Behavision/agent/pkg/cameras/live.go

package cameras

import (
	"bytes"
	"context"
	"crypto/sha256"
	"encoding/binary"
	"encoding/json"
	"fmt"
	"io"
	"log"
	"net/http"
	"time"
)

// Live relays camera frames to head office, but only while somebody is
// watching.
//
// The engine serves MJPEG on this PC's loopback and this PC sits behind a
// router with no inbound route, so head office cannot pull it. It can answer
// our outbound requests, which is the shape of everything else here: we ask
// "is anyone watching?", and push frames for as long as the answer is yes.
//
// It is a few frames a second of re-encoded JPEG, not 25 fps video. True video
// needs WebRTC and a TURN server; this needs neither, and answers the question
// somebody at head office is actually asking - what does that camera see right
// now - at a cost a shop's uplink can carry.
//
// **Nothing is uploaded when nobody is looking.** That is the entire cost
// argument, and it is why the wanted-check comes first and the push stops the
// moment the server says the last viewer has gone.
type Live struct {
	Engine    *EngineClient
	Cloud     *CloudClient
	Log       *log.Logger
	FPS       float64
	Width     int
	Quality   int
}

// Defaults, measured against the office camera rather than guessed.
//
// The engine produces ~12 distinct frames a second, so asking for more than
// that only re-sends pictures the viewer already has - which is why the poll
// runs slightly ahead of it and identical frames are dropped rather than sent.
// 640 px at quality 60 is ~20 KB, so a watcher costs ~200 KB/s at the full
// rate, and a camera nobody is watching costs nothing at all.
const (
	DefaultLiveFPS     = 15.0
	DefaultLiveWidth   = 640
	DefaultLiveQuality = 60
)

func NewLive(eng *EngineClient, cloud *CloudClient, logger *log.Logger) *Live {
	return &Live{Engine: eng, Cloud: cloud, Log: logger,
		FPS: DefaultLiveFPS, Width: DefaultLiveWidth, Quality: DefaultLiveQuality}
}

// Run waits for viewers and serves them until the context ends.
func (l *Live) Run(ctx context.Context) {
	if l.Engine == nil || l.Cloud == nil {
		return
	}
	for {
		if ctx.Err() != nil {
			return
		}
		wanted, err := l.Cloud.LiveWanted(ctx)
		if err != nil {
			// Unclaimed, offline, or head office is down. All three mean the
			// same thing here - nobody can be watching - so back off rather
			// than hammering, and keep the shop's own recognition untouched.
			if ctx.Err() != nil {
				return
			}
			l.sleep(ctx, 15*time.Second)
			continue
		}
		if len(wanted) == 0 {
			// The poll is held open by the server, so an empty answer already
			// means ~25 s passed. No extra delay.
			continue
		}
		for _, id := range wanted {
			if ctx.Err() != nil {
				return
			}
			l.serve(ctx, id)
		}
	}
}

// serve pushes frames for one camera until the server says stop.
func (l *Live) serve(ctx context.Context, cameraID string) {
	engineID, err := l.Cloud.LiveEngineID(ctx, cameraID)
	if err != nil {
		l.logf("live %s: %v", cameraID, err)
		l.sleep(ctx, 2*time.Second)
		return
	}
	interval := time.Duration(float64(time.Second) / l.fps())

	// A pipe so frames can be written as they are grabbed while one request
	// carries all of them. A request per frame would spend more on handshakes
	// and headers than on pictures.
	pr, pw := io.Pipe()
	done := make(chan error, 1)
	go func() { done <- l.Cloud.PushLive(ctx, cameraID, pr) }()

