subdomains: Don't compute realm_subdomain if not needed.

We were doing an unnecessary database query on every user registration
checking the availability of the user's subdomain, when in fact this
is only required for realm creation.

zerver/forms.py

@@ -99,9 +99,11 @@ class RegistrationForm(forms.Form):
 
     def clean_realm_subdomain(self):
         # type: () -> str
+        if not self.realm_creation:
+            return Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
         subdomain = self.cleaned_data['realm_subdomain']
         if not subdomain:
-            return ''
+            return Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
         check_subdomain_available(subdomain)
         return subdomain
 

zerver/tests/test_signup.py

@@ -305,7 +305,7 @@ class LoginTest(ZulipTestCase):
         with queries_captured() as queries:
             self.register(self.nonreg_email('test'), "test")
         # Ensure the number of queries we make is not O(streams)
-        self.assert_length(queries, 69)
+        self.assert_length(queries, 68)
         user_profile = self.nonreg_user('test')
         self.assertEqual(get_session_dict_user(self.client.session), user_profile.id)
         self.assertFalse(user_profile.enable_stream_desktop_notifications)