Use old purchase flow when user last name is blank

The customers app requires a last name for the creation of the customer
record. As the new purchase flow does not currently have a way for users
to set a last name, fallback to the old flow when the user does not have
a last name set. This was causing issues with users in the following
state:

  1. User does not have a last name (signed up before we split the name
     field) and their full name only has one word
  2. User does not have an existing customers dot account
  3. User is creating the subscription on .com
  4. User is creating the subscription for a group (ie, using the
     purchase flow that does not redirect to customers-dot)

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/298721

ee/app/helpers/billing_plans_helper.rb

@@ -25,6 +25,10 @@ module BillingPlansHelper
   end
 
   def use_new_purchase_flow?(namespace)
+    # new flow requires the user to already have a last name.
+    # This can be removed once https://gitlab.com/gitlab-org/gitlab/-/issues/298715 is complete.
+    return false unless current_user.last_name.present?
+
     namespace.group? && (namespace.actual_plan_name == Plan::FREE || namespace.trial_active?)
   end
 

ee/changelogs/unreleased/jh-old_purchase_flow_last_name.yml

+---
+title: Use old purchase flow when user last name is blank
+merge_request: 51730
+author:
+type: changed

ee/spec/helpers/billing_plans_helper_spec.rb

@@ -94,6 +94,36 @@ RSpec.describe BillingPlansHelper do
         is_expected.to be(result)
       end
     end
+
+    context 'when the group is on a plan eligible for the new purchase flow' do
+      let(:namespace) do
+        create(
+          :namespace,
+          type: Group,
+          gitlab_subscription: create(:gitlab_subscription, hosted_plan: create(:free_plan))
+        )
+      end
+
+      before do
+        allow(helper).to receive(:current_user).and_return(user)
+      end
+
+      context 'when the user has a last name' do
+        let(:user) { build(:user, last_name: 'Lastname') }
+
+        it 'returns true' do
+          expect(helper.use_new_purchase_flow?(namespace)).to eq true
+        end
+      end
+
+      context 'when the user does not have a last name' do
+        let(:user) { build(:user, last_name: nil, name: 'Firstname') }
+
+        it 'returns false' do
+          expect(helper.use_new_purchase_flow?(namespace)).to eq false
+        end
+      end
+    end
   end
 
   describe '#show_contact_sales_button?' do