frame_host/

server.rs

//! Static shell server for the browser client bundle.
//!
//! Serves exactly one directory of static files (the demo client's `dist/`:
//! `index.html` at the root plus hashed bundle files, typically under
//! `assets/`, including the `GraphMother` `.wasm`). Resolution is exact:
//!
//! - `GET /` serves `index.html`; every other path must name a real file.
//! - A missing asset is a typed, loud 404 page — NEVER an index.html
//!   fallback. SPA fallbacks silently mask broken asset paths (a `.wasm`
//!   request answered with HTML burned this exact demo once already).
//! - Traversal (`..`, absolute components, NUL) and malformed encodings are
//!   refused with typed 400 pages; symlink escapes are refused by
//!   canonical-prefix checking.
//!
//! `GET /frame/config.json` surfaces the bus endpoint to the page (as
//! `busEndpoint`, plus the deprecated duplicate `liminalEndpoint` for one
//! compatibility window). Per D3 the browser connects to the bus directly as
//! a participant; this server carries no feed bytes.
//!
//! `GET /frame/app/status.json` (2026-07-21 boot-visibility fix, see
//! [`crate::truth`]) serves the process's CURRENT application-truth
//! snapshot: every lifecycle transition and announced fact observed so far,
//! in order. A client fetches this on the SAME surface as `/frame/config.json`
//! — reachable the instant this server binds, before the announcer pump has
//! drained a single event onto the bus — then follows the live channel for
//! what happens next.
//!
//! `GET /frame/bus/{health,ready,metrics}` are the same-origin proxy
//! routes for the embedded bus health listener ([`crate::bus_proxy`]):
//! observability HTTP only, never feed bytes, forwarded verbatim with typed
//! bounded failure. The pre-rename routes `/frame/liminal/*` answer as
//! deprecated aliases (same handlers, loud per-hit `tracing::warn!`) for the
//! same window.

use std::net::SocketAddr;
use std::path::{Path, PathBuf};
use std::sync::Arc;

use axum::Router;
use axum::body::Body;
use axum::extract::{Request, State};
use axum::http::{HeaderValue, Method, StatusCode, header};
use axum::response::{IntoResponse, Response};
use axum::routing::get;
use serde::Serialize;
use tokio::net::TcpListener;

use crate::bus_proxy::{
    BUS_HEALTH_ROUTE, BUS_METRICS_ROUTE, BUS_READY_ROUTE, LEGACY_LIMINAL_HEALTH_ROUTE,
    LEGACY_LIMINAL_METRICS_ROUTE, LEGACY_LIMINAL_READY_ROUTE, PRODUCTION_DEADLINES,
    bus_metrics_body, failure_response, forward_health_request, passthrough_response,
};
use crate::error::HostError;
use crate::truth::AppTruth;

/// Route on which the page fetches its runtime configuration.
pub const CONFIG_ROUTE: &str = "/frame/config.json";

/// Route on which a client fetches the CURRENT application-truth snapshot
/// (2026-07-21 boot-visibility fix, [`crate::truth`]): every lifecycle
/// transition and announced fact observed so far, in order. Named alongside
/// [`CONFIG_ROUTE`] under the same `/frame/` namespace, matching this
/// module's own route-naming idiom.
pub const APP_STATUS_ROUTE: &str = "/frame/app/status.json";

/// Configuration for one shell server instance. No field has a default.
#[derive(Clone, Debug)]
pub struct ShellConfig {
    /// Socket address to bind.
    pub bind: SocketAddr,
    /// Directory containing the static client bundle.
    pub asset_root: PathBuf,
    /// Bus WebSocket endpoint surfaced verbatim to the page (D3).
    pub bus_endpoint: String,
    /// Bearer token surfaced verbatim as `authToken`; empty = open server.
    pub auth_token: String,
    /// Feed channel; omitted from the served config when `None` so the shell
    /// applies the SDK's `DEFAULT_FEED_CHANNEL`. Stays the console's PRIMARY
    /// (default-selected) channel.
    pub channel: Option<String>,
    /// The FULL channel list of the embedded liminal component, served verbatim
    /// as `channels` — the console subscribes every one of them. Must be
    /// non-empty, duplicate-free, all entries non-empty, and contain `channel`
    /// when one is provided.
    pub channels: Vec<String>,
    /// The embedded bus health listener's real bound address — the target
    /// of the same-origin `/frame/bus/*` proxy routes.
    pub bus_health: SocketAddr,
    /// The optional document-service advert (IRIDIUM-A3 C2's config.json
    /// growth): served as `document` when `[document]` is configured.
    pub document: Option<DocumentAdvert>,
    /// The process's application-truth recorder (2026-07-21
    /// boot-visibility fix): backs [`APP_STATUS_ROUTE`]. See [`crate::truth`].
    pub truth: Arc<AppTruth>,
}

