Behavision/server/internal/api/handlers_arrivals.go

package api

import (
	"encoding/base64"
	"fmt"
	"net/http"
	"strconv"
	"strings"
	"time"
)

const (
	// A poller asking for more than this is either paging history through the
	// wrong endpoint or has lost its cursor. Capped rather than rejected: a
	// mobile app coming back from a tunnel should get a big catch-up page and
	// a fresh cursor, not an error it has no way to recover from.
	maxArrivals     = 200
	defaultArrivals = 50

	// How often a live stream re-queries even if no doorbell rings. This is the
	// safety net for a second server instance whose ingest this process cannot
	// hear, so it is slow on purpose - it is the fallback, not the mechanism.
	streamFallback = 15 * time.Second
	// SSE comment sent on an idle connection. Mobile networks and reverse
	// proxies both close a stream that has been silent for a minute or two,
	// and a client that reconnects every ninety seconds is a client that
	// re-queries constantly.
	streamKeepalive = 20 * time.Second
)

// handleArrivals is the live feed: who walked in, with their photos, in one
// request.
//
// This is the endpoint a mobile app or a shop-floor screen actually needs, and
// it is the one thing the API could not previously answer. `GET /api/visitors`
// searches a customer list by name; it cannot tell you that four people just
// came through the door, and until it could, a client had no way to know which
// customer ids to ask about.
func (s *Server) handleArrivals(w http.ResponseWriter, r *http.Request) {
	p := PrincipalFrom(r.Context())

	q := ArrivalQuery{
		ClientID: p.ClientID,
		SiteID:   siteParam(r),
		Limit:    queryInt(r, "limit", defaultArrivals, maxArrivals),
	}
	if q.SiteID != "" {
		var ok bool
		if q.SiteID, ok = s.resolveSiteFilter(w, r, q.SiteID); !ok {
			return
		}
	}
	// The site is still filtered by client_id in SQL as well. A site_id from
	// the query string is caller-controlled, and this is a read of other
	// people's customers if it is ever trusted on its own.
	if cur := trim(r.URL.Query().Get("cursor")); cur != "" {
		seq, err := decodeCursor(cur)
		if err != nil {
			badRequest(w, "that cursor is not one of ours - drop it and poll again without one")
			return
		}
		q.AfterSeq = &seq
	} else if since := trim(r.URL.Query().Get("since")); since != "" {
		at, err := time.Parse(time.RFC3339, since)
		if err != nil {
			badRequest(w, "since must look like 2026-09-02T10:30:00Z")
			return
		}
		at = at.UTC()
		q.Since = &at
	}

	page, err := s.arrivalPage(r, q)
	if err != nil {
		s.serverError(w, "arrivals", err)
		return
	}
	writeJSON(w, http.StatusOK, page)
}

// arrivalPage runs the query, attaches photos and returns the next cursor.
// Shared by the poll and the stream so the two cannot answer differently.
func (s *Server) arrivalPage(r *http.Request, q ArrivalQuery) (ArrivalPage, error) {
	rows, err := s.Store.Arrivals(r.Context(), q)
	if err != nil {
		return ArrivalPage{}, err
	}
	s.attachImages(r, rows)

	page := ArrivalPage{
		Arrivals: rows,
		PolledAt: s.now().UTC().Format(time.RFC3339Nano),
	}
	if page.Arrivals == nil {
		// An empty list, never null. A client looping over the response should
		// not have to special-case a quiet minute.
		page.Arrivals = []Arrival{}
	}
	if n := len(rows); n > 0 {
		// The LAST row, and the rows are ascending by position, so this is the
		// highest position the caller has now seen.
		page.Cursor = encodeCursor(rows[n-1].Seq)
	} else if q.AfterSeq != nil {
		// Nothing new. Hand the caller its own position back rather than an
		// empty string, so a poll that returns nothing does not reset the feed
		// to the beginning on the next request.
		page.Cursor = encodeCursor(*q.AfterSeq)
	}
	return page, nil
}

// attachImages swaps each row's object key for a short-lived signed link.
//
// One audit row for the whole page, not one per photo. Every read of a face is
// worth recording - "who looked at my customers" has to be answerable - but a
// tablet polling this feed every two seconds would write tens of thousands of
// rows a day and bury the single deliberate look that an investigation is
// actually after. The row records how many faces were surfaced and to whom,
// which is the fact worth keeping.
func (s *Server) attachImages(r *http.Request, rows []Arrival) {
	p := PrincipalFrom(r.Context())
	seen := make([]string, 0, len(rows))

	for i := range rows {
		key := rows[i].ImageKey
		rows[i].ImageKey = ""
		rows[i].Image = s.imageFor(key)
		if rows[i].Image.Available && rows[i].VisitorID != "" {
			seen = append(seen, rows[i].VisitorID)
		}
	}

	if len(seen) == 0 {
		return
	}
	s.Store.Audit(r.Context(), AuditEntry{
		ClientID: p.ClientID, ActorID: p.UserID, ActorKind: "user",
		Action: "image.view.feed", Entity: "visits",
		Detail: map[string]any{"count": len(seen), "visitor_ids": seen},
	})
}