	tick := time.NewTicker(interval)
	defer tick.Stop()
	// The engine re-serves its latest frame until the pipeline produces a new
	// one, so polling faster than it encodes returns the SAME picture again.
	// Measured: 93 polls in 6 s yielded 72 distinct frames. Sending the
	// duplicates would cost a fifth of the bandwidth for nothing, so the poll
	// runs a little ahead of the engine and the repeats are dropped - which is
	// what lets the rate follow the camera instead of a guess.
	var lastSum [32]byte
	for {
		select {
		case <-ctx.Done():
			_ = pw.CloseWithError(context.Canceled)
			<-done
			return
		case err := <-done:
			// The server closed the request: the last viewer went away, or the
			// session cap was reached. Either way stop grabbing frames.
			_ = pw.Close()
			if err != nil {
				l.logf("live %s ended: %v", cameraID, err)
			}
			return
		case <-tick.C:
		}
		jpeg, err := l.Engine.Frame(ctx, engineID, l.Width, l.Quality)
		if err != nil || len(jpeg) == 0 {
			// A camera that is reconnecting has no frame. Keep the request
			// open - the viewer sees the last frame rather than a dropped
			// stream, and the next tick may well have one.
			continue
		}
		if sum := sha256.Sum256(jpeg); sum == lastSum {
			continue
		} else {
			lastSum = sum
		}
		var hdr [4]byte
		binary.BigEndian.PutUint32(hdr[:], uint32(len(jpeg)))
		if _, err := pw.Write(hdr[:]); err != nil {
			<-done
			return
		}
		if _, err := pw.Write(jpeg); err != nil {
			<-done
			return
		}
	}
}

func (l *Live) fps() float64 {
	if l.FPS <= 0 || l.FPS > 25 {
		// A ceiling rather than a target: duplicate frames are dropped, so
		// polling above what the engine encodes costs requests and no
		// bandwidth - but it is still work, on the PC doing the recognition.
		return DefaultLiveFPS
	}
	return l.FPS
}

func (l *Live) sleep(ctx context.Context, d time.Duration) {
	t := time.NewTimer(d)
	defer t.Stop()
	select {
	case <-ctx.Done():
	case <-t.C:
	}
}

func (l *Live) logf(format string, args ...any) {
	if l.Log != nil {
		l.Log.Printf(format, args...)
	}
}

// ------------------------------------------------------------------ wire --

// LiveWanted asks head office which of this site's cameras are being watched.
// The server holds the request open, so this returns promptly when somebody
// presses Live and after ~25 s when nobody has.
func (c *CloudClient) LiveWanted(ctx context.Context) ([]string, error) {
	var body struct {
		Cameras []string `json:"cameras"`
	}
	// Longer than the server's own wait, so a held request is not cut off by
	// our own client timeout and reported as a failure.
	ctx, cancel := context.WithTimeout(ctx, 60*time.Second)
	defer cancel()
	if err := c.do(ctx, http.MethodGet, "/api/agent/live", nil, &body); err != nil {
		return nil, err
	}
	return body.Cameras, nil
}

// LiveEngineID maps head office's camera uuid to the name the engine knows,
// which is the only name this PC can ask for a frame with.
func (c *CloudClient) LiveEngineID(ctx context.Context, cameraID string) (string, error) {
	desired, err := c.Desired(ctx)
	if err != nil {
		return "", err
	}
	for _, d := range desired {
		if d.ID == cameraID {
			return d.CameraID, nil
		}
	}
	return "", fmt.Errorf("camera %s is not one of this site's", cameraID)
}

// PushLive streams frames until the server stops reading.
func (c *CloudClient) PushLive(ctx context.Context, cameraID string, body io.Reader) error {
	req, err := http.NewRequestWithContext(ctx, http.MethodPost,
		c.Base+"/api/agent/cameras/"+cameraID+"/live", body)
	if err != nil {
		return err
	}
	req.Header.Set("Authorization", "Bearer "+c.Token)
	req.Header.Set("Content-Type", "application/octet-stream")
	resp, err := c.Client.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	blob, _ := io.ReadAll(io.LimitReader(resp.Body, 4<<10))
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		return fmt.Errorf("head office: %s: %s", resp.Status, bytes.TrimSpace(blob))
	}
	var out struct {
		Frames int `json:"frames"`
	}
	_ = json.Unmarshal(blob, &out)
	return nil
}