Domains: don't run CNAME checks on domain updates (#12564)

This check only makes sense when adding a new domain, not when updating.
And on .com, the target CNAME is a domain that points to another CNAME,
so the check always fails after a valid domain is added. The domain
field is set to read-only, so this is fine.

Author: stsewd

readthedocs/projects/forms.py

@@ -1020,7 +1020,9 @@ def clean_domain(self):
             if invalid_domain and domain_string.endswith(invalid_domain):
                 raise forms.ValidationError(f"{invalid_domain} is not a valid domain.")
 
-        self._check_for_suspicious_cname(domain_string)
+        # Run this check only on domain creation.
+        if not self.instance.pk:
+            self._check_for_suspicious_cname(domain_string)
 
         return domain_string
 

readthedocs/projects/tests/test_domain_views.py

@@ -173,6 +173,19 @@ def test_create_domain_with_single_cname(self, dns_resolve_mock):
         domain = self.project.domains.first()
         assert domain.domain == "docs.example.com"
 
+    @mock.patch("readthedocs.projects.forms.dns.resolver.resolve")
+    def test_editing_domain_doesnt_trigger_cname_validation(self, dns_resolve_mock):
+        domain = get(Domain, project=self.project, domain="docs.example.com")
+        assert not domain.canonical
+        resp = self.client.post(
+            reverse("projects_domains_edit", args=[self.project.slug, domain.pk]),
+            data={"canonical": True},
+        )
+        assert resp.status_code == 302
+        domain.refresh_from_db()
+        assert domain.canonical
+        dns_resolve_mock.assert_not_called()
+
     @override_settings(
         RTD_DEFAULT_FEATURES=dict(
             [RTDProductFeature(type=TYPE_CNAME, value=1).to_item()]