twitch: implement webhook challenge handling

twitch/eventsub.go

@@ -6,8 +6,21 @@ type EventSub[Event any] struct {
 	Subscription Subscription `json:"subscription"`
 	// Event is the event payload.
 	Event Event `json:"event"`
+	// Challenge is payload for verification challenges.
+	Challenge string `json:"challenge"`
 }
 
 // Subscription is the fields of an EventSub subscription
 // which are relevant to Kaiyan.
-type Subscription struct{}
+type Subscription struct {
+	// Status is the subscription status.
+	// For revocation messages, this holds the reason for revocation.
+	Status string `json:"status"`
+}
+
+// EventSub message types.
+const (
+	Notification = "notification"
+	Verification = "webhook_callback_verification"
+	Revocation   = "revocation"
+)

twitch/webhook.go

@@ -10,7 +10,10 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"strconv"
 	"sync"
+
+	"github.com/go-json-experiment/json"
 )
 
 // ErrVerificationFailed is the error returned when a webhook request does not
@@ -39,14 +42,26 @@ func Receive(b, secret []byte, req *http.Request) ([]byte, error) {
 
 	got := req.Header.Get("Twitch-Eventsub-Message-Signature")
 	if !hmac.Equal(want, []byte(got)) {
-		fmt.Printf("req message id: %q\n", req.Header.Get("Twitch-Eventsub-Message-Id"))
-		fmt.Printf("req timestamp: %q\n", req.Header.Get("Twitch-Eventsub-Message-Timestamp"))
-		fmt.Printf("req body: %q\n", b)
 		return nil, fmt.Errorf("%w: computed %q, header has %q", ErrVerificationFailed, want, got)
 	}
 	return b, nil
 }
 
+// HandleChallenge responds to an EventSub verification challenge.
+func HandleChallenge(w http.ResponseWriter, body []byte) error {
+	var ev EventSub[struct{}]
+	if err := json.Unmarshal(body, &ev); err != nil {
+		return err
+	}
+	if ev.Challenge == "" {
+		return errors.New("no challenge in body")
+	}
+	w.Header().Set("Content-Type", "text/plain")
+	w.Header().Set("Content-Length", strconv.Itoa(len(ev.Challenge)))
+	_, err := io.WriteString(w, ev.Challenge)
+	return err
+}
+
 // Secret returns a process-wide secret for webhook subscriptions.
 var Secret = sync.OnceValue(func() []byte {
 	b := make([]byte, 32)