Behavision/desktop/stream_proxy.go

package main

// streamProxy serves the engine's camera feeds to this app's own webview
// without putting a credential in the page.
//
// What this replaces: StreamURL used to build
// http://user:pass@127.0.0.1:8010/api/cameras/<id>/stream.mjpeg and hand it
// to an <img>, with a comment saying the credentials were inline "so an <img>
// tag can load it". It cannot. Chromium strips credentials from subresource
// URLs and has since M59, and WebView2 is Chromium - so on the one platform
// this product ships to, every camera tile on the shop floor renders as a
// broken image. Measured against the same running engine: the app's Go-side
// calls returned stats and people while an <img> on the very same URL failed,
// and curl proved the URL itself answered 200. The engine was never the
// problem; the browser was throwing the password away before it asked.
//
// So the password stays on this side of the process boundary. The webview
// asks this loopback listener, the listener attaches Basic auth and relays
// the engine's bytes back unchanged. It is the same reasoning the head-office
// web app already follows in Shot.jsx, where an <img> equally cannot carry a
// session and the bytes are fetched and handed over as an object URL.

import (
	"context"
	"crypto/rand"
	"crypto/subtle"
	"encoding/hex"
	"fmt"
	"net"
	"net/http"
	"net/url"
	"regexp"
	"strings"
	"sync"
	"time"
)

// A camera id reaches this from the engine and from a person typing into the
// Add Camera form. Validated rather than interpolated: without this a `..`
// would climb out of the two paths below and turn a camera relay into a proxy
// for any engine endpoint, with the credential helpfully attached.
var safeCameraIDChars = regexp.MustCompile(`^[A-Za-z0-9_.-]{1,64}$`)

// safeCameraID is the character check AND the two names that pass it and still
// mean something to a path resolver.
//
// The pattern allows `.` because real camera ids contain them - which means it
// also allows exactly `.` and `..`, and `/api/cameras/../stream.mjpeg` is not
// the endpoint anyone intended. The id can never contain a slash (the path is
// split on them before we get here), so these two strings are the entire
// remaining traversal surface. Found by the test, not by reading the regex.
func safeCameraID(id string) bool {
	if id == "." || id == ".." {
		return false
	}
	return safeCameraIDChars.MatchString(id)
}

type streamProxy struct {
	mu     sync.RWMutex
	ln     net.Listener
	srv    *http.Server
	client *http.Client
	token  string
	target string // engine origin, e.g. http://127.0.0.1:8010
	user   string
	pass   string

	// Opens head office's live relay for one camera. Set on a computer that
	// is signed in, whether or not an engine runs here - which is the whole
	// point: watching a camera in another building is precisely the case
	// where there is no engine on this machine to ask.
	live func(ctx context.Context, cameraID string) (*http.Response, error)
}

func newStreamProxy() *streamProxy { return &streamProxy{} }

// start binds a loopback listener and begins relaying. Calling it again while
// running is a no-op, so a restarted engine cannot leave two listeners behind.
func (p *streamProxy) start(base, user, pass string) error {
	if !strings.HasPrefix(base, "http://") && !strings.HasPrefix(base, "https://") {
		base = "http://" + base
	}
	if _, err := url.Parse(base); err != nil {
		return fmt.Errorf("engine base %q: %w", base, err)
	}
	if err := p.bind(); err != nil {
		return err
	}
	p.mu.Lock()
	defer p.mu.Unlock()
	p.target = strings.TrimRight(base, "/")
	p.user, p.pass = user, pass
	return nil
}

// watchRemote makes the relay able to serve head office's live view, and
// binds it if nothing else has.
//
// Separate from start() because the two are independent: a shop PC has both
// an engine and a session, an owner's laptop has only a session, and a PC
// still being set up has only an engine. Folding them together would mean a
// computer with no engine could not watch a camera at all - which is the one
// computer most likely to be trying to.
func (p *streamProxy) watchRemote(fn func(context.Context, string) (*http.Response, error)) error {
	if err := p.bind(); err != nil {
		return err
	}
	p.mu.Lock()
	defer p.mu.Unlock()
	p.live = fn
	return nil
}

// bind starts the loopback listener once. Calling it again while running is a
// no-op, so neither a restarted engine nor a second sign-in can leave two
// listeners behind.
func (p *streamProxy) bind() error {
	p.mu.Lock()
	defer p.mu.Unlock()
	if p.srv != nil {
		return nil
	}

	// The engine's own credential exists precisely so that the live face feed
	// is never served open - CLAUDE.md is explicit that an unauthenticated
	// listener would expose it. An unauthenticated loopback relay would hand
	// that same feed to any other process on this PC, which on a shop counter
	// is not a theoretical set. A per-run token, minted here and given only to
	// this app's own webview, keeps the relay as private as the engine is.
	raw := make([]byte, 32)
	if _, err := rand.Read(raw); err != nil {
		return fmt.Errorf("proxy token: %w", err)
	}

