billing: Fix the type annotation of Customer.stripe_customer_id.

This also fixes a bug in void_all_open_invoices function. If a realm
with a local Customer object but without an associated stripe.Customer
is passed to void_all_open_invoices, then the function will end up
voiding the last 10 invoices created by billing system instead of voiding
no invoices at all. This is because stripe.Invoice.list(customer=None)
return last 10 invoices across all customers.

But this bug won't cauuse any issue in production since
void_all_open_invoices can be only invoked from /support page. And we
show the option to void invoices in support page only if the realm
has a paid plan. And it's not really possible for a realm to have
a paid plan without having an associated stripe_customer_id. Plus I
went through the void events in stripe stream since the PR to add
void invoices was merged and there does not seems to be any suspicious
events.

corporate/lib/stripe.py

@@ -319,6 +319,7 @@ def do_replace_payment_source(
 ) -> stripe.Customer:
     customer = get_customer_by_realm(user.realm)
     assert customer is not None  # for mypy
+    assert customer.stripe_customer_id is not None  # for mypy
 
     stripe_customer = stripe_get_customer(customer.stripe_customer_id)
     stripe_customer.source = stripe_token
@@ -533,6 +534,8 @@ def process_initial_upgrade(
 ) -> None:
     realm = user.realm
     customer = update_or_create_stripe_customer(user, stripe_token=stripe_token)
+    assert customer.stripe_customer_id is not None  # for mypy
+
     charge_automatically = stripe_token is not None
     free_trial = is_free_trial_offer_enabled()
 
@@ -710,6 +713,11 @@ def update_license_ledger_if_needed(realm: Realm, event_time: datetime) -> None:
 def invoice_plan(plan: CustomerPlan, event_time: datetime) -> None:
     if plan.invoicing_status == CustomerPlan.STARTED:
         raise NotImplementedError("Plan with invoicing_status==STARTED needs manual resolution.")
+    if not plan.customer.stripe_customer_id:
+        raise BillingError(
+            f"Realm {plan.customer.realm.string_id} has a paid plan without a Stripe customer."
+        )
+
     make_end_of_cycle_updates_if_needed(plan, event_time)
 
     if plan.invoicing_status == CustomerPlan.INITIAL_INVOICE_TO_BE_SENT:
@@ -957,6 +965,8 @@ def void_all_open_invoices(realm: Realm) -> int:
     customer = get_customer_by_realm(realm)
     if customer is None:
         return 0
+    if customer.stripe_customer_id is None:
+        return 0
     invoices = stripe.Invoice.list(customer=customer.stripe_customer_id)
     voided_invoices_count = 0
     for invoice in invoices:

corporate/models.py

@@ -16,7 +16,7 @@ class Customer(models.Model):
     """
 
     realm: Realm = models.OneToOneField(Realm, on_delete=CASCADE)
-    stripe_customer_id: str = models.CharField(max_length=255, null=True, unique=True)
+    stripe_customer_id: Optional[str] = models.CharField(max_length=255, null=True, unique=True)
     sponsorship_pending: bool = models.BooleanField(default=False)
     # A percentage, like 85.
     default_discount: Optional[Decimal] = models.DecimalField(

corporate/tests/test_stripe.py

@@ -1990,6 +1990,7 @@ class StripeTest(StripeTestCase):
         monthly_plan.refresh_from_db()
         self.assertEqual(monthly_plan.next_invoice_date, None)
 
+        assert customer.stripe_customer_id
         [invoice0, invoice1, invoice2] = stripe.Invoice.list(customer=customer.stripe_customer_id)
 
         [invoice_item0, invoice_item1] = invoice0.get("lines")
@@ -2154,6 +2155,7 @@ class StripeTest(StripeTestCase):
         self.assertEqual(annual_plan.next_invoice_date, add_months(self.next_month, 12))
         self.assertEqual(annual_plan.invoicing_status, CustomerPlan.DONE)
 
+        assert customer.stripe_customer_id
         [invoice0, invoice1] = stripe.Invoice.list(customer=customer.stripe_customer_id)
 
