Fix Namespace.find_by_pages_host - return results only for subdomains

of the configured Pages domain.

Related to https://gitlab.com/gitlab-org/gitlab/issues/28781.

app/models/namespace.rb

@@ -123,8 +123,10 @@ class Namespace < ApplicationRecord
 
     def find_by_pages_host(host)
       gitlab_host = "." + Settings.pages.host.downcase
-      name = host.downcase.delete_suffix(gitlab_host)
+      host = host.downcase
+      return unless host.ends_with?(gitlab_host)
 
+      name = host.delete_suffix(gitlab_host)
       Namespace.find_by_full_path(name)
     end
   end

spec/models/namespace_spec.rb

@@ -199,6 +199,13 @@ describe Namespace do
 
       expect(described_class.find_by_pages_host(host)).to eq(namespace)
     end
+
+    it "returns no result if the provided host is not subdomain of the Pages host" do
+      create(:namespace, name: 'namespace.io')
+      host = "namespace.io"
+
+      expect(described_class.find_by_pages_host(host)).to eq(nil)
+    end
   end
 
   describe '#ancestors_upto' do