Prevent 500 on parallel service instance creation (cloudfoundry#3899)

We observed 500 responses on POST /v3/service_instances when requests
run in parallel and the db is small.
This occurs if the one POST bypasses the other one and inserts the
record into the db after the first POST has passed the validate function
but before the actual insert. This leads to a PG::UniqueViolation: ERROR:
duplicate key value violates unique constraint which is not catched and
thus results in a 500 response. To prevent this, we add a
Sequel::UniqueConstraintViolation rescue to catch the PG::UniqueViolation error and to return an adequate 422 error "The service instance name is taken.."

app/models/services/service_instance.rb

@@ -110,6 +110,15 @@ def name_clashes
       end
     end
 
+    def around_save
+      yield
+    rescue Sequel::UniqueConstraintViolation => e
+      raise e unless e.message.include?('si_space_id_name_index')
+
+      errors.add(:name, :unique)
+      raise validation_failed_error
+    end
+
     def validate
       validates_presence :name
       validates_presence :space

spec/unit/actions/v3/service_instance_create_managed_spec.rb

@@ -214,6 +214,23 @@ module V3
           end
         end
 
+        context 'parallel creation of managed service instances' do
+          it 'ensures one creation is successful and the other fails due to name conflict' do
+            # First request, should succeed
+            expect do
+              action.precursor(message:, service_plan:)
+            end.not_to raise_error
+
+            # Mock the validation for the second request to simulate the race condition and trigger a unique constraint violation
+            allow_any_instance_of(ManagedServiceInstance).to receive(:validate).and_return(true)
+
+            # Second request, should fail with correct error message
+            expect do
+              action.precursor(message:, service_plan:)
+            end.to raise_error(ServiceInstanceCreateManaged::InvalidManagedServiceInstance, 'The service instance name is taken: si-test-name.')
+          end
+        end
+
         context 'quotas' do
           context 'when service instance limit has been reached for the space' do
             before do