dosh/src/persist.rs

//! Session persistence across server restarts.
//!
//! A persistent dosh session's shell does not run as a child of `dosh-server`.
//! Instead the server spawns a tiny per-session *holder* process (the same
//! `dosh-server` binary re-exec'd as `dosh-server hold ...`). The holder calls
//! `setsid()` to leave the server's process group/session, opens a PTY, spawns
//! the shell as ITS own child, and listens on a Unix socket in a per-session
//! runtime directory. The server connects to that socket and the holder hands it
//! the PTY master fd over `SCM_RIGHTS`.
//!
//! Because the shell belongs to the holder (which is in its own session and is
//! not waited on by the server), killing or restarting `dosh-server` leaves the
//! holder + shell alive. On startup the server scans the runtime directory,
//! reconnects to each live holder, receives the master fd again, and rebuilds the
//! in-memory session so clients reattach to the very same shell.
//!
//! Screen / scrollback state lives only in server memory, so it is also mirrored
//! to disk (atomically) and restored on re-adoption, letting a reattaching client
//! repaint the screen exactly as it was before the restart.

use anyhow::{Context, Result, anyhow, bail};
use std::io::{Read, Write};
use std::os::fd::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
use std::os::unix::net::{UnixListener, UnixStream};
use std::path::{Path, PathBuf};
use std::time::Duration;

/// One-byte commands a server sends to a holder over its control socket after
/// the holder has handed back the master fd.
const HOLDER_CMD_SHUTDOWN: u8 = b'X';
const HOLDER_STARTUP_TIMEOUT: Duration = Duration::from_millis(750);

/// Magic written into a holder's `meta` file, bumped if the on-disk layout
/// changes so a stale holder from an incompatible build is ignored.
const META_MAGIC: &str = "dosh-holder-1";

/// Per-session runtime metadata persisted next to the holder socket.
#[derive(Debug, Clone)]
pub struct HolderMeta {
    pub session: String,
    pub shell_pid: i32,
}

/// Root runtime directory holding one subdirectory per persistent session.
/// Lives under `sessions_dir/run` so it shares the session storage location and
/// can be wiped with it.
pub fn runtime_root(sessions_dir: &Path) -> PathBuf {
    sessions_dir.join("run")
}

/// Map a session name to a filesystem-safe directory name. Session names are
/// user-controlled, so hex-encode them rather than trusting them as path
/// components (avoids traversal, slashes, NULs, length issues).
fn session_dir_name(session: &str) -> String {
    let mut out = String::with_capacity(session.len() * 2);
    for byte in session.as_bytes() {
        out.push(char::from_digit((byte >> 4) as u32, 16).unwrap());
        out.push(char::from_digit((byte & 0xf) as u32, 16).unwrap());
    }
    out
}

fn decode_session_dir_name(name: &str) -> Option<String> {
    if !name.len().is_multiple_of(2) {
        return None;
    }
    let mut bytes = Vec::with_capacity(name.len() / 2);
    let raw = name.as_bytes();
    let mut i = 0;
    while i < raw.len() {
        let hi = (raw[i] as char).to_digit(16)?;
        let lo = (raw[i + 1] as char).to_digit(16)?;
        bytes.push(((hi << 4) | lo) as u8);
        i += 2;
    }
    String::from_utf8(bytes).ok()
}

/// Directory for one session's holder runtime state.
pub fn session_runtime_dir(sessions_dir: &Path, session: &str) -> PathBuf {
    runtime_root(sessions_dir).join(session_dir_name(session))
}

fn holder_sock_path(dir: &Path) -> PathBuf {
    dir.join("holder.sock")
}

fn meta_path(dir: &Path) -> PathBuf {
    dir.join("meta")
}

fn screen_path(dir: &Path) -> PathBuf {
    dir.join("screen")
}

/// Create the runtime directory tree with private (0700) permissions.
pub fn ensure_runtime_dir(sessions_dir: &Path, session: &str) -> Result<PathBuf> {
    use std::os::unix::fs::PermissionsExt;
    let root = runtime_root(sessions_dir);
    std::fs::create_dir_all(&root).with_context(|| format!("create {}", root.display()))?;
    let _ = std::fs::set_permissions(&root, std::fs::Permissions::from_mode(0o700));
    let dir = session_runtime_dir(sessions_dir, session);
    std::fs::create_dir_all(&dir).with_context(|| format!("create {}", dir.display()))?;
    std::fs::set_permissions(&dir, std::fs::Permissions::from_mode(0o700))
        .with_context(|| format!("chmod {}", dir.display()))?;
    Ok(dir)
}