/// What the authoring page needs to bind the `frame:code-editor@v1`
/// component: the document id, the component instance, and the two
/// channels (C2: feed + authoring per document).
#[derive(Clone, Debug, Serialize)]
#[serde(rename_all = "camelCase")]
pub struct DocumentAdvert {
    /// The bound document id.
    pub id: String,
    /// The component instance id on every feed envelope.
    pub component_id: String,
    /// The feed channel (service publishes; pages subscribe).
    pub feed_channel: String,
    /// The authoring channel (pages publish; the service subscribes).
    pub authoring_channel: String,
    /// The component's declared cursor-blink interval (T4), milliseconds.
    pub blink_interval_ms: u64,
}

/// Runtime configuration served to the page at [`CONFIG_ROUTE`], matching
/// the shell's validation in `examples/frame-demo/src/config.ts` exactly:
/// non-empty `busEndpoint`, string `authToken` (empty = open server),
/// `channel` present only when the operator provided one, and `channels` —
/// the embedded bus's full configured channel list — always present.
#[derive(Clone, Debug, Serialize)]
#[serde(rename_all = "camelCase")]
struct ShellRuntimeConfig {
    /// The embedded bus's WebSocket endpoint — the primary field.
    bus_endpoint: String,
    /// DEPRECATED duplicate of `bus_endpoint`, served as `liminalEndpoint`
    /// for one compatibility window; remove with the `[liminal]` config alias.
    liminal_endpoint: String,
    auth_token: String,
    #[serde(skip_serializing_if = "Option::is_none")]
    channel: Option<String>,
    channels: Vec<String>,
    /// The document-service advert; absent when no `[document]` is
    /// configured (the page fails loudly if it needs one).
    #[serde(skip_serializing_if = "Option::is_none")]
    document: Option<DocumentAdvert>,
}

struct ShellState {
    asset_root: PathBuf,
    config: ShellRuntimeConfig,
    bus_health: SocketAddr,
    truth: Arc<AppTruth>,
}

/// A bound-but-not-yet-serving shell server.
pub struct ShellServer {
    listener: TcpListener,
    router: Router,
    addr: SocketAddr,
}

impl ShellServer {
    /// Validates the asset root and the served config contract, then binds
    /// the listener.
    ///
    /// # Errors
    ///
    /// Refuses an unreadable or non-directory asset root, a root without
    /// `index.html`, a config the shell's contract would refuse (empty
    /// `busEndpoint`, empty provided `channel`), and any bind failure —
    /// all typed. Serving a config the shell will refuse at parse time is a
    /// composition error the host catches here, not in the browser.
    pub async fn bind(config: ShellConfig) -> Result<Self, HostError> {
        let bind_addr = config.bind;
        let router = Self::prepare(config)?;
        let listener = TcpListener::bind(bind_addr)
            .await
            .map_err(|source| HostError::Bind {
                addr: bind_addr,
                source,
            })?;
        let addr = listener.local_addr().map_err(|source| HostError::Bind {
            addr: bind_addr,
            source,
        })?;
        Ok(Self {
            listener,
            router,
            addr,
        })
    }

