// Package host is the warp-admin HTTP server: it serves the built panel and
// proxies the few homeserver paths the panel uses.
//
// The panel runs apart from the homeserver so an operator chooses whether
// to have one at all, and where. Proxying, rather than having the browser
// call the homeserver, keeps the panel same-origin (no CORS, no homeserver
// URL baked into the build) and lets a deployment keep /_warp/admin off the
// public listener: only warp-admin needs to reach it.
package host

import (
	"bytes"
	"encoding/json"
	"errors"
	"io/fs"
	"log/slog"
	"net"
	"net/http"
	"net/http/httputil"
	"net/url"
	"path"
	"strings"
	"time"
)

// Options configures the handler.
type Options struct {
	// Homeserver is where the proxied requests go, e.g. http://warp:8008.
	Homeserver *url.URL
	// Panel is the built panel rooted at its index.html; nil serves a page
	// saying it has not been built.
	Panel fs.FS
	// Logger receives proxy errors. Defaults to slog.Default().
	Logger *slog.Logger
	// Transport overrides the proxy's round tripper, for tests.
	Transport http.RoundTripper
}

// proxiedExact are the Client-Server endpoints the panel calls for its own
// session. Nothing else of the C-S API is reachable through warp-admin, so
// it can't be used as a general client front door.
var proxiedExact = map[string]bool{
	"/_matrix/client/versions":          true,
	"/_matrix/client/v3/login":          true,
	"/_matrix/client/v3/logout":         true,
	"/_matrix/client/v3/account/whoami": true,
}

const adminPrefix = "/_warp/admin/v1/"

// contentSecurityPolicy allows the panel's own scripts, styles and fonts and
// nothing else. Inline styles are allowed because Vue binds style attributes.
const contentSecurityPolicy = "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; " +
	"img-src 'self' data: blob:; font-src 'self'; connect-src 'self'; object-src 'none'; " +
	"base-uri 'none'; form-action 'self'; frame-ancestors 'none'"

// New returns the warp-admin handler.
func New(opts Options) (http.Handler, error) {
	if opts.Homeserver == nil || opts.Homeserver.Scheme == "" || opts.Homeserver.Host == "" {
		return nil, errors.New("host: homeserver must be an absolute URL")
	}
	if opts.Logger == nil {
		opts.Logger = slog.Default()
	}

	h := &handler{opts: opts}

	transport := opts.Transport
	if transport == nil {
		transport = &http.Transport{
			Proxy:                 http.ProxyFromEnvironment,
			DialContext:           (&net.Dialer{Timeout: 10 * time.Second, KeepAlive: 30 * time.Second}).DialContext,
			ForceAttemptHTTP2:     true,
			MaxIdleConns:          32,
			IdleConnTimeout:       90 * time.Second,
			TLSHandshakeTimeout:   10 * time.Second,
			ResponseHeaderTimeout: 2 * time.Minute,
		}
	}
	upstream := opts.Homeserver
	h.proxy = &httputil.ReverseProxy{
		Rewrite: func(pr *httputil.ProxyRequest) {
			pr.SetURL(upstream)
			pr.SetXForwarded()
			// The panel has no cookies of its own to send; don't pass on any
			// a shared domain might have set.
			pr.Out.Header.Del("Cookie")
		},
		Transport: transport,
		ModifyResponse: func(res *http.Response) error {
			res.Header.Del("Set-Cookie")
			if res.Header.Get("Cache-Control") == "" {
				res.Header.Set("Cache-Control", "no-store")
			}
			return nil
		},
		ErrorHandler: func(w http.ResponseWriter, r *http.Request, err error) {
			opts.Logger.Warn("homeserver unreachable", "path", r.URL.Path, "err", err)
			writeError(w, http.StatusBadGateway, "M_UNKNOWN", "The admin service can’t reach the homeserver.")
		},
	}

	if opts.Panel != nil {
		index, err := fs.ReadFile(opts.Panel, "index.html")
		if err != nil {
			return nil, err
		}
		h.index = index
	}
	return h, nil
}

