//! Per-agent `send` allow-list enforcement. Driven by the operator config at
//! `/etc/hyperhive/send-allow.json` (written by the NixOS harness module); the
//! MCP server calls [`check_send_allowed`] before forwarding a `send` tool call
//! so a blocked recipient surfaces as a claude-readable tool result rather than
//! a silent drop.

/// Where the NixOS module writes the per-agent send allow-list (see
/// `nix/templates/harness/`). Empty list = unrestricted (the
/// default). Non-empty list constrains `mcp__hyperhive__send`'s `to`
/// field; the manager is always implicitly permitted regardless of
/// the list contents.
const SEND_ALLOW_PATH: &str = "/etc/hyperhive/send-allow.json";

/// Enforce the per-agent send allow-list. Returns `Ok` when the
/// recipient is permitted (no list configured, `<parent>` sentinel
/// always allowed, or `to` is in the list); returns `Err(refusal)`
/// with a claude-readable string when blocked — the harness surfaces
/// the refusal as the tool result so claude knows the message didn't
/// land and can react (e.g. route via `<parent>` instead).
pub fn check_send_allowed(to: &str) -> Result<(), String> {
    if to == hive_sh4re::PARENT_RECIPIENT {
        // Always allow `<parent>` — the allow-list constrains peer
        // chatter, not the structural reporting line; the operator
        // can rewire who the parent IS via `set_parent` without
        // having to remember to update the per-agent allow-list.
        // The broker resolves the sentinel to the real parent label
        // on the host side per topology.json (falls back to `operator`
        // for root agents).
        return Ok(());
    }
    let Ok(raw) = std::fs::read_to_string(SEND_ALLOW_PATH) else {
        return Ok(()); // file missing → no policy configured → unrestricted
    };
    let allow: Vec<String> = match serde_json::from_str(&raw) {
        Ok(v) => v,
        Err(e) => {
            tracing::warn!(
                path = SEND_ALLOW_PATH,
                error = ?e,
                "send allow-list parse failed; falling back to unrestricted",
            );
            return Ok(());
        }
    };
    if allow.is_empty() {
        return Ok(()); // empty list = unrestricted (back-compat)
    }
    if allow.iter().any(|n| n == to) {
        return Ok(());
    }
    Err(format!(
        "send refused: recipient '{to}' not in hyperhive.allowedRecipients \
         (configured in agent.nix). Allowed: {allow:?}. Your structural \
         parent is always reachable — route through `send(to: \"{}\", …)` \
         if you need to reach someone outside the allow-list.",
        hive_sh4re::PARENT_RECIPIENT
    ))
}