    /// Adopts an ALREADY-BOUND listener (from [`crate::page::PageServer`])
    /// instead of binding its own, then validates the served config contract
    /// exactly as [`Self::bind`] does.
    ///
    /// The page-server port is resolved and BOUND up front (prefer-and-walk, or
    /// stated-and-exact) before the embedded bus boots, so its real address can
    /// seed the bus's derived origin allow-list. Handing the held listener
    /// straight here — rather than probing a port, dropping it, and re-binding
    /// — is what makes that resolution race-free: no other process can claim
    /// the port in between. `config.bind` is ignored (the listener is already
    /// bound); the real bound address is read from the listener itself.
    ///
    /// # Errors
    ///
    /// Refuses the same config-contract and asset-root violations as
    /// [`Self::bind`], and any failure converting the adopted std listener to a
    /// non-blocking async listener.
    pub fn adopt(listener: std::net::TcpListener, config: ShellConfig) -> Result<Self, HostError> {
        let router = Self::prepare(config)?;
        listener.set_nonblocking(true).map_err(|source| {
            let addr = listener
                .local_addr()
                .unwrap_or_else(|_| std::net::SocketAddr::from(([127, 0, 0, 1], 0)));
            HostError::Bind { addr, source }
        })?;
        let addr = listener.local_addr().map_err(|source| HostError::Bind {
            addr: std::net::SocketAddr::from(([127, 0, 0, 1], 0)),
            source,
        })?;
        let listener =
            TcpListener::from_std(listener).map_err(|source| HostError::Bind { addr, source })?;
        Ok(Self {
            listener,
            router,
            addr,
        })
    }

    /// Validates the served config contract and asset root, then builds the
    /// router — the shared body of [`Self::bind`] and [`Self::adopt`], which
    /// differ only in where the listener comes from.
    fn prepare(config: ShellConfig) -> Result<Router, HostError> {
        if config.bus_endpoint.is_empty() {
            return Err(HostError::ConfigContract {
                detail: "the bus endpoint is empty: it is derived from the embedded \
                         WebSocket listener's bound address, which must never be empty — this is \
                         an internal error, not an operator one"
                    .to_owned(),
            });
        }
        if let Some(channel) = &config.channel
            && channel.is_empty()
        {
            return Err(HostError::ConfigContract {
                detail: "[frame].channel, when set, must be non-empty: the console refuses an \
                         empty channel with CONFIG_INVALID; omit the key to use the SDK's default \
                         channel"
                    .to_owned(),
            });
        }
        validate_channels_contract(&config.channels, config.channel.as_deref())?;
        let asset_root =
            config
                .asset_root
                .canonicalize()
                .map_err(|error| HostError::AssetRoot {
                    path: config.asset_root.clone(),
                    detail: error.to_string(),
                })?;
        if !asset_root.is_dir() {
            return Err(HostError::AssetRoot {
                path: config.asset_root.clone(),
                detail: "not a directory".to_owned(),
            });
        }
        if !asset_root.join("index.html").is_file() {
            return Err(HostError::MissingIndex {
                path: config.asset_root.clone(),
            });
        }
        let state = Arc::new(ShellState {
            asset_root,
            config: ShellRuntimeConfig {
                bus_endpoint: config.bus_endpoint.clone(),
                // Deprecated duplicate, same value — one compatibility window.
                liminal_endpoint: config.bus_endpoint,
                auth_token: config.auth_token,
                channel: config.channel,
                channels: config.channels,
                document: config.document,
            },
            bus_health: config.bus_health,
            truth: config.truth,
        });
        Ok(Router::new()
            .route(CONFIG_ROUTE, get(serve_config))
            .route(APP_STATUS_ROUTE, get(serve_app_status))
            .route(BUS_HEALTH_ROUTE, get(proxy_bus_health))
            .route(BUS_READY_ROUTE, get(proxy_bus_ready))
            .route(BUS_METRICS_ROUTE, get(proxy_bus_metrics))
            // DEPRECATED aliases (compatibility window): same handlers, loud
            // per-hit deprecation warning — never a silent legacy acceptance.
            .route(LEGACY_LIMINAL_HEALTH_ROUTE, get(proxy_legacy_health))
            .route(LEGACY_LIMINAL_READY_ROUTE, get(proxy_legacy_ready))
            .route(LEGACY_LIMINAL_METRICS_ROUTE, get(proxy_legacy_metrics))
            .fallback(serve_asset)
            .with_state(state))
    }

    /// The address actually bound (resolves an ephemeral port request).
    #[must_use]
    pub const fn local_addr(&self) -> SocketAddr {
        self.addr
    }

    /// Serves until `shutdown` resolves.
    ///
    /// # Errors
    ///
    /// Propagates accept-loop failures as typed serve errors.
    pub async fn serve(
        self,
        shutdown: impl Future<Output = ()> + Send + 'static,
    ) -> Result<(), HostError> {
        axum::serve(self.listener, self.router)
            .with_graceful_shutdown(shutdown)
            .await
            .map_err(|source| HostError::Serve { source })
    }
}