type handler struct {
	opts  Options
	proxy *httputil.ReverseProxy
	index []byte
}

func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	hdr := w.Header()
	hdr.Set("X-Content-Type-Options", "nosniff")
	hdr.Set("Referrer-Policy", "no-referrer")
	hdr.Set("X-Frame-Options", "DENY")

	p := r.URL.Path
	switch {
	case p == "/healthz":
		hdr.Set("Content-Type", "text/plain; charset=utf-8")
		hdr.Set("Cache-Control", "no-store")
		_, _ = w.Write([]byte("ok\n"))
		return

	case strings.HasPrefix(p, "/_matrix/") || strings.HasPrefix(p, "/_warp/"):
		// A path that cleans to something else (dot segments, doubled
		// slashes) is refused rather than normalised: what is allowed
		// here must be exactly what the homeserver routes.
		if path.Clean(p) != p || !(proxiedExact[p] || strings.HasPrefix(p, adminPrefix)) {
			writeError(w, http.StatusNotFound, "M_UNRECOGNIZED", "Not served by the admin service.")
			return
		}
		h.proxy.ServeHTTP(w, r)
		return
	}

	if r.Method != http.MethodGet && r.Method != http.MethodHead {
		hdr.Set("Allow", "GET, HEAD")
		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
		return
	}
	hdr.Set("Content-Security-Policy", contentSecurityPolicy)
	h.serveStatic(w, r)
}

func (h *handler) serveStatic(w http.ResponseWriter, r *http.Request) {
	if h.opts.Panel == nil {
		w.Header().Set("Content-Type", "text/html; charset=utf-8")
		w.Header().Set("Cache-Control", "no-store")
		w.WriteHeader(http.StatusServiceUnavailable)
		_, _ = w.Write([]byte(notBuiltPage))
		return
	}

	name := strings.TrimPrefix(path.Clean("/"+r.URL.Path), "/")
	if name != "" && name != "index.html" {
		if st, err := fs.Stat(h.opts.Panel, name); err == nil && st.Mode().IsRegular() {
			if strings.HasPrefix(name, "assets/") {
				// Vite puts a content hash in every file name under assets/.
				w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
			} else {
				w.Header().Set("Cache-Control", "public, max-age=3600")
			}
			http.ServeFileFS(w, r, h.opts.Panel, name)
			return
		}
		if strings.HasPrefix(name, "assets/") || (!strings.Contains(name, "/") && path.Ext(name) != "") {
			// A missing asset is a 404, not the app: answering a stale
			// script URL with HTML only produces a confusing MIME error.
			// Deeper paths are routes even with a dot in them, like
			// /users/@alice:warp.example.
			http.NotFound(w, r)
			return
		}
	}

	// Everything else is a route of the single-page app.
	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	w.Header().Set("Cache-Control", "no-cache")
	http.ServeContent(w, r, "index.html", time.Time{}, bytes.NewReader(h.index))
}

func writeError(w http.ResponseWriter, status int, errcode, msg string) {
	w.Header().Set("Content-Type", "application/json")
	w.Header().Set("Cache-Control", "no-store")
	w.WriteHeader(status)
	_ = json.NewEncoder(w).Encode(map[string]string{"errcode": errcode, "error": msg})
}

const notBuiltPage = `<!doctype html>
<html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1">
<title>Warp admin</title>
<style>body{font:14px/20px system-ui,sans-serif;max-width:560px;margin:15vh auto;padding:0 16px;color:#111114}
code{font:13px/20px ui-monospace,monospace;background:#f3f5f4;padding:1px 4px;border-radius:4px}
@media (prefers-color-scheme:dark){body{background:#0f1213;color:#eceeee}code{background:#161a1c}}</style></head>
<body><h1 style="font-size:20px">The admin panel isn’t built into this binary</h1>
<p>This warp-admin was compiled before the frontend. Build it, then the binary:</p>
<p><code>pnpm --filter @warp/admin build</code><br><code>make admin</code></p></body></html>
`