Fix PlantUML integration in GFM

Fixes #30557
Use "lang" attribute instead of "class" in PlantumlFilter.

changelogs/unreleased/plantuml-filter-after-highlight.yml

+---
+title: Fix PlantUML integration in GFM
+merge_request: 10651
+author:

lib/banzai/filter/plantuml_filter.rb

@@ -7,14 +7,14 @@ module Banzai
     #
     class PlantumlFilter < HTML::Pipeline::Filter
       def call
-        return doc unless doc.at('pre.plantuml') && settings.plantuml_enabled
+        return doc unless doc.at('pre > code[lang="plantuml"]') && settings.plantuml_enabled
 
         plantuml_setup
 
-        doc.css('pre.plantuml').each do |el|
+        doc.css('pre > code[lang="plantuml"]').each do |node|
           img_tag = Nokogiri::HTML::DocumentFragment.parse(
-            Asciidoctor::PlantUml::Processor.plantuml_content(el.content, {}))
-          el.replace img_tag
+            Asciidoctor::PlantUml::Processor.plantuml_content(node.content, {}))
+          node.parent.replace(img_tag)
         end
 
         doc

spec/lib/banzai/filter/plantuml_filter_spec.rb

@@ -5,7 +5,7 @@ describe Banzai::Filter::PlantumlFilter, lib: true do
 
   it 'should replace plantuml pre tag with img tag' do
     stub_application_setting(plantuml_enabled: true, plantuml_url: "http://localhost:8080")
-    input = '<pre class="plantuml"><code>Bob -> Sara : Hello</code><pre>'
+    input = '<pre><code lang="plantuml">Bob -> Sara : Hello</code></pre>'
     output = '<div class="imageblock"><div class="content"><img class="plantuml" src="http://localhost:8080/png/U9npoazIqBLJ24uiIbImKl18pSd91m0rkGMq"></div></div>'
     doc = filter(input)
 
@@ -14,8 +14,8 @@ describe Banzai::Filter::PlantumlFilter, lib: true do
 
   it 'should not replace plantuml pre tag with img tag if disabled' do
     stub_application_setting(plantuml_enabled: false)
-    input = '<pre class="plantuml"><code>Bob -> Sara : Hello</code><pre>'
-    output = '<pre class="plantuml"><code>Bob -&gt; Sara : Hello</code><pre></pre></pre>'
+    input = '<pre><code lang="plantuml">Bob -> Sara : Hello</code></pre>'
+    output = '<pre><code lang="plantuml">Bob -&gt; Sara : Hello</code></pre>'
     doc = filter(input)
 
     expect(doc.to_s).to eq output
@@ -23,7 +23,7 @@ describe Banzai::Filter::PlantumlFilter, lib: true do
 
   it 'should not replace plantuml pre tag with img tag if url is invalid' do
     stub_application_setting(plantuml_enabled: true, plantuml_url: "invalid")
-    input = '<pre class="plantuml"><code>Bob -> Sara : Hello</code><pre>'
+    input = '<pre><code lang="plantuml">Bob -> Sara : Hello</code></pre>'
     output = '<div class="listingblock"><div class="content"><pre class="plantuml plantuml-error"> PlantUML Error: cannot connect to PlantUML server at "invalid"</pre></div></div>'
     doc = filter(input)