/// Refuses a served `channels` roster the console's own config contract would
/// reject: it must be non-empty (the console subscribes every entry), every
/// entry must be non-empty, entries must be unique (a duplicate would open two
/// identical subscriptions — liminal's own validator refuses the same shape),
/// and it must contain `channel` when one is provided (the console refuses a
/// roster that omits its primary channel with `CONFIG_INVALID`).
fn validate_channels_contract(channels: &[String], channel: Option<&str>) -> Result<(), HostError> {
    if channels.is_empty() {
        return Err(HostError::ConfigContract {
            detail: "the served channels roster is empty: the console subscribes every entry of \
                     `channels`, so an empty roster leaves it with no feed — it is derived from \
                     [bus].channels, which must declare at least one channel"
                .to_owned(),
        });
    }
    let mut seen = std::collections::HashSet::new();
    for entry in channels {
        if entry.is_empty() {
            return Err(HostError::ConfigContract {
                detail: "the served channels roster contains an empty channel name: the console \
                         refuses an empty channel with CONFIG_INVALID"
                    .to_owned(),
            });
        }
        if !seen.insert(entry.as_str()) {
            return Err(HostError::ConfigContract {
                detail: format!(
                    "the served channels roster names \"{entry}\" more than once: a duplicate \
                     would open two identical subscriptions, and liminal's own config validation \
                     refuses duplicate channel names"
                ),
            });
        }
    }
    if let Some(channel) = channel
        && !channels.iter().any(|entry| entry == channel)
    {
        return Err(HostError::ConfigContract {
            detail: format!(
                "the served channels roster [{roster}] does not contain the primary channel \
                 \"{channel}\": the console refuses that pair with CONFIG_INVALID",
                roster = channels.join(", ")
            ),
        });
    }
    Ok(())
}

async fn serve_config(State(state): State<Arc<ShellState>>) -> Response {
    tracing::info!(route = CONFIG_ROUTE, "serving shell runtime configuration");
    axum::Json(state.config.clone()).into_response()
}

/// Serves the CURRENT application-truth snapshot (2026-07-21
/// boot-visibility fix, [`crate::truth`]): every lifecycle transition and
/// announced fact this process has produced so far, in order. Reachable the
/// instant this server binds — before the announcer pump has drained a
/// single event onto the bus — so a client can observe the full boot
/// history without racing the bus at all.
async fn serve_app_status(State(state): State<Arc<ShellState>>) -> Response {
    let snapshot = state.truth.snapshot();
    tracing::info!(
        route = APP_STATUS_ROUTE,
        transitions = snapshot.transitions.len(),
        facts = snapshot.facts.len(),
        "serving application-truth snapshot"
    );
    axum::Json(snapshot).into_response()
}

async fn proxy_bus_health(State(state): State<Arc<ShellState>>) -> Response {
    proxy_bus(&state, BUS_HEALTH_ROUTE, "/health").await
}

async fn proxy_bus_ready(State(state): State<Arc<ShellState>>) -> Response {
    proxy_bus(&state, BUS_READY_ROUTE, "/ready").await
}

async fn proxy_bus_metrics(State(state): State<Arc<ShellState>>) -> Response {
    proxy_bus(&state, BUS_METRICS_ROUTE, "/metrics").await
}

async fn proxy_legacy_health(State(state): State<Arc<ShellState>>) -> Response {
    warn_legacy_route(LEGACY_LIMINAL_HEALTH_ROUTE, BUS_HEALTH_ROUTE);
    proxy_bus(&state, LEGACY_LIMINAL_HEALTH_ROUTE, "/health").await
}

async fn proxy_legacy_ready(State(state): State<Arc<ShellState>>) -> Response {
    warn_legacy_route(LEGACY_LIMINAL_READY_ROUTE, BUS_READY_ROUTE);
    proxy_bus(&state, LEGACY_LIMINAL_READY_ROUTE, "/ready").await
}

async fn proxy_legacy_metrics(State(state): State<Arc<ShellState>>) -> Response {
    warn_legacy_route(LEGACY_LIMINAL_METRICS_ROUTE, BUS_METRICS_ROUTE);
    proxy_bus(&state, LEGACY_LIMINAL_METRICS_ROUTE, "/metrics").await
}

