Crussell/backend/handlers/webhooks/square.go

package webhooks

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"encoding/json"
	"io"
	"log"
	"net/http"
	"os"
	"sync"
)

type SquareWebhookEvent struct {
	Type       string          `json:"type"`
	EventID    string          `json:"event_id"`
	CreatedAt  string          `json:"created_at"`
	Data       json.RawMessage `json:"data"`
	LocationID string          `json:"location_id"`
}

// squareWebhookDedup is a bounded, mutex-guarded set of recently handled
// event IDs. Square redelivers signed webhooks on retries (or a replay); once
// the handlers mutate state a duplicate delivery would double-apply, so drop
// replays while keeping the set bounded.
type squareWebhookDedup struct {
	mu    sync.Mutex
	seen  map[string]struct{}
	order []string
	max   int
}

func newSquareWebhookDedup(max int) *squareWebhookDedup {
	return &squareWebhookDedup{
		seen:  make(map[string]struct{}),
		order: make([]string, 0, max),
		max:   max,
	}
}

// register reports whether id was already handled: false on first occurrence
// (recording id, evicting the oldest once the cap is reached), true on a
// replay (set untouched, preserving insertion order). Mutex-guarded — the
// handler may be hit concurrently.
func (d *squareWebhookDedup) register(id string) bool {
	d.mu.Lock()
	defer d.mu.Unlock()
	if _, ok := d.seen[id]; ok {
		return true
	}
	d.seen[id] = struct{}{}
	d.order = append(d.order, id)
	if len(d.order) > d.max {
		oldest := d.order[0]
		d.order = d.order[1:]
		delete(d.seen, oldest)
	}
	return false
}

// 1000 IDs far exceeds Square's redelivery window while capping memory.
var squareWebhookEventsSeen = newSquareWebhookDedup(1000)

func HandleSquareWebhook(w http.ResponseWriter, r *http.Request) {
	r.Body = http.MaxBytesReader(w, r.Body, 512*1024)
	body, err := io.ReadAll(r.Body)
	if err != nil {
		log.Printf("Failed to read webhook body: %v", err)
		http.Error(w, "request body too large or unreadable", http.StatusRequestEntityTooLarge)
		return
	}
	defer r.Body.Close()

	// Verification logic per Square spec (HMAC-SHA256, base64, notificationURL + body).
	// Production setup: set SQUARE_WEBHOOK_SIGNATURE_KEY and SQUARE_WEBHOOK_NOTIFICATION_URL
	// in env vars (see Square Developer Console → Webhooks → Subscription).
	// Reference: https://developer.squareup.com/docs/webhooks/step3validate

	// Fail closed: a missing signing key means the webhook cannot be verified,
	// so reject rather than process unauthenticated events (S-4). Square
	// always sends the signature header, so an unset key in production is a
	// misconfiguration that must not silently accept forged events.
	signingKey := os.Getenv("SQUARE_WEBHOOK_SIGNATURE_KEY")
	notificationURL := os.Getenv("SQUARE_WEBHOOK_NOTIFICATION_URL")
	if notificationURL == "" {
		notificationURL = "http://localhost:8080/webhooks/square"
	}
	if signingKey == "" {
		log.Printf("SQUARE_WEBHOOK_SIGNATURE_KEY is not set — rejecting webhook (fail-closed)")
		http.Error(w, "webhook signature verification unavailable", http.StatusServiceUnavailable)
		return
	}
	signature := r.Header.Get("x-square-hmacsha256-signature")
	if signature == "" {
		log.Printf("Missing Square webhook signature header")
		http.Error(w, "Invalid signature", http.StatusForbidden)
		return
	}
	if !verifySquareSignature(body, signature, signingKey, notificationURL) {
		log.Printf("Invalid Square webhook signature")
		http.Error(w, "Invalid signature", http.StatusForbidden)
		return
	}

	var event SquareWebhookEvent
	if err := json.Unmarshal(body, &event); err != nil {
		log.Printf("Failed to parse webhook event: %v", err)
		http.Error(w, "Invalid event", http.StatusBadRequest)
		return
	}

	// Dedup BEFORE dispatch: a correctly signed replay of a handled event
	// must not re-enter the handlers (which will mutate state once wired).
	// Returns 200 to acknowledge delivery without processing.
	if event.EventID != "" && squareWebhookEventsSeen.register(event.EventID) {
		log.Printf("[SQUARE-WEBHOOK] Duplicate event_id %s; skipping (already processed)", event.EventID)
		w.WriteHeader(http.StatusOK)
		_, _ = w.Write([]byte("ok"))
		return
	}

	log.Printf("[SQUARE-WEBHOOK] Received event: %s", event.Type)

	switch event.Type {
	case "payment.updated":
		handlePaymentUpdated(event.Data)
	case "refund.updated":
		handleRefundUpdated(event.Data)
	case "dispute.created":
		log.Printf("[SQUARE-WEBHOOK] Dispute created: %s", event.EventID)
	default:
		log.Printf("[SQUARE-WEBHOOK] Unknown event type: %s", event.Type)
	}

	w.WriteHeader(http.StatusOK)
	_, _ = w.Write([]byte("ok"))
}

func verifySquareSignature(body []byte, signature, signingKey, notificationURL string) bool {
	mac := hmac.New(sha256.New, []byte(signingKey))
	mac.Write([]byte(notificationURL))
	mac.Write(body)
	expected := base64.StdEncoding.EncodeToString(mac.Sum(nil))
	return hmac.Equal([]byte(signature), []byte(expected))
}

func handlePaymentUpdated(data json.RawMessage) {
	log.Printf("[SQUARE-WEBHOOK] payment.updated: %s", string(data))
}

func handleRefundUpdated(data json.RawMessage) {
	log.Printf("[SQUARE-WEBHOOK] refund.updated: %s", string(data))
}