/// Atomically write `data` to `path` (write temp + rename), so a concurrent
/// reader (e.g. a restarting server) never sees a half-written file.
fn atomic_write(path: &Path, data: &[u8]) -> Result<()> {
    use std::os::unix::fs::PermissionsExt;
    let tmp = path.with_extension("tmp");
    {
        let mut file =
            std::fs::File::create(&tmp).with_context(|| format!("create {}", tmp.display()))?;
        let _ = file.set_permissions(std::fs::Permissions::from_mode(0o600));
        file.write_all(data)
            .with_context(|| format!("write {}", tmp.display()))?;
        file.sync_all().ok();
    }
    std::fs::rename(&tmp, path)
        .with_context(|| format!("rename {} -> {}", tmp.display(), path.display()))?;
    Ok(())
}

/// Persist the vt100 screen snapshot plus recent raw output for a session so a
/// post-restart reattach repaints correctly. `snapshot` is the bytes a fresh
/// attach would receive (alt-screen toggle + `state_formatted`).
pub fn save_screen(
    sessions_dir: &Path,
    session: &str,
    cols: u16,
    rows: u16,
    output_seq: u64,
    snapshot: &[u8],
) -> Result<()> {
    let dir = session_runtime_dir(sessions_dir, session);
    if !dir.exists() {
        // No holder runtime for this session (non-persistent): nothing to do.
        return Ok(());
    }
    let mut buf = Vec::with_capacity(snapshot.len() + 32);
    buf.extend_from_slice(&cols.to_be_bytes());
    buf.extend_from_slice(&rows.to_be_bytes());
    buf.extend_from_slice(&output_seq.to_be_bytes());
    buf.extend_from_slice(&(snapshot.len() as u32).to_be_bytes());
    buf.extend_from_slice(snapshot);
    atomic_write(&screen_path(&dir), &buf)
}

/// A restored screen for a re-adopted session.
pub struct SavedScreen {
    pub cols: u16,
    pub rows: u16,
    pub output_seq: u64,
    pub snapshot: Vec<u8>,
}

/// Load a previously persisted screen, if any.
pub fn load_screen(sessions_dir: &Path, session: &str) -> Option<SavedScreen> {
    let dir = session_runtime_dir(sessions_dir, session);
    let data = std::fs::read(screen_path(&dir)).ok()?;
    if data.len() < 16 {
        return None;
    }
    let cols = u16::from_be_bytes(data[0..2].try_into().ok()?);
    let rows = u16::from_be_bytes(data[2..4].try_into().ok()?);
    let output_seq = u64::from_be_bytes(data[4..12].try_into().ok()?);
    let len = u32::from_be_bytes(data[12..16].try_into().ok()?) as usize;
    if data.len() < 16 + len {
        return None;
    }
    Some(SavedScreen {
        cols,
        rows,
        output_seq,
        snapshot: data[16..16 + len].to_vec(),
    })
}

fn write_meta(dir: &Path, meta: &HolderMeta) -> Result<()> {
    let body = format!("{META_MAGIC}\n{}\n{}\n", meta.shell_pid, meta.session);
    atomic_write(&meta_path(dir), body.as_bytes())
}

fn read_meta(dir: &Path) -> Option<HolderMeta> {
    let body = std::fs::read_to_string(meta_path(dir)).ok()?;
    let mut lines = body.lines();
    if lines.next()? != META_MAGIC {
        return None;
    }
    let shell_pid: i32 = lines.next()?.parse().ok()?;
    let session = lines.next()?.to_string();
    Some(HolderMeta { session, shell_pid })
}

/// Spawn a holder process for `session`: re-exec this binary as `dosh-server
/// hold` with the runtime dir, shell, terminal size, and accepted env. The
/// holder daemonizes (setsid, owns the PTY) and listens on its control socket.
/// Returns once the holder has signalled readiness by creating its socket.
#[allow(clippy::too_many_arguments)]
pub fn spawn_holder(
    sessions_dir: &Path,
    session: &str,
    shell: &str,
    cols: u16,
    rows: u16,
    env: &[(String, String)],
) -> Result<()> {
    let dir = ensure_runtime_dir(sessions_dir, session)?;
    let sock = holder_sock_path(&dir);
    // Clear any stale socket left by a crashed holder with the same name.
    let _ = std::fs::remove_file(&sock);

