Reduce Markdown cache updates during deploy

Skips saving of Markdown cache if persisted data is newer than the
current version. This happens when some Markdown cache is generated on a
newer node that has the cache version incremented during a rolling
deploy or Canary deployment and then viewed again on an older node.

Changelog: performance

lib/gitlab/markdown_cache.rb

@@ -2,10 +2,14 @@
 
 module Gitlab
   module MarkdownCache
-    # Increment this number every time the renderer changes its output.
+    # Increment this number to invalidate cached HTML from Markdown documents.
+    # Even when reverting an MR, we should increment this because we only
+    # persist the cache when the new version is higher.
+    #
     # Changing this value puts strain on the database, as every row with
-    # cached markdown needs to be updated. As a result, this line should
-    # not be changed.
+    # cached markdown needs to be updated. As a result, avoid changing
+    # this if the change to the renderer output is a new feature or a
+    # minor bug fix.
     # See: https://gitlab.com/gitlab-org/gitlab/-/issues/330313
     CACHE_COMMONMARK_VERSION        = 28
     CACHE_COMMONMARK_VERSION_START  = 10

lib/gitlab/markdown_cache/active_record/extension.rb

@@ -39,6 +39,7 @@ module Gitlab
 
         def save_markdown(updates)
           return unless persisted? && Gitlab::Database.read_write?
+          return if cached_markdown_version < cached_markdown_version_in_database
 
           update_columns(updates)
         end

spec/lib/gitlab/markdown_cache/active_record/extension_spec.rb

@@ -216,4 +216,16 @@ RSpec.describe Gitlab::MarkdownCache::ActiveRecord::Extension do
       end
     end
   end
+
+  context 'when persisted cache is newer than current version' do
+    before do
+      thing.update_column(:cached_markdown_version, thing.cached_markdown_version + 1)
+    end
+
+    it 'does not save the generated HTML' do
+      expect(thing).not_to receive(:update_columns)
+
+      thing.refresh_markdown_cache!
+    end
+  end
 end