converge-relay: phase 1 — webhook-to-SSE relay server in Go

- Pub/sub broker with fan-out to SSE clients
- POST /webhook — receives Gitea push events, optional HMAC verification
- GET /events — SSE stream of push events
- GET /health — health check endpoint
- Zero external dependencies (stdlib only)
- Docker-based build via goctl wrapper (golang:1.26-alpine)
- systemd unit, README with deployment docs
- Tests: broker pub/sub, slow subscriber drop, ref parsing, signature verification
- 6.0M static binary

converge-relay/sse.go

@@ -0,0 +1,59 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"log/slog"
+	"net/http"
+)
+
+func sseHandler(broker *Broker) http.HandlerFunc {
+	return func(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") // disable nginx buffering
+
+		ch := broker.Subscribe()
+		defer broker.Unsubscribe(ch)
+
+		slog.Debug("SSE client connected")
+
+		ctx := r.Context()
+		for {
+			select {
+			case <-ctx.Done():
+				slog.Debug("SSE client disconnected")
+				return
+
+			case evt, ok := <-ch:
+				if !ok {
+					// Broker shut down
+					return
+				}
+				writeSSEEvent(w, flusher, "push", evt)
+			}
+		}
+	}
+}
+
+func writeSSEEvent(w http.ResponseWriter, flusher http.Flusher, eventType string, data any) {
+	jsonData, err := json.Marshal(data)
+	if err != nil {
+		slog.Warn("failed to marshal SSE event", "error", err)
+		return
+	}
+
+	_, err = fmt.Fprintf(w, "event: %s\ndata: %s\n\n", eventType, jsonData)
+	if err != nil {
+		slog.Debug("failed to write SSE event, client likely disconnected", "error", err)
+		return
+	}
+	flusher.Flush()
+}