    let exe = std::env::current_exe().context("locate current executable")?;
    let mut cmd = std::process::Command::new(exe);
    cmd.arg("hold")
        .arg("--runtime-dir")
        .arg(&dir)
        .arg("--session")
        .arg(session)
        .arg("--shell")
        .arg(shell)
        .arg("--cols")
        .arg(cols.to_string())
        .arg("--rows")
        .arg(rows.to_string());
    for (name, value) in env {
        cmd.arg("--env").arg(format!("{name}={value}"));
    }
    cmd.stdin(std::process::Stdio::null())
        .stdout(std::process::Stdio::null())
        .stderr(std::process::Stdio::null());
    let mut child = cmd.spawn().context("spawn holder process")?;

    // Wait (briefly) for the holder to come up. The holder forks/setsids before
    // creating the socket; once the socket exists we can connect. We also reap
    // the short-lived launcher child so it never becomes a zombie.
    let deadline = std::time::Instant::now() + HOLDER_STARTUP_TIMEOUT;
    loop {
        if sock.exists() {
            break;
        }
        if let Some(status) = child.try_wait().context("wait holder launcher")? {
            bail!("holder launcher for session {session} exited before socket was ready: {status}");
        }
        if std::time::Instant::now() >= deadline {
            let _ = child.kill();
            let _ = child.wait();
            bail!("holder for session {session} did not come up in time");
        }
        std::thread::sleep(Duration::from_millis(20));
    }
    // The launcher exits immediately after the grandchild setsids; reap it.
    let _ = child.wait();
    Ok(())
}

/// Connect to a session's holder and receive the PTY master fd over SCM_RIGHTS.
/// Returns the raw fd (caller owns it) and the holder control socket, kept open
/// so the server can later ask the holder to shut down. Returns an error if the
/// holder is gone (caller then degrades to a fresh, non-persistent session).
pub fn adopt_holder(sessions_dir: &Path, session: &str) -> Result<(RawFd, UnixStream)> {
    let dir = session_runtime_dir(sessions_dir, session);
    let sock = holder_sock_path(&dir);
    let mut stream = UnixStream::connect(&sock)
        .with_context(|| format!("connect holder socket {}", sock.display()))?;
    stream.set_read_timeout(Some(Duration::from_secs(5))).ok();
    let fd = recv_fd(&mut stream).context("receive master fd from holder")?;
    Ok((fd, stream))
}

/// Ask a holder to terminate its shell and exit, then clean its runtime dir.
/// Used when reaping a truly-abandoned persistent session.
pub fn request_shutdown(sessions_dir: &Path, session: &str, control: Option<&mut UnixStream>) {
    if let Some(stream) = control {
        let _ = stream.write_all(&[HOLDER_CMD_SHUTDOWN]);
        let _ = stream.flush();
    } else {
        let dir = session_runtime_dir(sessions_dir, session);
        if let Ok(mut stream) = UnixStream::connect(holder_sock_path(&dir)) {
            // Drain the fd the holder sends on connect, then send shutdown.
            let _ = recv_fd(&mut stream);
            let _ = stream.write_all(&[HOLDER_CMD_SHUTDOWN]);
            let _ = stream.flush();
        }
    }
    // Give the holder a moment to tear down, then remove its runtime dir.
    std::thread::sleep(Duration::from_millis(50));
    let dir = session_runtime_dir(sessions_dir, session);
    let _ = std::fs::remove_dir_all(&dir);
}

/// Remove a session's runtime directory unconditionally (best effort).
pub fn remove_runtime_dir(sessions_dir: &Path, session: &str) {
    let dir = session_runtime_dir(sessions_dir, session);
    let _ = std::fs::remove_dir_all(&dir);
}

/// Scan the runtime root for holders left behind by a previous server. Returns
/// `(session_name, meta)` for each one whose holder process still appears alive.
/// Stale entries (no live process) are cleaned up.
pub fn scan_existing_holders(sessions_dir: &Path) -> Vec<HolderMeta> {
    let root = runtime_root(sessions_dir);
    let mut found = Vec::new();
    let Ok(entries) = std::fs::read_dir(&root) else {
        return found;
    };
    for entry in entries.flatten() {
        let dir = entry.path();
        if !dir.is_dir() {
            continue;
        }
        let Some(name) = dir.file_name().and_then(|n| n.to_str()) else {
            continue;
        };
        // Decode the on-disk name back to a session name; skip junk dirs.
        if decode_session_dir_name(name).is_none() {
            continue;
        }
        match read_meta(&dir) {
            Some(meta) if process_alive(meta.shell_pid) && holder_sock_path(&dir).exists() => {
                found.push(meta);
            }
            _ => {
                // Holder gone or meta unreadable: clean up so we don't try to
                // adopt a dead session (degrade to fresh on next attach).
                let _ = std::fs::remove_dir_all(&dir);
            }
        }
    }
    found
}