/// The loud, per-hit deprecation on a legacy `/frame/liminal/*` alias — a
/// legacy-name acceptance is never silent.
fn warn_legacy_route(route: &'static str, replacement: &'static str) {
    tracing::warn!(
        route,
        replacement,
        "DEPRECATED route served: /frame/liminal/* is the legacy alias of /frame/bus/*; update \
         the client to the bus route — the alias is removed after one compatibility window"
    );
}

/// Forwards one same-origin health-proxy request to the embedded bus
/// health listener, passing the upstream response through verbatim (the
/// metrics body alone is re-vocabularied by [`bus_metrics_body`]) and
/// rendering any forwarding failure as its typed, bounded 502 body.
async fn proxy_bus(state: &ShellState, route: &'static str, upstream_path: &str) -> Response {
    match forward_health_request(state.bus_health, upstream_path, PRODUCTION_DEADLINES).await {
        Ok(mut upstream) => {
            if upstream_path == "/metrics" {
                upstream.body = bus_metrics_body(&upstream.body);
            }
            tracing::info!(
                route,
                upstream_path,
                status = upstream.status,
                bytes = upstream.body.len(),
                "proxied bus health route"
            );
            passthrough_response(upstream)
        }
        Err(failure) => {
            tracing::error!(
                route,
                upstream_path,
                code = failure.code(),
                detail = failure.detail(),
                "bus health proxy failed typed"
            );
            failure_response(upstream_path, &failure)
        }
    }
}

async fn serve_asset(State(state): State<Arc<ShellState>>, request: Request) -> Response {
    let method = request.method().clone();
    let raw_path = request.uri().path().to_owned();
    if method != Method::GET && method != Method::HEAD {
        tracing::warn!(%method, path = %raw_path, "method not allowed on static shell");
        return error_page(
            StatusCode::METHOD_NOT_ALLOWED,
            "METHOD NOT ALLOWED",
            &format!("The static shell serves GET and HEAD only; {method} was refused."),
        );
    }
    let relative = match sanitize_request_path(&raw_path) {
        Ok(relative) => relative,
        Err(refusal) => {
            tracing::warn!(path = %raw_path, %refusal, "refusing malformed asset path");
            return error_page(StatusCode::BAD_REQUEST, "BAD ASSET PATH", &refusal);
        }
    };
    let candidate = state.asset_root.join(&relative);
    let resolved = match candidate.canonicalize() {
        Ok(resolved) => resolved,
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => {
            tracing::warn!(path = %raw_path, "asset not found");
            return not_found_page(&raw_path, &state.asset_root);
        }
        Err(error) => {
            tracing::error!(path = %raw_path, %error, "asset resolution failed");
            return error_page(
                StatusCode::INTERNAL_SERVER_ERROR,
                "ASSET RESOLUTION FAILED",
                &format!("Resolving {raw_path} failed: {error}"),
            );
        }
    };
    if !resolved.starts_with(&state.asset_root) {
        tracing::warn!(path = %raw_path, resolved = %resolved.display(), "asset escapes the asset root");
        return error_page(
            StatusCode::BAD_REQUEST,
            "BAD ASSET PATH",
            "The requested path resolves outside the configured asset directory.",
        );
    }
    if !resolved.is_file() {
        tracing::warn!(path = %raw_path, "asset path is not a file");
        return not_found_page(&raw_path, &state.asset_root);
    }
    match tokio::fs::read(&resolved).await {
        Ok(bytes) => {
            let mime = mime_for(&resolved);
            tracing::info!(path = %raw_path, bytes = bytes.len(), mime, "served asset");
            let mut response = Response::new(Body::from(bytes));
            response
                .headers_mut()
                .insert(header::CONTENT_TYPE, HeaderValue::from_static(mime));
            response
        }
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => {
            tracing::warn!(path = %raw_path, "asset vanished between resolution and read");
            not_found_page(&raw_path, &state.asset_root)
        }
        Err(error) => {
            tracing::error!(path = %raw_path, %error, "asset read failed");
            error_page(
                StatusCode::INTERNAL_SERVER_ERROR,
                "ASSET READ FAILED",
                &format!("Reading {raw_path} failed: {error}"),
            )
        }
    }
}

