Behavision/server/internal/api/handlers_live.go

package api

import (
	"encoding/base64"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"net/http"
	"time"
)

const (
	// liveMaxFrame bounds one frame. The agent re-encodes to ~640 px before
	// sending, so a frame is tens of KB; 1 MB is a ceiling, not a target.
	liveMaxFrame = 1 << 20
	// liveSession caps one push. A browser tab left open for a week must not
	// leave a shop uploading for a week; the agent simply asks again while
	// anyone is still watching, so the cap costs a reconnect, not the stream.
	liveSession = 5 * time.Minute
	// liveWaitForWork is how long the agent's poll is held open. Long enough
	// that an idle site makes ~2 requests a minute; short enough to sit well
	// inside any proxy's idle timeout.
	liveWaitForWork = 25 * time.Second
)

// handleWatchLive streams one camera's frames to a signed-in user over SSE.
//
// SSE rather than serving MJPEG directly, for the same reason the snapshot is
// not a signed link: an <img> cannot send an Authorization header, and minting
// a URL that works without a session - for LIVE video of a shop floor, no less
// - would be a much worse trade than the 33% base64 costs.
func (s *Server) handleWatchLive(w http.ResponseWriter, r *http.Request) {
	p := PrincipalFrom(r.Context())
	id, ok := s.resolveCamera(w, r, r.PathValue("id"))
	if !ok {
		return
	}
	// Ownership is checked HERE, once, before anything is streamed. Everything
	// after this point is keyed on a camera id, and a hub does not know whose
	// camera it is holding.
	if _, _, err := s.Store.CameraRef(r.Context(), p.ClientID, id); err != nil {
		writeErr(w, http.StatusNotFound, "not_found", "No such camera.")
		return
	}
	flusher, ok := w.(http.Flusher)
	if !ok {
		s.serverError(w, "live", errors.New("this server cannot stream"))
		return
	}

	frames, release := s.live().Watch(id)
	defer release()

	h := w.Header()
	h.Set("Content-Type", "text/event-stream")
	h.Set("Cache-Control", "no-store")
	h.Set("Connection", "keep-alive")
	// Without this a proxy buffers the stream into one response that arrives
	// when the connection closes - which for live video means never.
	h.Set("X-Accel-Buffering", "no")
	w.WriteHeader(http.StatusOK)
	// Told up front, so a viewer can say "waiting for the shop PC" rather than
	// showing an empty box while the agent is still being asked.
	fmt.Fprint(w, "event: waiting\ndata: {}\n\n")
	flusher.Flush()

	ctx := r.Context()
	// Refreshed as we go rather than once at the start: this is what tells the
	// agent somebody is still there, and a viewer that has gone away stops a
	// shop uploading within seconds without having to announce anything.
	keep := time.NewTicker(liveIdle / 3)
	defer keep.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case <-keep.C:
			s.live().Keep(id)
		case frame, ok := <-frames:
			if !ok {
				return
			}
			s.live().Keep(id)
			if _, err := fmt.Fprintf(w, "event: frame\ndata: %s\n\n",
				base64.StdEncoding.EncodeToString(frame)); err != nil {
				return
			}
			flusher.Flush()
		}
	}
}

// handleAgentLiveWanted is the shop PC asking whether anyone is watching.
//
// Held open rather than answered immediately: an agent polling every few
// seconds would put a floor under how quickly a live view can start, and one
// polling slowly would put a ceiling on it. Holding the request means pressing
// "Live" reaches the shop PC at once, and an idle site costs about two requests
// a minute.
func (s *Server) handleAgentLiveWanted(w http.ResponseWriter, r *http.Request, ap AgentPrincipal) {
	ids, err := s.Store.SiteCameraIDs(r.Context(), ap.SiteID)
	if err != nil {
		s.serverError(w, "live wanted", err)
		return
	}
	if wanted := s.live().WantedAmong(ids); len(wanted) > 0 {
		writeJSON(w, http.StatusOK, map[string]any{"cameras": wanted})
		return
	}
	select {
	case <-r.Context().Done():
		return
	case <-s.live().Bell(ids):
	case <-time.After(liveWaitForWork):
	}
	writeJSON(w, http.StatusOK, map[string]any{
		"cameras": s.live().WantedAmong(ids)})
}

// handleAgentPushLive receives frames for as long as somebody is watching.
//
// One request carrying many frames, each prefixed with its length, rather than
// a request per frame: at a few frames a second the per-request overhead and
// the TLS handshakes would cost more than the pictures.
func (s *Server) handleAgentPushLive(w http.ResponseWriter, r *http.Request, ap AgentPrincipal) {
	id := r.PathValue("camera")
	if !looksLikeUUID(id) {
		writeErr(w, http.StatusNotFound, "not_found", "No such camera.")
		return
	}
	// The camera must belong to the AGENT's own site. Without this an agent
	// could push its own pictures into another site's live view - a camera id
	// is not a secret, and the agent supplies this one.
	siteID, _, err := s.Store.CameraRefBySite(r.Context(), ap.SiteID, id)
	if err != nil || siteID != ap.SiteID {
		writeErr(w, http.StatusNotFound, "not_found", "No such camera.")
		return
	}

	deadline := time.Now().Add(liveSession)
	body := r.Body
	var header [4]byte
	frames := 0
	for {
		if time.Now().After(deadline) {
			break
		}
		if _, err := io.ReadFull(body, header[:]); err != nil {
			break
		}
		n := binary.BigEndian.Uint32(header[:])
		if n == 0 || n > liveMaxFrame {
			// A length this side cannot trust ends the stream rather than
			// allocating what it was told to.
			writeErr(w, http.StatusBadRequest, "bad_frame", "Frame size out of range.")
			return
		}
		frame := make([]byte, n)
		if _, err := io.ReadFull(body, frame); err != nil {
			break
		}
		frames++
		if !s.live().Publish(id, frame) {
			// Nobody is watching any more. Saying so in the response is what
			// stops the shop uploading; the agent goes back to waiting.
			break
		}
	}
	writeJSON(w, http.StatusOK, map[string]any{"frames": frames})
}