Conditionally cache Snippet content

Not all Snippets contain Markdown content and so
the `content` field should not always be cached.

This change will attempt to determine if the content
is Markdown based on the Snippet filename and only cache
if it is

app/models/concerns/cache_markdown_field.rb

@@ -20,6 +20,10 @@ module CacheMarkdownField
     false
   end
 
+  def can_cache_field?(field)
+    true
+  end
+
   # Returns the default Banzai render context for the cached markdown field.
   def banzai_render_context(field)
     raise ArgumentError.new("Unknown field: #{field.inspect}") unless
@@ -38,17 +42,23 @@ module CacheMarkdownField
     context
   end
 
-  # Update every column in a row if any one is invalidated, as we only store
-  # one version per row
-  def refresh_markdown_cache
+  def rendered_field_content(markdown_field)
+    return unless can_cache_field?(markdown_field)
+
     options = { skip_project_check: skip_project_check? }
+    Banzai::Renderer.cacheless_render_field(self, markdown_field, options)
+  end
 
+  # Update every applicable column in a row if any one is invalidated, as we only store
+  # one version per row
+  def refresh_markdown_cache
     updates = cached_markdown_fields.markdown_fields.map do |markdown_field|
       [
         cached_markdown_fields.html_field(markdown_field),
-        Banzai::Renderer.cacheless_render_field(self, markdown_field, options)
+        rendered_field_content(markdown_field)
       ]
     end.to_h
+
     updates['cached_markdown_version'] = latest_cached_markdown_version
 
     updates.each { |field, data| write_markdown_field(field, data) }

app/models/snippet.rb

@@ -301,6 +301,10 @@ class Snippet < ApplicationRecord
     repository.update!(shard_name: repository_storage, disk_path: disk_path)
   end
 
+  def can_cache_field?(field)
+    field != :content || MarkupHelper.gitlab_markdown?(file_name)
+  end
+
   class << self
     # Searches for snippets with a matching title or file name.
     #

changelogs/unreleased/207976-stop-markdown-caching-of-non-markdown-snippet-content.yml

+---
+title: Fix Snippet content incorrectly caching
+merge_request: 25985
+author:
+type: fixed

spec/models/concerns/cache_markdown_field_spec.rb

@@ -230,6 +230,26 @@ describe CacheMarkdownField, :clean_gitlab_redis_cache do
         end
       end
     end
+
+    describe '#rendered_field_content' do
+      let(:thing) { klass.new(description: markdown, description_html: nil, cached_markdown_version: cache_version) }
+
+      context 'when a field can be cached' do
+        it 'returns the html' do
+          thing.description = updated_markdown
+
+          expect(thing.rendered_field_content(:description)).to eq updated_html
+        end
+      end
+
+      context 'when a field cannot be cached' do
+        it 'returns nil' do
+          allow(thing).to receive(:can_cache_field?).with(:description).and_return false
+
+          expect(thing.rendered_field_content(:description)).to eq nil
+        end
+      end
+    end
   end
 
   context 'for Active record classes' do

spec/models/snippet_spec.rb

@@ -631,4 +631,26 @@ describe Snippet do
       end
     end
   end
+
+  describe '#can_cache_field?' do
+    using RSpec::Parameterized::TableSyntax
+
+    let(:snippet) { create(:snippet, file_name: file_name) }
+
+    subject { snippet.can_cache_field?(field) }
+
+    where(:field, :file_name, :result) do
+      :title       | nil           | true
+      :title       | 'foo.bar'     | true
+      :description | nil           | true
+      :description | 'foo.bar'     | true
+      :content     | nil           | false
+      :content     | 'bar.foo'     | false
+      :content     | 'markdown.md' | true
+    end
+
+    with_them do
+      it { is_expected.to eq result }
+    end
+  end
 end