updates

middlewares/idempotency.go

@@ -70,10 +70,27 @@ func Idempotency() fiber.Handler {
 			return err
 		}
 
-		// Cache only deterministic outcomes (2xx/4xx). A 5xx is transient — the
-		// retry should get a genuine second attempt, not a cached failure.
+		// Cache SUCCESS only.
+		//
+		// This used to store any status below 500, on the reasoning that a 4xx
+		// is deterministic. A 4xx is not deterministic — it is a refusal made
+		// against state that moves. POST /customer/auth/otp/verify returns 401
+		// when the submitted code does not match the one in Redis, and the whole
+		// point of that screen is that the customer then gets the code right.
+		// With the refusal cached for 24 hours, the retry that should have
+		// worked replayed the old 401 instead — confirmed live against
+		// api.doormile.com, where the second attempt came back carrying
+		// Idempotent-Replay: true. One typo locked a customer out for a day.
+		// The same shape applies to 403 after a permission is granted, 404 after
+		// a record is created, and 429 after a window rolls over.
+		//
+		// Nothing is lost by narrowing it. This middleware exists to stop a retry
+		// repeating a SIDE EFFECT — a second pickup, a second COD collection, a
+		// second session. A request that ended 4xx performed no side effect, so
+		// re-executing it is exactly as safe as the first attempt was, and
+		// strictly more correct than replaying a stale no.
 		status := c.Response().StatusCode()
-		if status < 500 {
+		if isCacheableStatus(status) {
 			body := string(c.Response().Body())
 			db.Rdb.Set(context.Background(), base, strconv.Itoa(status)+sep+body, ttl)
 		}
@@ -82,6 +99,12 @@ func Idempotency() fiber.Handler {
 	}
 }
 
+// isCacheableStatus reports whether a response may be stored and replayed to
+// a later request carrying the same key. Only a 2xx may — see above.
+func isCacheableStatus(status int) bool {
+	return status >= 200 && status < 300
+}
+
 // idempotencyScope namespaces a key so one caller's stored response can never
 // be replayed to another.
 //