Add SSE broadcaster

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/web/broadcaster.go

@@ -0,0 +1,42 @@
+package web
+
+import "sync"
+
+// Broadcaster fans a string message out to all SSE subscribers. Slow
+// subscribers drop messages rather than blocking publishers.
+type Broadcaster struct {
+	mu   sync.Mutex
+	subs map[chan string]struct{}
+}
+
+func NewBroadcaster() *Broadcaster {
+	return &Broadcaster{subs: make(map[chan string]struct{})}
+}
+
+func (b *Broadcaster) Subscribe() chan string {
+	ch := make(chan string, 8)
+	b.mu.Lock()
+	b.subs[ch] = struct{}{}
+	b.mu.Unlock()
+	return ch
+}
+
+func (b *Broadcaster) Unsubscribe(ch chan string) {
+	b.mu.Lock()
+	if _, ok := b.subs[ch]; ok {
+		delete(b.subs, ch)
+		close(ch)
+	}
+	b.mu.Unlock()
+}
+
+func (b *Broadcaster) Publish(msg string) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	for ch := range b.subs {
+		select {
+		case ch <- msg:
+		default: // drop for slow subscriber
+		}
+	}
+}

internal/web/broadcaster_test.go

@@ -0,0 +1,42 @@
+package web
+
+import (
+	"testing"
+	"time"
+)
+
+func TestBroadcasterDeliversToSubscribers(t *testing.T) {
+	b := NewBroadcaster()
+	ch := b.Subscribe()
+	defer b.Unsubscribe(ch)
+
+	b.Publish("hello")
+
+	select {
+	case msg := <-ch:
+		if msg != "hello" {
+			t.Fatalf("got %q want hello", msg)
+		}
+	case <-time.After(time.Second):
+		t.Fatal("subscriber did not receive message")
+	}
+}
+
+func TestBroadcasterDropsWhenSubscriberFull(t *testing.T) {
+	b := NewBroadcaster()
+	ch := b.Subscribe()
+	defer b.Unsubscribe(ch)
+	// Publishing many messages without reading must not block (slow client).
+	done := make(chan struct{})
+	go func() {
+		for i := 0; i < 100; i++ {
+			b.Publish("x")
+		}
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("Publish blocked on a full subscriber")
+	}
+}