         [invoice_item] = invoice0.get("lines")
@@ -2632,12 +2634,17 @@ class StripeTest(StripeTestCase):
     @mock_stripe()
     def test_void_all_open_invoices(self, *mock: Mock) -> None:
         iago = self.example_user("iago")
-        self.assertEqual(void_all_open_invoices(iago.realm), 0)
-        customer = update_or_create_stripe_customer(iago)
+        king = self.lear_user("king")
 
+        self.assertEqual(void_all_open_invoices(iago.realm), 0)
+
+        zulip_customer = update_or_create_stripe_customer(iago)
+        lear_customer = update_or_create_stripe_customer(king)
+
+        assert zulip_customer.stripe_customer_id
         stripe.InvoiceItem.create(
             currency="usd",
-            customer=customer.stripe_customer_id,
+            customer=zulip_customer.stripe_customer_id,
             description="Zulip standard upgrade",
             discountable=False,
             unit_amount=800,
@@ -2646,14 +2653,45 @@ class StripeTest(StripeTestCase):
         stripe_invoice = stripe.Invoice.create(
             auto_advance=True,
             billing="send_invoice",
-            customer=customer.stripe_customer_id,
+            customer=zulip_customer.stripe_customer_id,
+            days_until_due=30,
+            statement_descriptor="Zulip Standard",
+        )
+        stripe.Invoice.finalize_invoice(stripe_invoice)
+
+        assert lear_customer.stripe_customer_id
+        stripe.InvoiceItem.create(
+            currency="usd",
+            customer=lear_customer.stripe_customer_id,
+            description="Zulip standard upgrade",
+            discountable=False,
+            unit_amount=800,
+            quantity=8,
+        )
+        stripe_invoice = stripe.Invoice.create(
+            auto_advance=True,
+            billing="send_invoice",
+            customer=lear_customer.stripe_customer_id,
             days_until_due=30,
             statement_descriptor="Zulip Standard",
         )
         stripe.Invoice.finalize_invoice(stripe_invoice)
 
         self.assertEqual(void_all_open_invoices(iago.realm), 1)
-        invoices = stripe.Invoice.list(customer=customer.stripe_customer_id)
+        invoices = stripe.Invoice.list(customer=zulip_customer.stripe_customer_id)
+        self.assert_length(invoices, 1)
+        for invoice in invoices:
+            self.assertEqual(invoice.status, "void")
+
+        lear_stripe_customer_id = lear_customer.stripe_customer_id
+        lear_customer.stripe_customer_id = None
+        lear_customer.save(update_fields=["stripe_customer_id"])
+        self.assertEqual(void_all_open_invoices(king.realm), 0)
+
+        lear_customer.stripe_customer_id = lear_stripe_customer_id
+        lear_customer.save(update_fields=["stripe_customer_id"])
+        self.assertEqual(void_all_open_invoices(king.realm), 1)
+        invoices = stripe.Invoice.list(customer=lear_customer.stripe_customer_id)
         self.assert_length(invoices, 1)
         for invoice in invoices:
             self.assertEqual(invoice.status, "void")
@@ -3207,6 +3245,17 @@ class InvoiceTest(StripeTestCase):
         with self.assertRaises(NotImplementedError):
             invoice_plan(CustomerPlan.objects.first(), self.now)
 
+    def test_invoice_plan_without_stripe_customer(self) -> None:
+        self.local_upgrade(self.seat_count, True, CustomerPlan.ANNUAL)
+        plan = get_current_plan_by_realm(get_realm("zulip"))
+        assert plan and plan.customer
+        plan.customer.stripe_customer_id = None
+        plan.customer.save(update_fields=["stripe_customer_id"])
+        with self.assertRaisesRegex(
+            BillingError, "Realm zulip has a paid plan without a Stripe customer"
+        ):
+            invoice_plan(plan, timezone_now())
+
     @mock_stripe()
     def test_invoice_plan(self, *mocks: Mock) -> None:
         user = self.example_user("hamlet")

corporate/views.py

@@ -317,6 +317,7 @@ def billing_home(request: HttpRequest) -> HttpResponse:
             )
             renewal_cents = renewal_amount(plan, now)
             charge_automatically = plan.charge_automatically
+            assert customer.stripe_customer_id is not None  # for mypy
             stripe_customer = stripe_get_customer(customer.stripe_customer_id)
             if charge_automatically:
                 payment_method = payment_method_string(stripe_customer)