Vetting/internal/events/events.go

// Package events provides an in-process SSE fan-out hub. Browser
// clients subscribe via GET /events; the orchestrator publishes
// pre-rendered HTML fragments that HTMX swaps into the DOM.
package events

import (
	"context"
	"fmt"
	"net/http"
	"sync"
	"sync/atomic"
	"time"
)

// Event is a typed event published on the internal bus. In Phase 1 the
// payload is an already-rendered HTML fragment; later phases will wrap
// structured run state in this same Event envelope.
type Event struct {
	Name    string // SSE event name (e.g. "heartbeat", "tile-update", "log-line")
	Payload string // pre-rendered HTML, ready to write as SSE data
}

type subscriber struct {
	id int64
	ch chan Event
}

const (
	defaultSubscriberBuffer = 32
	heartbeatInterval       = 15 * time.Second
)

// Hub is an in-process fan-out for SSE subscribers.
type Hub struct {
	mu        sync.RWMutex
	nextID    int64
	subs      map[int64]*subscriber
	buffer    int
	heartbeat time.Duration
	done      chan struct{}
	closeOnce sync.Once
}

func NewHub() *Hub {
	h := &Hub{
		subs:      map[int64]*subscriber{},
		buffer:    defaultSubscriberBuffer,
		heartbeat: heartbeatInterval,
		done:      make(chan struct{}),
	}
	go h.heartbeatLoop()
	return h
}

func (h *Hub) Publish(ev Event) {
	h.mu.RLock()
	defer h.mu.RUnlock()
	for _, s := range h.subs {
		select {
		case s.ch <- ev:
		default:
			// Slow subscriber: drop the event rather than stall other clients.
		}
	}
}

func (h *Hub) Subscribe() (id int64, ch <-chan Event, cancel func()) {
	id = atomic.AddInt64(&h.nextID, 1)
	s := &subscriber{id: id, ch: make(chan Event, h.buffer)}
	h.mu.Lock()
	h.subs[id] = s
	h.mu.Unlock()
	return id, s.ch, func() {
		h.mu.Lock()
		delete(h.subs, id)
		h.mu.Unlock()
		close(s.ch)
	}
}

func (h *Hub) heartbeatLoop() {
	t := time.NewTicker(h.heartbeat)
	defer t.Stop()
	for {
		select {
		case <-h.done:
			return
		case <-t.C:
			h.Publish(Event{
				Name:    "heartbeat",
				Payload: fmt.Sprintf(`<span data-heartbeat="%d"></span>`, time.Now().Unix()),
			})
		}
	}
}

// ServeSSE writes server-sent events for a single subscriber for the
// lifetime of the request. Each Event becomes one SSE message.
func (h *Hub) ServeSSE(w http.ResponseWriter, r *http.Request) {
	flusher, ok := w.(http.Flusher)
	if !ok {
		http.Error(w, "streaming not supported", http.StatusInternalServerError)
		return
	}
	w.Header().Set("Content-Type", "text/event-stream")
	w.Header().Set("Cache-Control", "no-cache")
	w.Header().Set("Connection", "keep-alive")
	w.Header().Set("X-Accel-Buffering", "no")

	_, eventsCh, cancel := h.Subscribe()
	defer cancel()

	fmt.Fprintf(w, "event: hello\ndata: ok\n\n")
	flusher.Flush()

	ctx := r.Context()
	for {
		select {
		case <-ctx.Done():
			return
		case ev, ok := <-eventsCh:
			if !ok {
				return
			}
			writeSSE(w, ev)
			flusher.Flush()
		}
	}
}

func writeSSE(w http.ResponseWriter, ev Event) {
	if ev.Name != "" {
		fmt.Fprintf(w, "event: %s\n", ev.Name)
	}
	for _, line := range splitLines(ev.Payload) {
		fmt.Fprintf(w, "data: %s\n", line)
	}
	fmt.Fprint(w, "\n")
}

func splitLines(s string) []string {
	if s == "" {
		return []string{""}
	}
	out := []string{}
	start := 0
	for i := 0; i < len(s); i++ {
		if s[i] == '\n' {
			out = append(out, s[start:i])
			start = i + 1
		}
	}
	if start <= len(s) {
		out = append(out, s[start:])
	}
	return out
}

// Shutdown stops the heartbeat goroutine and closes all subscriber channels.
func (h *Hub) Shutdown(_ context.Context) error {
	h.closeOnce.Do(func() {
		close(h.done)
		h.mu.Lock()
		for id, s := range h.subs {
			close(s.ch)
			delete(h.subs, id)
		}
		h.mu.Unlock()
	})
	return nil
}