Validate certificate chain only if it's changed

This validation prevents the domain from being saved from the UI
e.g. when user tries to enable Let's Encrypt integration

app/models/pages_domain.rb

@@ -22,7 +22,7 @@ class PagesDomain < ApplicationRecord
 
   validate :validate_pages_domain
   validate :validate_matching_key, if: ->(domain) { domain.certificate.present? || domain.key.present? }
-  validate :validate_intermediates, if: ->(domain) { domain.certificate.present? }
+  validate :validate_intermediates, if: ->(domain) { domain.certificate.present? && domain.certificate_changed? }
 
   attr_encrypted :key,
     mode: :per_attribute_iv_and_salt,

changelogs/unreleased/64870-can-t-save-pages-domain-form-with-let-s-encrypt-enabled-if-current-certificate-is-outdated.yml

+---
+title: Fix "Certificate misses intermediates" UI error when enabling Let's Encrypt
+  integration for pages domain
+merge_request: 30995
+author:
+type: fixed

spec/factories/pages_domains.rb

@@ -166,6 +166,90 @@ pu/xO28QOG8=
 -----END CERTIFICATE-----'
     end
 
+    trait :with_trusted_expired_chain do
+      # This contains
+      # Let's Encrypt Authority X3
+      # DST Root CA X3
+      certificate '-----BEGIN CERTIFICATE-----
+MIIFSjCCBDKgAwIBAgISAw24xGWrFotvTBa6AZI/pzq1MA0GCSqGSIb3DQEBCwUA
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+7NuX+D2esUNos/D6uJg=
+-----END CERTIFICATE-----
+-----BEGIN CERTIFICATE-----
+MIIEkjCCA3qgAwIBAgIQCgFBQgAAAVOFc2oLheynCDANBgkqhkiG9w0BAQsFADA/
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+-----END CERTIFICATE-----
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----'
+    end
+
     trait :with_expired_certificate do
       certificate '-----BEGIN CERTIFICATE-----
 MIIBsDCCARmgAwIBAgIBATANBgkqhkiG9w0BAQUFADAeMRwwGgYDVQQDExNleHBp

spec/models/pages_domain_spec.rb

@@ -127,6 +127,30 @@ describe PagesDomain do
 
       it { is_expected.not_to be_valid }
     end
+
+    context 'when certificate is expired' do
+      let(:domain) do
+        build(:pages_domain, :with_trusted_expired_chain)
+      end
+
+      context 'when certificate is being changed' do
+        it "adds error to certificate" do
+          domain.valid?
+
+          expect(domain.errors.keys).to contain_exactly(:key, :certificate)
+        end
+      end
+
+      context 'when certificate is already saved' do
+        it "doesn't add error to certificate" do
+          domain.save(validate: false)
+
+          domain.valid?
+
+          expect(domain.errors.keys).to contain_exactly(:key)
+        end
+      end
+    end
   end
 
   describe 'validations' do