Fix bug in wiki link rewriter filter

If the url inside an html element is relative and it points
to the wiki base path, we still append the wiki base path
to the url, resulting in invalid urls.

changelogs/unreleased/fj-fix-bug-with-wiki-link-rewriter.yml

+---
+title: Fix bug in wiki link rewriter filter
+merge_request: 56636
+author:
+type: fixed

lib/banzai/filter/wiki_link_filter/rewriter.rb

@@ -6,7 +6,7 @@ module Banzai
       class Rewriter
         def initialize(link_string, wiki:, slug:)
           @uri = Addressable::URI.parse(link_string)
-          @wiki_base_path = wiki && wiki.wiki_base_path
+          @wiki_base_path = wiki&.wiki_base_path
           @slug = slug
         end
 
@@ -41,7 +41,8 @@ module Banzai
         # Any link _not_ of the form `http://example.com/`
         def apply_relative_link_rules!
           if @uri.relative? && @uri.path.present?
-            link = ::File.join(@wiki_base_path, @uri.path)
+            link = @uri.path
+            link = ::File.join(@wiki_base_path, link) unless link.starts_with?(@wiki_base_path)
             link = "#{link}##{@uri.fragment}" if @uri.fragment
             @uri = Addressable::URI.parse(link)
           end

spec/lib/banzai/filter/wiki_link_filter_spec.rb

@@ -22,6 +22,15 @@ RSpec.describe Banzai::Filter::WikiLinkFilter do
     expect(filtered_link.attribute('href').value).to eq('/uploads/a.test')
   end
 
+  describe 'when links point to the relative wiki path' do
+    it 'does not rewrite links' do
+      path = "#{wiki.wiki_base_path}/#{repository_upload_folder}/a.jpg"
+      filtered_link = filter("<a href='#{path}'>Link</a>", wiki: wiki, page_slug: 'home').children[0]
+
+      expect(filtered_link.attribute('href').value).to eq(path)
+    end
+  end
+
   describe "when links point to the #{Wikis::CreateAttachmentService::ATTACHMENT_PATH} folder" do
     context 'with an "a" html tag' do
       it 'rewrites links' do