fix(multi-tenancy): check existence before banning namespace (#7887)

Before, the deleting a non-existent namespace would just ban the
namespace. This would essentially not allow creating namespaces after
this banned namespace. Now, we fail instead of banning a namespace that
has not been even created yet.

Co-authored-by: Aman Mangal <aman@dgraph.io>

edgraph/multi_tenancy_ee.go

@@ -138,5 +138,8 @@ func createGuardianAndGroot(ctx context.Context, namespace uint64, passwd string
 // Authorization is handled by middlewares.
 func (s *Server) DeleteNamespace(ctx context.Context, namespace uint64) error {
 	glog.Info("Deleting namespace", namespace)
+	if _, ok := schema.State().Namespaces()[namespace]; !ok {
+		return errors.Errorf("error deleting non-existing namespace %#x", namespace)
+	}
 	return worker.ProcessDeleteNsRequest(ctx, namespace)
 }

systest/multi-tenancy/basic_test.go

@@ -292,6 +292,15 @@ func TestDeleteNamespace(t *testing.T) {
 	err = testutil.DeleteNamespace(t, galaxyToken, x.GalaxyNamespace)
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "Cannot delete default namespace")
+
+	// Deleting a non-existent namespace should error out
+	err = testutil.DeleteNamespace(t, galaxyToken, 20)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "error deleting non-existing namespace")
+	for i := 0; i < 20; i++ {
+		ns, err = testutil.CreateNamespaceWithRetry(t, galaxyToken)
+		require.NoError(t, err)
+	}
 }
 
 type liveOpts struct {