// handleArrivalStream is the same feed pushed instead of polled.
//
// Server-sent events rather than websockets: this direction is one-way, SSE is
// stdlib with no dependency, it survives the reverse proxy in front of this
// server unchanged, and browsers and mobile HTTP clients reconnect it on their
// own. A websocket would buy bidirectionality that nothing here wants.
func (s *Server) handleArrivalStream(w http.ResponseWriter, r *http.Request) {
	flusher, ok := w.(http.Flusher)
	if !ok {
		// Without flushing this is not a stream, it is a response that arrives
		// at the end of the day. Say so rather than appearing to work.
		writeErr(w, http.StatusInternalServerError, "server_error",
			"streaming is not available on this connection")
		return
	}
	p := PrincipalFrom(r.Context())

	q := ArrivalQuery{
		ClientID: p.ClientID,
		SiteID:   siteParam(r),
		Limit:    queryInt(r, "limit", defaultArrivals, maxArrivals),
	}
	if q.SiteID != "" {
		var ok bool
		if q.SiteID, ok = s.resolveSiteFilter(w, r, q.SiteID); !ok {
			return
		}
	}
	// Last-Event-ID is what the browser's EventSource resends automatically on
	// a dropped connection, so honouring it is what makes a reconnect lossless
	// without the client writing any recovery code. ?cursor= is the same thing
	// for a native client that cannot set the header.
	cur := trim(r.Header.Get("Last-Event-ID"))
	if cur == "" {
		cur = trim(r.URL.Query().Get("cursor"))
	}
	if cur != "" {
		if seq, err := decodeCursor(cur); err == nil {
			q.AfterSeq = &seq
		}
		// A cursor we cannot read is not worth failing a reconnect over: the
		// client falls back to the recent window, which is the same thing it
		// would get on a fresh connection.
	}

	h := w.Header()
	h.Set("Content-Type", "text/event-stream")
	h.Set("Cache-Control", "no-cache")
	h.Set("Connection", "keep-alive")
	// Traefik and nginx both buffer by default, which turns an event stream
	// into a file that arrives when the connection closes.
	h.Set("X-Accel-Buffering", "no")
	w.WriteHeader(http.StatusOK)
	flusher.Flush()

	bell, release := s.hub().Subscribe()
	defer release()

	ctx := r.Context()
	fallback := time.NewTicker(streamFallback)
	defer fallback.Stop()
	keepalive := time.NewTicker(streamKeepalive)
	defer keepalive.Stop()

	// Send whatever is already there before waiting for a doorbell, so a client
	// that connects after people have walked in is not blind until the next
	// one does.
	send := func() bool {
		page, err := s.arrivalPage(r, q)
		if err != nil {
			s.logf("ERROR arrival stream: %v", err)
			// Keep the connection: a transient database error should not log
			// a shop's screen out and start a reconnect storm across an estate.
			return true
		}
		if len(page.Arrivals) == 0 {
			return true
		}
		if page.Cursor != "" {
			// The SSE id becomes the client's Last-Event-ID, so the cursor
			// rides the protocol's own reconnect machinery instead of needing
			// application-level recovery.
			fmt.Fprintf(w, "id: %s\n", page.Cursor)
			// Advance our own position from the rows we just sent, so the next
			// wake-up asks for what comes after them.
			last := page.Arrivals[len(page.Arrivals)-1].Seq
			q.AfterSeq = &last
		}
		fmt.Fprint(w, "event: arrivals\ndata: ")
		if err := writeCompactJSON(w, page); err != nil {
			return false
		}
		fmt.Fprint(w, "\n\n")
		flusher.Flush()
		return true
	}
	if !send() {
		return
	}

	for {
		select {
		case <-ctx.Done():
			return
		case id, open := <-bell:
			if !open {
				return
			}
			// One process serves every tenant, so a doorbell for somebody
			// else's shop must not cost this connection a query.
			if id != p.ClientID {
				continue
			}
			if !send() {
				return
			}
		case <-fallback.C:
			if !send() {
				return
			}
		case <-keepalive.C:
			// A comment line. Keeps proxies and mobile radios from deciding
			// the connection is dead, and is ignored by every SSE client.
			fmt.Fprint(w, ": keepalive\n\n")
			flusher.Flush()
		}
	}
}

func (s *Server) hub() *Hub {
	if s.Hub == nil {
		// A server built without one still streams; it just has no doorbell,
		// so it falls back to the slow tick. A nil map panic on an endpoint
		// somebody forgot to wire is a worse outcome than a slower feed.
		return nil
	}
	return s.Hub
}

// ---------------------------------------------------------------- cursors

// Cursors are opaque on purpose, and base64 is what makes them look it. A
// caller that reads one starts depending on the ordering column, and changing
// that later then breaks every deployed mobile app rather than just this file -
// which is exactly what happened once already, when the feed was ordered by a
// timestamp and a random uuid.
func encodeCursor(seq int64) string {
	return base64.RawURLEncoding.EncodeToString(
		[]byte("v1:" + strconv.FormatInt(seq, 10)))
}

func decodeCursor(s string) (int64, error) {
	raw, err := base64.RawURLEncoding.DecodeString(s)
	if err != nil {
		return 0, fmt.Errorf("cursor is not base64: %w", err)
	}
	// The version prefix is what lets the ordering change again without
	// silently misreading cursors already held by deployed clients: an old
	// cursor fails to parse and the client restarts cleanly from the recent
	// window, rather than resuming at a position that now means something else.
	body, ok := strings.CutPrefix(string(raw), "v1:")
	if !ok {
		return 0, fmt.Errorf("cursor is not a v1 cursor")
	}
	seq, err := strconv.ParseInt(body, 10, 64)
	if err != nil || seq < 0 {
		return 0, fmt.Errorf("cursor position is not a number")
	}
	return seq, nil
}