/// Decodes and validates a request path into a root-relative file path.
///
/// `/` maps to `index.html`. Every component must be a plain name: empty,
/// `.`, `..`, NUL-bearing, and backslash-bearing components are refused, as
/// is any malformed percent-encoding.
fn sanitize_request_path(raw: &str) -> Result<PathBuf, String> {
    let decoded = percent_decode(raw)?;
    let Some(stripped) = decoded.strip_prefix('/') else {
        return Err("request path must start with '/'".to_owned());
    };
    if stripped.is_empty() {
        return Ok(PathBuf::from("index.html"));
    }
    let mut relative = PathBuf::new();
    for component in stripped.split('/') {
        if component.is_empty() {
            return Err("empty path component (doubled or trailing '/')".to_owned());
        }
        if component == "." || component == ".." {
            return Err("relative path components are refused".to_owned());
        }
        if component.contains('\0') || component.contains('\\') {
            return Err("path component contains a refused character".to_owned());
        }
        relative.push(component);
    }
    Ok(relative)
}

/// Strict RFC 3986 percent-decoding: every `%` must head two hex digits and
/// the decoded bytes must be valid UTF-8.
fn percent_decode(raw: &str) -> Result<String, String> {
    if !raw.contains('%') {
        return Ok(raw.to_owned());
    }
    let bytes = raw.as_bytes();
    let mut decoded = Vec::with_capacity(bytes.len());
    let mut index = 0;
    while index < bytes.len() {
        if bytes[index] == b'%' {
            let (Some(high), Some(low)) = (
                bytes.get(index + 1).and_then(|byte| hex_value(*byte)),
                bytes.get(index + 2).and_then(|byte| hex_value(*byte)),
            ) else {
                return Err("malformed percent-encoding".to_owned());
            };
            decoded.push(high * 16 + low);
            index += 3;
        } else {
            decoded.push(bytes[index]);
            index += 1;
        }
    }
    String::from_utf8(decoded).map_err(|_| "percent-encoding decodes to invalid UTF-8".to_owned())
}

const fn hex_value(byte: u8) -> Option<u8> {
    match byte {
        b'0'..=b'9' => Some(byte - b'0'),
        b'a'..=b'f' => Some(byte - b'a' + 10),
        b'A'..=b'F' => Some(byte - b'A' + 10),
        _ => None,
    }
}

/// Explicit MIME map. `application/wasm` is load-bearing: browsers refuse
/// `WebAssembly.instantiateStreaming` on anything else.
fn mime_for(path: &Path) -> &'static str {
    let extension = path
        .extension()
        .and_then(|extension| extension.to_str())
        .map(str::to_ascii_lowercase);
    match extension.as_deref() {
        Some("html") => "text/html; charset=utf-8",
        Some("js" | "mjs") => "text/javascript; charset=utf-8",
        Some("css") => "text/css; charset=utf-8",
        Some("wasm") => "application/wasm",
        Some("json" | "map") => "application/json; charset=utf-8",
        Some("svg") => "image/svg+xml",
        Some("png") => "image/png",
        Some("jpg" | "jpeg") => "image/jpeg",
        Some("gif") => "image/gif",
        Some("webp") => "image/webp",
        Some("ico") => "image/x-icon",
        Some("txt") => "text/plain; charset=utf-8",
        Some("woff") => "font/woff",
        Some("woff2") => "font/woff2",
        Some("ttf") => "font/ttf",
        Some("otf") => "font/otf",
        _ => "application/octet-stream",
    }
}

fn not_found_page(raw_path: &str, asset_root: &Path) -> Response {
    error_page(
        StatusCode::NOT_FOUND,
        "ASSET NOT FOUND",
        &format!(
            "No file named {raw_path} exists under the configured asset directory \
             {root}. This host never falls back to index.html: a missing asset is \
             this loud page, not a silently mis-served shell.",
            root = asset_root.display()
        ),
    )
}

fn error_page(status: StatusCode, title: &str, detail: &str) -> Response {
    let body = format!(
        "<!doctype html>\n<html lang=\"en\">\n<head><meta charset=\"utf-8\">\
         <title>frame-host: {status} {title}</title></head>\n<body style=\"font-family: monospace; \
         background: #1a0505; color: #ffb4b4; padding: 2rem;\">\n<h1>FRAME HOST — {status} {title}</h1>\n\
         <p>{detail}</p>\n</body>\n</html>\n",
        status = status.as_u16(),
    );
    let mut response = Response::new(Body::from(body));
    *response.status_mut() = status;
    response.headers_mut().insert(
        header::CONTENT_TYPE,
        HeaderValue::from_static("text/html; charset=utf-8"),
    );
    response
}

