FIX: Rescue `ActiveRecord::ReadOnlyError` when baking theme field (#29776)

Firstly, we need to understand that ActiveRecord can be
connected to a role which prevent writes and this happens in Discourse when a
replica database has been setup for failover purposes. When a role
prevent writes from happening, ActiveRecord will raise the
`ActiveRecord::ReadOnlyError` if a write query is attempted.

Secondly, theme fields are baked at runtime within GET requests. The
baking process involves writing the baked value to the
`ThemeField#baked_value` column in the database.

If we combine the two points above, we can see how the writing of the
baked value to the database will trigger a `ActiveRecord::ReadOnlyError`
in a GET requests when the database is connected to a role preventing
writes. However, failing to bake a theme is not the end of the world and
should not cause GET requests to fail. Therefore, this commit adds a rescue
for `ActiveRecord::ReadOnlyError` in the `ThemeField#ensure_baked!`
method.

app/models/theme_field.rb

@@ -475,6 +475,9 @@ class ThemeField < ActiveRecord::Base
         error: error,
       )
     end
+  rescue ActiveRecord::ReadOnlyError
+    # Just noop if ActiveRecord is preventing writes for now. In an ideal world, this method will not be called in GET
+    # requests.
   end
 
   def compile_scss(prepended_scss = nil)

lib/scheduler/defer.rb

@@ -6,6 +6,8 @@ module Scheduler
     DEFAULT_TIMEOUT = 90
     STATS_CACHE_SIZE = 100
 
+    attr_reader :async
+
     def initialize
       @async = !Rails.env.test?
       @queue =

spec/integration/activerecord_preventing_writes_spec.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+RSpec.describe "When ActiveRecord is preventing writes" do
+  before do
+    @original_async = Scheduler::Defer.async
+    Scheduler::Defer.async = true
+  end
+
+  after { Scheduler::Defer.async = @original_async }
+
+  it "should not result in an error response when there is a theme field that needs to be baked" do
+    theme_field =
+      Fabricate(
+        :theme_field,
+        type_id: ThemeField.types[:html],
+        target_id: Theme.targets[:common],
+        name: "head_tag",
+        value: <<~HTML,
+        <script type="text/discourse-plugin" version="0.1">
+          console.log(settings.uploads.imajee);
+        </script>
+      HTML
+      )
+
+    SiteSetting.default_theme_id = theme_field.theme_id
+
+    ActiveRecord::Base.connected_to(role: ActiveRecord.writing_role, prevent_writes: true) do
+      get "/latest"
+
+      expect(request.env[:resolved_theme_id]).to eq(theme_field.theme_id)
+      expect(response.status).to eq(200)
+    end
+  end
+end

spec/rails_helper.rb

@@ -713,6 +713,7 @@ RSpec.configure do |config|
     unfreeze_time
     ActionMailer::Base.deliveries.clear
     Discourse.redis.flushdb
+    Scheduler::Defer.do_all_work
   end
 
   config.after(:each, type: :system) do |example|