Fixed offer not found case during Stripe checkout (#20322)

fixes https://linear.app/tryghost/issue/SLO-135

- handles edge cases when an invalid `offerId` is provided during Stripe
checkout

ghost/core/test/e2e-frontend/members.test.js

@@ -125,11 +125,10 @@ describe('Front-end members behavior', function () {
                 .expect(400);
         });
 
-        //TODO: Remove 500 expect once tests are wired up with Stripe
-        it('should not throw 400 for using offer id on members create checkout session endpoint', async function () {
+        it('should error for using an invalid offer id on members create checkout session endpoint', async function () {
             await request.post('/members/api/create-stripe-checkout-session')
                 .send({
-                    offerId: '62826b1b6dccb3e3e997ebd4',
+                    offerId: 'invalid',
                     identity: null,
                     metadata: {
                         name: 'Jamie Larsen'
@@ -139,7 +138,7 @@ describe('Front-end members behavior', function () {
                     tierId: null,
                     cadence: null
                 })
-                .expect(500);
+                .expect(400);
         });
 
         it('should error for invalid data on members create update session endpoint', async function () {

ghost/members-api/lib/controllers/RouterController.js

@@ -8,6 +8,7 @@ const messages = {
     emailRequired: 'Email is required.',
     badRequest: 'Bad Request.',
     notFound: 'Not Found.',
+    offerNotFound: 'This offer does not exist.',
     offerArchived: 'This offer is archived.',
     tierArchived: 'This tier is archived.',
     existingSubscription: 'A subscription exists for this Member.',
@@ -247,6 +248,14 @@ module.exports = class RouterController {
         // Fetch tier and offer
         if (offerId) {
             offer = await this._offersAPI.getOffer({id: offerId});
+
+            if (!offer) {
+                throw new BadRequestError({
+                    message: tpl(messages.offerNotFound),
+                    context: 'Offer with id "' + offerId + '" not found'
+                });
+            }
+
             tier = await this._tiersService.api.read(offer.tier.id);
             cadence = offer.cadence;
         } else {

ghost/members-api/test/unit/lib/controllers/router.test.js

@@ -161,6 +161,29 @@ describe('RouterController', function () {
                     assert.equal(error.context, 'Expected cadence to be "month" or "year", received "day"');
                 }
             });
+
+            it('returns a BadRequestError if offer is not found', async function () {
+                offersAPI = {
+                    getOffer: sinon.stub().resolves(null)
+                };
+
+                const routerController = new RouterController({
+                    tiersService,
+                    paymentsService,
+                    offersAPI,
+                    stripeAPIService,
+                    labsService
+                });
+
+                try {
+                    await routerController._getSubscriptionCheckoutData({offerId: 'invalid'});
+
+                    assert.fail('Expected function to throw BadRequestError');
+                } catch (error) {
+                    assert(error instanceof errors.BadRequestError, 'Error should be an instance of BadRequestError');
+                    assert.equal(error.context, 'Offer with id "invalid" not found');
+                }
+            });
         });
 
         afterEach(function () {