#[cfg(test)]
mod tests {
    use super::{mime_for, percent_decode, sanitize_request_path, validate_channels_contract};
    use crate::error::HostError;
    use std::path::{Path, PathBuf};

    fn roster(entries: &[&str]) -> Vec<String> {
        entries.iter().map(|entry| (*entry).to_owned()).collect()
    }

    /// The channels contract: non-empty roster, non-empty unique entries, and
    /// primary-channel membership — each refusal typed and loud.
    #[test]
    fn channels_contract_accepts_valid_rosters() {
        assert!(validate_channels_contract(&roster(&["telemetry.stream"]), None).is_ok());
        assert!(
            validate_channels_contract(
                &roster(&["telemetry.stream", "telemetry.east"]),
                Some("telemetry.east"),
            )
            .is_ok()
        );
    }

    #[test]
    fn channels_contract_refuses_empty_roster() {
        let refusal = validate_channels_contract(&[], None);
        assert!(matches!(refusal, Err(HostError::ConfigContract { .. })));
    }

    #[test]
    fn channels_contract_refuses_empty_and_duplicate_entries()
    -> Result<(), Box<dyn std::error::Error>> {
        let empty_entry = validate_channels_contract(&roster(&["telemetry.stream", ""]), None);
        assert!(matches!(empty_entry, Err(HostError::ConfigContract { .. })));

        let duplicate = validate_channels_contract(
            &roster(&["telemetry.stream", "telemetry.stream"]),
            Some("telemetry.stream"),
        );
        let Err(HostError::ConfigContract { detail }) = duplicate else {
            return Err("expected ConfigContract for a duplicate entry".into());
        };
        assert!(
            detail.contains("more than once"),
            "duplicate refusal must state the duplication: {detail}"
        );
        Ok(())
    }

    #[test]
    fn channels_contract_refuses_a_primary_outside_the_roster()
    -> Result<(), Box<dyn std::error::Error>> {
        let refusal = validate_channels_contract(
            &roster(&["telemetry.stream", "telemetry.east"]),
            Some("telemetry.west"),
        );
        let Err(HostError::ConfigContract { detail }) = refusal else {
            return Err("expected ConfigContract for a foreign primary".into());
        };
        assert!(
            detail.contains("telemetry.west"),
            "refusal must name the missing primary: {detail}"
        );
        Ok(())
    }

    #[test]
    fn root_maps_to_index_only() {
        assert_eq!(
            sanitize_request_path("/").ok(),
            Some(PathBuf::from("index.html"))
        );
        assert_eq!(
            sanitize_request_path("/assets/app.js").ok(),
            Some(PathBuf::from("assets/app.js"))
        );
    }

    #[test]
    fn traversal_and_malformed_paths_are_refused() {
        assert!(sanitize_request_path("/../secret").is_err());
        assert!(sanitize_request_path("/a/../b").is_err());
        assert!(sanitize_request_path("/%2e%2e/secret").is_err());
        assert!(sanitize_request_path("//double").is_err());
        assert!(sanitize_request_path("/a/./b").is_err());
        assert!(sanitize_request_path("/a%00b").is_err());
        assert!(sanitize_request_path("/a%zz").is_err());
        assert!(sanitize_request_path("no-leading-slash").is_err());
    }

    #[test]
    fn percent_decoding_is_strict() {
        assert_eq!(percent_decode("/plain").ok().as_deref(), Some("/plain"));
        assert_eq!(percent_decode("/a%20b").ok().as_deref(), Some("/a b"));
        assert!(percent_decode("/broken%2").is_err());
        assert!(percent_decode("/broken%").is_err());
        assert!(percent_decode("/bad%ff%fe").is_err());
    }

    #[test]
    fn wasm_and_core_types_map_exactly() {
        assert_eq!(mime_for(Path::new("a/app.wasm")), "application/wasm");
        assert_eq!(
            mime_for(Path::new("index.html")),
            "text/html; charset=utf-8"
        );
        assert_eq!(
            mime_for(Path::new("assets/index-abc.js")),
            "text/javascript; charset=utf-8"
        );
        assert_eq!(mime_for(Path::new("noext")), "application/octet-stream");
    }
}