Merge branch 'debug-banzai-cache' into 'master'

Add DEBUG_BANZAI_CACHE env var to debug Banzai cache issue.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/4130#note_3012511

See merge request !2283

lib/banzai/renderer.rb

 module Banzai
   module Renderer
-    CACHE_ENABLED = false
-
     # Convert a Markdown String into an HTML-safe String of HTML
     #
     # Note that while the returned HTML will have been sanitized of dangerous
@@ -20,13 +18,22 @@ module Banzai
       cache_key = context.delete(:cache_key)
       cache_key = full_cache_key(cache_key, context[:pipeline])
 
-      if cache_key && CACHE_ENABLED
-        Rails.cache.fetch(cache_key) do
-          cacheless_render(text, context)
+      cacheless = cacheless_render(text, context)
+
+      if cache_key && ENV["DEBUG_BANZAI_CACHE"]
+        cached = Rails.cache.fetch(cache_key) { cacheless }
+
+        if cached != cacheless
+          Rails.logger.warn "Banzai cache mismatch"
+          Rails.logger.warn "Text: #{text.inspect}"
+          Rails.logger.warn "Context: #{context.inspect}"
+          Rails.logger.warn "Cache key: #{cache_key.inspect}"
+          Rails.logger.warn "Cacheless: #{cacheless.inspect}"
+          Rails.logger.warn "With cache: #{cached.inspect}"
         end
-      else
-        cacheless_render(text, context)
       end
+
+      cacheless
     end
 
     def self.render_result(text, context = {})