get(), 9);

        for filesystem_error in [false, true] {
            let stat_called = Cell::new(false);
            let result = authority_from_pidfd_with(
                file.as_fd(),
                BootId::new([1; 16]),
                |_| {
                    if filesystem_error {
                        Err(io::Error::other("injected fstatfs error"))
                    } else {
                        Ok(0)
                    }
                },
                |_| {
                    stat_called.set(true);
                    Ok((1, 1))
                },
            );
            assert!(result.is_err());
            assert!(!stat_called.get());
        }

        assert!(
            authority_from_pidfd_with(
                file.as_fd(),
                BootId::new([1; 16]),
                |_| Ok(PID_FS_MAGIC),
                |_| Err(io::Error::other("injected fstat error")),
            )
            .is_err()
        );
    }

    #[test]
    fn sigchld_probe_epipe_terminates_and_reaps_exact_child() {
        let events = Rc::new(std::cell::RefCell::new(Vec::new()));
        let terminate_events = Rc::clone(&events);
        let wait_events = Rc::clone(&events);
        let result = cleanup_sigchld_probe_with(
            io::Error::new(io::ErrorKind::BrokenPipe, "probe died before release"),
            move || {
                terminate_events.borrow_mut().push("terminate-pidfd");
                Err(io::Error::from_raw_os_error(3))
            },
            move || {
                wait_events.borrow_mut().push("wait-pidfd");
                Ok(())
            },
        );
        assert_eq!(result.unwrap_err().kind(), io::ErrorKind::BrokenPipe);
        assert_eq!(&*events.borrow(), &["terminate-pidfd", "wait-pidfd"]);
    }

    #[test]
    fn sigchld_probe_failed_wait_retries_cleanup_until_not_waitable() {
        let waits = Cell::new(0_u8);
        let result = cleanup_sigchld_probe_with(
            io::Error::other("injected semantic wait failure"),
            || Ok(()),
            || {
                waits.set(waits.get() + 1);
                match waits.get() {
                    1 => Err(io::Error::from(io::ErrorKind::Interrupted)),
                    _ => Err(io::Error::from_raw_os_error(
                        rustix::io::Errno::CHILD.raw_os_error(),
                    )),
                }
            },
        );
        assert_eq!(result.unwrap_err().kind(), io::ErrorKind::Other);
        assert_eq!(waits.get(), 2);
    }
}