Ensure DB is writable before continuing jobs

In the context of a Geo setup, some jobs can be
running on a Geo secondary where the database
is read-only and therefore we should guard
against various jobs attempting to write.

app/workers/pages_domain_verification_cron_worker.rb

@@ -5,6 +5,8 @@ class PagesDomainVerificationCronWorker
   include CronjobQueue
 
   def perform
+    return if Gitlab::Database.read_only?
+
     PagesDomain.needs_verification.find_each do |domain|
       PagesDomainVerificationWorker.perform_async(domain.id)
     end

app/workers/pages_domain_verification_worker.rb

@@ -5,6 +5,8 @@ class PagesDomainVerificationWorker
 
   # rubocop: disable CodeReuse/ActiveRecord
   def perform(domain_id)
+    return if Gitlab::Database.read_only?
+
     domain = PagesDomain.find_by(id: domain_id)
 
     return unless domain

spec/workers/pages_domain_verification_cron_worker_spec.rb

@@ -10,6 +10,13 @@ describe PagesDomainVerificationCronWorker do
     let!(:reverify) { create(:pages_domain, :reverify) }
     let!(:disabled) { create(:pages_domain, :disabled) }
 
+    it 'does nothing if the database is read-only' do
+      allow(Gitlab::Database).to receive(:read_only?).and_return(true)
+      expect(PagesDomainVerificationWorker).not_to receive(:perform_async).with(reverify.id)
+
+      worker.perform
+    end
+
     it 'enqueues a PagesDomainVerificationWorker for domains needing verification' do
       [reverify, disabled].each do |domain|
         expect(PagesDomainVerificationWorker).to receive(:perform_async).with(domain.id)

spec/workers/pages_domain_verification_worker_spec.rb

@@ -8,6 +8,13 @@ describe PagesDomainVerificationWorker do
   let(:domain) { create(:pages_domain) }
 
   describe '#perform' do
+    it 'does nothing if the database is read-only' do
+      allow(Gitlab::Database).to receive(:read_only?).and_return(true)
+      expect(PagesDomain).not_to receive(:find_by).with(id: domain.id)
+
+      worker.perform(domain.id)
+    end
+
     it 'does nothing for a non-existent domain' do
       domain.destroy