/// Whether a pid refers to a live process (signal 0 probe).
fn process_alive(pid: i32) -> bool {
    if pid <= 0 {
        return false;
    }
    unsafe {
        libc::kill(pid, 0) == 0
            || std::io::Error::last_os_error().raw_os_error() == Some(libc::EPERM)
    }
}

// ---------------------------------------------------------------------------
// Holder process entry point
// ---------------------------------------------------------------------------

/// Run as a holder process. Daemonizes (double-fork + setsid), opens a PTY,
/// spawns the shell as its own child, and serves the control socket: every
/// accepted connection is handed the master fd (SCM_RIGHTS); a subsequent
/// SHUTDOWN byte (or the shell exiting) tears everything down.
///
/// This function does not return on success — it `exit`s the process. Errors
/// before the daemonization point are returned to the launcher.
#[allow(clippy::too_many_arguments)]
pub fn run_holder(
    runtime_dir: &Path,
    session: &str,
    shell: &str,
    cols: u16,
    rows: u16,
    env: &[(String, String)],
) -> Result<()> {
    use portable_pty::{NativePtySystem, PtySize, PtySystem};

    // Detach from the launching server: own session + process group so a server
    // exit (even a process-group kill of the service) does not take us down.
    daemonize().context("daemonize holder")?;

    let pty_system = NativePtySystem::default();
    let pair = pty_system
        .openpty(PtySize {
            rows,
            cols,
            pixel_width: 0,
            pixel_height: 0,
        })
        .context("holder open pty")?;
    let cmd = crate::pty::build_shell_command(shell, env);
    let mut child = pair
        .slave
        .spawn_command(cmd)
        .context("holder spawn shell")?;
    drop(pair.slave);
    let master_fd = pair
        .master
        .as_raw_fd()
        .ok_or_else(|| anyhow!("holder master has no raw fd"))?;
    let shell_pid = child.process_id().map(|p| p as i32).unwrap_or(-1);

    write_meta(
        runtime_dir,
        &HolderMeta {
            session: session.to_string(),
            shell_pid,
        },
    )?;

    let sock = holder_sock_path(runtime_dir);
    let _ = std::fs::remove_file(&sock);
    let listener =
        UnixListener::bind(&sock).with_context(|| format!("holder bind {}", sock.display()))?;
    {
        use std::os::unix::fs::PermissionsExt;
        let _ = std::fs::set_permissions(&sock, std::fs::Permissions::from_mode(0o600));
    }
    listener
        .set_nonblocking(false)
        .context("holder listener blocking")?;

    // A watcher thread reaps the shell: when it exits, the holder cleans up and
    // exits too, so an abandoned shell does not linger forever.
    let runtime_owned = runtime_dir.to_path_buf();
    std::thread::spawn(move || {
        let _ = child.wait();
        // Shell exited: remove runtime dir and exit the holder process.
        let _ = std::fs::remove_dir_all(&runtime_owned);
        std::process::exit(0);
    });

    // Accept loop: each connecting server gets the master fd; a SHUTDOWN byte
    // from any of them tears the holder down. Multiple servers never run at once
    // in practice (one service), but serving repeated connections lets a server
    // restart re-adopt cleanly.
    for stream in listener.incoming() {
        let Ok(mut stream) = stream else { continue };
        if send_fd(&mut stream, master_fd).is_err() {
            continue;
        }
        // Wait for an optional command byte. EOF (server dropped the control
        // socket on its own exit) just means "keep running, await re-adoption".
        let mut cmd = [0u8; 1];
        match stream.read(&mut cmd) {
            Ok(1) if cmd[0] == HOLDER_CMD_SHUTDOWN => {
                let _ = std::fs::remove_dir_all(runtime_dir);
                std::process::exit(0);
            }
            _ => {
                // Server detached (exit/restart) or sent nothing actionable:
                // loop back and wait for the next server to re-adopt us.
            }
        }
    }
    Ok(())
}

