//! `hivectl agent <name> choom` — drop into an interactive Claude session inside
//! an agent container by exec-ing `machinectl shell` running claude as the
//! agent user, mirroring the harness's per-turn claude invocation.

#[cfg(unix)]
use std::os::unix::process::CommandExt as _;
use std::path::Path;

use anyhow::{Result, bail};

use crate::util::agent_exists;

/// Whether the caller's supplementary groups include `hive-admin`.
///
/// hive-c0re's polkit rule (`nix/host-modules/hive-c0re/default.nix`) grants
/// `org.freedesktop.machine1.shell` to this group, so a member reaches the
/// same outcome as root without one. `getgrnam`/`getgroups` read the
/// *process's* actual credentials, the same ones the kernel would check —
/// not a fresh `/etc/group` lookup by name — so this can say no for a user
/// who was just added to the group but hasn't logged back in yet, same
/// caveat as the host-socket grant (`hivectl/src/client.rs::connect_hint`).
fn in_hive_admin_group() -> bool {
    // SAFETY: `getgrnam` takes a valid NUL-terminated string literal and
    // returns either null (no such group) or a pointer to static storage
    // owned by libc, which is read once, immediately, before any other
    // libc call could invalidate it.
    let gid = unsafe {
        let grp = libc::getgrnam(c"hive-admin".as_ptr());
        if grp.is_null() {
            return false;
        }
        (*grp).gr_gid
    };
    // SAFETY: a zero-length call with a null buffer is documented to return
    // the caller's supplementary-group count without writing anything, so
    // the real buffer below is sized to exactly what the second call needs.
    let count = unsafe { libc::getgroups(0, std::ptr::null_mut()) };
    let Ok(count) = usize::try_from(count) else {
        return false;
    };
    let mut groups = vec![0u32; count];
    let Ok(capacity) = i32::try_from(groups.len()) else {
        return false;
    };
    // SAFETY: `capacity` is exactly `groups.len()`, so this write cannot
    // overflow the buffer.
    let n = unsafe { libc::getgroups(capacity, groups.as_mut_ptr()) };
    let Ok(n) = usize::try_from(n) else {
        return false;
    };
    groups.truncate(n);
    groups.contains(&gid)
}

/// Refuse early when the caller can't actually run `machinectl shell`.
///
/// Root always can. So can a `hive-admin` member, via the polkit grant
/// above. Anyone else hits an opaque polkit authentication prompt/refusal
/// from systemd that names neither hivectl nor the fix — naming both here
/// instead is the entire point of this check.
fn require_privilege() -> Result<()> {
    // SAFETY: `geteuid` takes no arguments, reads a process attribute the
    // kernel always has, and cannot fail.
    let euid = unsafe { libc::geteuid() };
    if euid != 0 && !in_hive_admin_group() {
        bail!(
            "choom needs root or `hive-admin` group membership: it runs `machinectl shell`, \
             gated by a polkit rule for that group. If you were just added to `hive-admin`, \
             log out and back in first — secondary group membership applies at login, so a \
             shell opened before the change still won't have it. `id -nG` shows what your \
             running shell actually has"
        );
    }
    Ok(())
}

/// Drop into an interactive Claude session in the agent container.
///
/// Execs `machinectl shell <name>@h-<name>` running claude as the agent
/// user from its state dir, reproducing the harness's per-turn claude
/// invocation (flags, session selection, why it never collides with the
/// live harness session). See `docs/tools/hivectl.md` (Choom) for the
/// full rationale. Inherits the caller's PTY; requires root or `hive-admin`
/// group membership, plus a running container.
///
/// The existence check goes over the daemon socket even though the exec
/// itself doesn't need it: the agents root isn't readable by the
/// operator group, so checking locally reported every rootless invocation
/// as a permission problem with the state dir rather than with the shell.
pub(crate) async fn choom(socket: &Path, name: &str, resume_session: Option<&str>) -> Result<()> {
    if !agent_exists(socket, name).await? {
        bail!(
            "no such agent: '{name}' (no state dir under {}/)",
            hive_host_sock::AGENTS_ROOT
        );
    }
    require_privilege()?;
    let container = hive_host_sock::container_name(name);
    // Enter as the agent's unix user (== agent name) so claude reads the
    // right `$HOME/.claude`.
    let target = format!("{name}@{container}");
    let claude = "/run/current-system/sw/bin/claude";
    // Bind-mounted state dir; matches `hive-agent::paths::state_dir()`.
    let state_dir = format!("/agents/{name}/state");
    // Per-turn config the harness writes; matches `paths::config_dir()`.
    let cfg = "/run/hive-config";
    // `--resume <value>` passes through as `claude --resume <value>` —
    // the rejoin-by-id surface. (claude's `--continue` is a bare flag
    // that resumes the cwd's latest session — the harness's — and would
    // consume a trailing value as the first PROMPT; choom never uses
    // it.) The value is single-quoted into the shell script, so reject
    // an embedded single quote (the only char that breaks
    // single-quoting) to rule out injection — session ids never
    // contain one.
    let session_arg = match resume_session {
        Some(val) => {
            if val.contains('\'') {
                bail!("invalid --resume value '{val}': must not contain a single quote");
            }
            format!("set -- --resume '{val}';")
        }
        None => "set --;".to_string(),
    };
    // Build claude's argv as the harness does, each flag included only
    // when its file is present so a half-up container degrades to a bare
    // session. `name` is `[a-z0-9._-]` so there's nothing to quote.
    let inner = format!(
        "cd {state_dir} || exit 1; {session_arg} \
         [ -f {cfg}/claude-settings.json ] && set -- \"$@\" --settings {cfg}/claude-settings.json; \
         [ -f {cfg}/claude-mcp-config.json ] && set -- \"$@\" --mcp-config {cfg}/claude-mcp-config.json; \
         [ -f {cfg}/claude-system-prompt.md ] && set -- \"$@\" --system-prompt-file {cfg}/claude-system-prompt.md; \
         exec {claude} \"$@\""
    );
    let mut cmd = std::process::Command::new("machinectl");
    cmd.arg("shell")
        .arg(&target)
        .arg("/bin/sh")
        .arg("-lc")
        .arg(&inner);
    // exec() replaces the current process — we inherit stdin/stdout/stderr
    // (the caller's PTY) so the Claude session is fully interactive.
    // This call only returns on error.
    let err = cmd.exec();
    Err(anyhow::anyhow!("exec machinectl: {err}"))
}