Refuse Checkout when a subscription can still bill

A student who already had Pro could open Checkout again and start a second Stripe subscription on the same customer. Checkout now stops when a subscription is active, trialing, past due, unpaid, or paused, and tells them to manage it from Settings.

Co-authored-by: Gáborjáni-Szabó Ádám <Programcode@users.noreply.github.com>

Author: cursoragent

src/test/checkout-security.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from "vitest";
 import {
   findCustomerOwnedByUser,
+  hasOpenSubscription,
   isAllowedCheckoutRedirectUrl,
 } from "../../supabase/functions/create-checkout/checkout-security";
 
@@ -28,4 +29,15 @@ describe("checkout security", () => {
     expect(findCustomerOwnedByUser(customers, "user-123")).toBe("cus_correct");
     expect(findCustomerOwnedByUser(customers, "missing-user")).toBeNull();
   });
+
+  it("treats a subscription that can still bill as a reason to refuse another checkout", () => {
+    expect(hasOpenSubscription([{ status: "active" }, { status: "canceled" }])).toBe(true);
+    expect(hasOpenSubscription([{ status: "trialing" }])).toBe(true);
+    expect(hasOpenSubscription([{ status: "past_due" }])).toBe(true);
+    expect(hasOpenSubscription([{ status: "paused" }])).toBe(true);
+    expect(hasOpenSubscription([{ status: "unpaid" }])).toBe(true);
+    expect(hasOpenSubscription([{ status: "canceled" }, { status: "incomplete" }, { status: "incomplete_expired" }])).toBe(false);
+    expect(hasOpenSubscription([])).toBe(false);
+    expect(hasOpenSubscription(null)).toBe(false);
+  });
 });

supabase/functions/create-checkout/checkout-security.ts

@@ -24,6 +24,13 @@ export function isAllowedCheckoutRedirectUrl(value: string): boolean {
   }
 }
 
+const OPEN_SUBSCRIPTION_STATUSES = new Set(["active", "trialing", "past_due", "unpaid", "paused"]);
+
+/** True when this user already has a Stripe subscription that can still bill them. */
+export function hasOpenSubscription(rows: Array<{ status?: string | null }> | null | undefined): boolean {
+  return (rows ?? []).some((row) => typeof row.status === "string" && OPEN_SUBSCRIPTION_STATUSES.has(row.status));
+}
+
 export function findCustomerOwnedByUser(
   customers: StripeCustomerLike[],
   userId: string,

supabase/functions/create-checkout/index.ts

@@ -7,6 +7,7 @@ import { HttpError, jsonError, requireUser } from "../_shared/auth.ts";
 import { getCorsHeaders } from "../_shared/cors.ts";
 import {
   findCustomerOwnedByUser,
+  hasOpenSubscription,
   isAllowedCheckoutRedirectUrl,
 } from "./checkout-security.ts";
 
@@ -249,6 +250,30 @@ export async function handleRequest(req: Request): Promise<Response> {
 
     const resolvedPrice = await resolveCheckoutPriceId();
 
+    // A second Checkout session creates a second subscription on the same
+    // customer. Refuse while one can still bill, and send the student to
+    // Settings to manage the one they have.
+    const { data: existingSubscriptions, error: subscriptionReadError } = await supabase
+      .from("subscriptions")
+      .select("status")
+      .eq("user_id", user.id);
+    if (subscriptionReadError) {
+      console.error(`[CHECKOUT] Could not read subscriptions for ${user.id}: ${subscriptionReadError.code}`);
+      return new Response(JSON.stringify({ error: "Failed to create checkout session" }), {
+        status: 500,
+        headers: { ...corsHeaders, "Content-Type": "application/json" },
+      });
+    }
+    if (hasOpenSubscription(existingSubscriptions)) {
+      return new Response(JSON.stringify({
+        error: "You already have a subscription. Manage it from Settings.",
+        code: "already_subscribed",
+      }), {
+        status: 409,
+        headers: { ...corsHeaders, "Content-Type": "application/json" },
+      });
+    }
+
     // Get or create Stripe customer
     let customerId = await findCustomerIdByUser({ id: user.id, email: user.email });
     if (!customerId) {