/// Double-fork + `setsid` so the holder runs in its own session, detached from
/// the server's controlling terminal and process group. Without this a
/// `systemctl restart` (which signals the whole service cgroup/process group)
/// could take the holder down with the server.
fn daemonize() -> Result<()> {
    // First fork: parent (launcher) returns to reap; child continues.
    match unsafe { libc::fork() } {
        -1 => bail!("fork: {}", std::io::Error::last_os_error()),
        0 => {}
        _ => {
            // Parent process: exit so the launcher's wait() returns promptly and
            // the grandchild is reparented to init.
            std::process::exit(0);
        }
    }
    // New session: detaches from controlling tty and the server's process group.
    if unsafe { libc::setsid() } == -1 {
        bail!("setsid: {}", std::io::Error::last_os_error());
    }
    // Second fork: ensures we are not a session leader, so we can never
    // re-acquire a controlling terminal.
    match unsafe { libc::fork() } {
        -1 => bail!("fork2: {}", std::io::Error::last_os_error()),
        0 => Ok(()),
        _ => std::process::exit(0),
    }
}

// ---------------------------------------------------------------------------
// SCM_RIGHTS file-descriptor passing over a Unix domain socket
// ---------------------------------------------------------------------------

/// Send a single fd over `stream` using SCM_RIGHTS, with one byte of normal data
/// (sendmsg requires at least one iovec byte for the ancillary data to ride on).
fn send_fd(stream: &mut UnixStream, fd: RawFd) -> Result<()> {
    let dummy: [u8; 1] = [0];
    let mut iov = libc::iovec {
        iov_base: dummy.as_ptr() as *mut libc::c_void,
        iov_len: 1,
    };
    let mut cmsg_buf = [0u8; cmsg_space_one_fd()];
    let mut msg: libc::msghdr = unsafe { std::mem::zeroed() };
    msg.msg_iov = &mut iov;
    msg.msg_iovlen = 1;
    msg.msg_control = cmsg_buf.as_mut_ptr() as *mut libc::c_void;
    msg.msg_controllen = cmsg_buf.len() as _;

    unsafe {
        let cmsg = libc::CMSG_FIRSTHDR(&msg);
        if cmsg.is_null() {
            bail!("CMSG_FIRSTHDR null");
        }
        (*cmsg).cmsg_level = libc::SOL_SOCKET;
        (*cmsg).cmsg_type = libc::SCM_RIGHTS;
        (*cmsg).cmsg_len = libc::CMSG_LEN(std::mem::size_of::<RawFd>() as u32) as _;
        std::ptr::copy_nonoverlapping(
            &fd as *const RawFd as *const u8,
            libc::CMSG_DATA(cmsg),
            std::mem::size_of::<RawFd>(),
        );
        let n = libc::sendmsg(stream.as_raw_fd(), &msg, 0);
        if n < 0 {
            return Err(std::io::Error::last_os_error()).context("sendmsg SCM_RIGHTS");
        }
    }
    Ok(())
}

/// Receive a single fd sent via SCM_RIGHTS. Returns a fresh fd owned by the
/// caller.
fn recv_fd(stream: &mut UnixStream) -> Result<RawFd> {
    let mut dummy = [0u8; 1];
    let mut iov = libc::iovec {
        iov_base: dummy.as_mut_ptr() as *mut libc::c_void,
        iov_len: 1,
    };
    let mut cmsg_buf = [0u8; cmsg_space_one_fd()];
    let mut msg: libc::msghdr = unsafe { std::mem::zeroed() };
    msg.msg_iov = &mut iov;
    msg.msg_iovlen = 1;
    msg.msg_control = cmsg_buf.as_mut_ptr() as *mut libc::c_void;
    msg.msg_controllen = cmsg_buf.len() as _;

    unsafe {
        let n = libc::recvmsg(stream.as_raw_fd(), &mut msg, 0);
        if n < 0 {
            return Err(std::io::Error::last_os_error()).context("recvmsg SCM_RIGHTS");
        }
        if n == 0 {
            bail!("holder closed connection before sending fd");
        }
        let cmsg = libc::CMSG_FIRSTHDR(&msg);
        if cmsg.is_null() {
            bail!("no ancillary data (fd) received from holder");
        }
        if (*cmsg).cmsg_level != libc::SOL_SOCKET || (*cmsg).cmsg_type != libc::SCM_RIGHTS {
            bail!("unexpected ancillary message from holder");
        }
        let mut fd: RawFd = -1;
        std::ptr::copy_nonoverlapping(
            libc::CMSG_DATA(cmsg),
            &mut fd as *mut RawFd as *mut u8,
            std::mem::size_of::<RawFd>(),
        );
        if fd < 0 {
            bail!("invalid fd received from holder");
        }
        Ok(fd)
    }
}