	// Port 0: the OS picks a free one. A fixed port would collide with
	// whatever else a shop PC happens to be running, and the failure would be
	// "the cameras stopped working" with nothing pointing at the cause.
	ln, err := net.Listen("tcp", "127.0.0.1:0")
	if err != nil {
		return fmt.Errorf("stream proxy listen: %w", err)
	}

	p.ln = ln
	p.token = hex.EncodeToString(raw)
	// No client timeout: an MJPEG stream is endless by design and any deadline
	// would cut the picture off mid-shift. The request context ends it when
	// the webview navigates away or the tile is replaced.
	p.client = &http.Client{
		Transport: &http.Transport{
			DialContext:         (&net.Dialer{Timeout: 5 * time.Second}).DialContext,
			TLSHandshakeTimeout: 5 * time.Second,
		},
	}
	srv := &http.Server{Handler: http.HandlerFunc(p.handle)}
	p.srv = srv

	// srv and ln are captured, not read off the struct inside the goroutine:
	// stop() sets both to nil, so a serve loop that reached for them after a
	// quick start/stop would dereference nil and take the whole app down. The
	// test that stops the relay found exactly that.
	go func() { _ = srv.Serve(ln) }()
	return nil
}

func (p *streamProxy) stop() {
	p.mu.Lock()
	srv, ln := p.srv, p.ln
	p.srv, p.ln, p.token, p.live = nil, nil, "", nil
	p.mu.Unlock()
	if srv != nil {
		_ = srv.Close()
	}
	if ln != nil {
		_ = ln.Close()
	}
}

// urlFor returns the loopback URL for one camera resource, or "" when the
// proxy is not running so the caller can fall back.
func (p *streamProxy) urlFor(cameraID, file string) string {
	p.mu.RLock()
	defer p.mu.RUnlock()
	if p.ln == nil || p.token == "" || !safeCameraID(cameraID) {
		return ""
	}
	return fmt.Sprintf("http://%s/s/%s/%s/%s",
		p.ln.Addr().String(), p.token, cameraID, file)
}

func (p *streamProxy) handle(w http.ResponseWriter, r *http.Request) {
	p.mu.RLock()
	token, target, user, pass, client := p.token, p.target, p.user, p.pass, p.client
	liveFn := p.live
	p.mu.RUnlock()
	if token == "" || client == nil {
		http.NotFound(w, r)
		return
	}

	// /s/<token>/<camera>/<file>
	parts := strings.Split(strings.TrimPrefix(r.URL.Path, "/"), "/")
	if len(parts) != 4 || parts[0] != "s" {
		http.NotFound(w, r)
		return
	}
	// Constant time: the token is the only thing standing between another
	// local process and a live view of customers' faces.
	if subtle.ConstantTimeCompare([]byte(parts[1]), []byte(token)) != 1 {
		// 404 rather than 403. There is nothing here to tell an unwelcome
		// caller they have found the right door with the wrong key.
		http.NotFound(w, r)
		return
	}
	cameraID := parts[2]
	if !safeCameraID(cameraID) {
		http.NotFound(w, r)
		return
	}

	// An allow-list, not a prefix match. Everything else the engine serves -
	// the identity list, the gallery, erasure - stays unreachable through here
	// even for a caller holding the token.
	//
	// frame.jpg is listed although no screen asks for one yet. It is reachable
	// only through urlFor, which is internal, so it adds no bound API nobody
	// calls; it is here so that adding a still later is a change to a screen
	// rather than a change to the one file where a mistake is a credentialed
	// proxy onto the biometric API.
	// Head office's relay, not the engine. The two are different machines and
	// different credentials, so this returns rather than falling through.
	if parts[3] == "live.mjpeg" {
		if liveFn == nil {
			http.Error(w, "not signed in to head office", http.StatusBadGateway)
			return
		}
		p.relayRemote(w, r, cameraID, liveFn)
		return
	}

	var enginePath string
	switch parts[3] {
	case "stream.mjpeg":
		enginePath = "/api/cameras/" + cameraID + "/stream.mjpeg"
	case "frame.jpg":
		enginePath = "/api/cameras/" + cameraID + "/frame.jpg"
	default:
		http.NotFound(w, r)
		return
	}

	req, err := http.NewRequestWithContext(r.Context(), http.MethodGet, target+enginePath, nil)
	if err != nil {
		http.Error(w, "bad upstream request", http.StatusInternalServerError)
		return
	}
	// frame.jpg takes width and quality; the engine re-encodes on demand.
	req.URL.RawQuery = r.URL.RawQuery
	if user != "" {
		req.SetBasicAuth(user, pass)
	}

	resp, err := client.Do(req)
	if err != nil {
		http.Error(w, "engine unreachable", http.StatusBadGateway)
		return
	}
	defer resp.Body.Close()

	for _, h := range []string{"Content-Type", "Cache-Control", "Pragma", "Expires"} {
		if v := resp.Header.Get(h); v != "" {
			w.Header().Set(h, v)
		}
	}
	w.WriteHeader(resp.StatusCode)

	// Copied by hand rather than with io.Copy so every chunk is flushed. An
	// MJPEG stream never ends, so anything buffered waiting for a full buffer
	// is a tile that stays blank forever - which is the same symptom as the
	// bug this file exists to fix, and would look like it had not worked.
	flusher, _ := w.(http.Flusher)
	buf := make([]byte, 32*1024)
	for {
		n, rerr := resp.Body.Read(buf)
		if n > 0 {
			if _, werr := w.Write(buf[:n]); werr != nil {
				return // webview went away
			}
			if flusher != nil {
				flusher.Flush()
			}
		}
		if rerr != nil {
			return
		}
	}
}