/// Space, in bytes, needed for a control-message buffer carrying exactly one fd.
const fn cmsg_space_one_fd() -> usize {
    // CMSG_SPACE is not const in libc; this is the equivalent for one RawFd.
    // cmsghdr is aligned to size_of::<usize>(); add data length rounded up.
    let data = std::mem::size_of::<RawFd>();
    let hdr = std::mem::size_of::<libc::cmsghdr>();
    let align = std::mem::size_of::<usize>();
    // round(hdr) + round(data)
    ((hdr + align - 1) & !(align - 1)) + ((data + align - 1) & !(align - 1))
}

/// Helper used by `adopt_holder` callers to turn the received raw fd into an
/// owned `File`-like object if they need RAII (the server hands it to
/// `pty::adopt_pty_from_fd`, which takes ownership of the raw fd).
pub fn fd_into_file(fd: RawFd) -> std::fs::File {
    unsafe { std::fs::File::from_raw_fd(fd) }
}

/// Convert a `File` back into a raw fd it no longer owns (so it can be handed to
/// `adopt_pty_from_fd`). Currently unused outside tests but kept symmetric.
pub fn file_into_fd(file: std::fs::File) -> RawFd {
    file.into_raw_fd()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn session_dir_name_round_trips() {
        for name in ["default", "work", "a/b/../c", "weird name", "日本語"] {
            let encoded = session_dir_name(name);
            assert!(encoded.bytes().all(|b| b.is_ascii_hexdigit()));
            assert_eq!(decode_session_dir_name(&encoded).as_deref(), Some(name));
        }
    }

    #[test]
    fn decode_rejects_non_hex() {
        assert!(decode_session_dir_name("zz").is_none());
        assert!(decode_session_dir_name("abc").is_none());
    }

    #[test]
    fn save_and_load_screen_round_trips() {
        let tmp = tempfile::tempdir().unwrap();
        let sessions_dir = tmp.path();
        ensure_runtime_dir(sessions_dir, "work").unwrap();
        let snap = b"\x1b[?1049lhello world".to_vec();
        save_screen(sessions_dir, "work", 100, 40, 7, &snap).unwrap();
        let loaded = load_screen(sessions_dir, "work").expect("screen restored");
        assert_eq!(loaded.cols, 100);
        assert_eq!(loaded.rows, 40);
        assert_eq!(loaded.output_seq, 7);
        assert_eq!(loaded.snapshot, snap);
    }

    #[test]
    fn load_screen_absent_is_none() {
        let tmp = tempfile::tempdir().unwrap();
        assert!(load_screen(tmp.path(), "missing").is_none());
    }

    #[test]
    fn scan_skips_dead_and_junk_entries() {
        let tmp = tempfile::tempdir().unwrap();
        let sessions_dir = tmp.path();
        // A meta pointing at a definitely-dead pid is cleaned up, not returned.
        let dir = ensure_runtime_dir(sessions_dir, "ghost").unwrap();
        write_meta(
            &dir,
            &HolderMeta {
                session: "ghost".to_string(),
                shell_pid: 2_000_000_000, // not a live pid
            },
        )
        .unwrap();
        // A junk (non-hex) directory is ignored.
        std::fs::create_dir_all(runtime_root(sessions_dir).join("not-hex")).unwrap();
        let found = scan_existing_holders(sessions_dir);
        assert!(found.is_empty(), "dead/junk holders must be skipped");
        assert!(!dir.exists(), "dead holder dir should be cleaned up");
    }

    #[test]
    fn send_and_recv_fd_round_trips() {
        use std::io::Seek;
        // Pass a temp file's fd across a socketpair and confirm both ends point
        // at the same open file (write via one, read via the other).
        let (mut a, mut b) = UnixStream::pair().unwrap();
        let mut file = tempfile::tempfile().unwrap();
        writeln!(file, "shared-fd-marker").unwrap();
        file.flush().unwrap();
        send_fd(&mut a, file.as_raw_fd()).unwrap();
        let received = recv_fd(&mut b).unwrap();
        let mut got = fd_into_file(received);
        got.rewind().unwrap();
        let mut contents = String::new();
        got.read_to_string(&mut contents).unwrap();
        assert!(contents.contains("shared-fd-marker"));
    }
}