Fix broken links on help page

Update the help controller to correctly handle relative links on the help pages
when the relative link is before an external link on the same line in the
markdown file.

Test cases have been implement to check for
 - relative links before external link on same line,
 - HTTPS in query part of link,
 - URLs without '//' and
 - protocol-relative links.

app/controllers/help_controller.rb

@@ -13,9 +13,11 @@ class HelpController < ApplicationController
     # Remove YAML frontmatter so that it doesn't look weird
     @help_index = File.read(Rails.root.join('doc', 'README.md')).sub(YAML_FRONT_MATTER_REGEXP, '')
 
-    # Prefix Markdown links with `help/` unless they are external links
-    # See http://rubular.com/r/X3baHTbPO2
-    @help_index.gsub!(%r{(?<delim>\]\()(?!.+://)(?!/)(?<link>[^\)\(]+\))}) do
+    # Prefix Markdown links with `help/` unless they are external links.
+    # RFC3986 allows alphanumeric, '+', '-' and '.' for URL schema;
+    # '//' not necessarily part of URL, e.g., mailto:mail@example.com
+    # See https://rubular.com/r/C5wTz0gE5r6x0f
+    @help_index.gsub!(%r{(?<delim>\]\()(?![a-zA-Z0-9.+-]+:)(?!/)(?<link>[^\)\(]+\))}) do
       "#{$~[:delim]}#{Gitlab.config.gitlab.relative_url_root}/help/#{$~[:link]}"
     end
   end

spec/controllers/help_controller_spec.rb

@@ -37,6 +37,46 @@ describe HelpController do
         expect(assigns[:help_index]).to eq '[external](https://some.external.link)'
       end
     end
+
+    context 'when relative url with external on same line' do
+      it 'prefix it with /help/' do
+        stub_readme("[API](api/README.md) [external](https://some.external.link)")
+
+        get :index
+
+        expect(assigns[:help_index]).to eq '[API](/help/api/README.md) [external](https://some.external.link)'
+      end
+    end
+
+    context 'when relative url with http:// in query' do
+      it 'prefix it with /help/' do
+        stub_readme("[API](api/README.md?go=https://example.com/)")
+
+        get :index
+
+        expect(assigns[:help_index]).to eq '[API](/help/api/README.md?go=https://example.com/)'
+      end
+    end
+
+    context 'when mailto URL' do
+      it 'do not change it' do
+        stub_readme("[report bug](mailto:bugs@example.com)")
+
+        get :index
+
+        expect(assigns[:help_index]).to eq '[report bug](mailto:bugs@example.com)'
+      end
+    end
+
+    context 'when protocol-relative link' do
+      it 'do not change it' do
+        stub_readme("[protocol-relative](//example.com)")
+
+        get :index
+
+        expect(assigns[:help_index]).to eq '[protocol-relative](//example.com)'
+      end
+    end
   end
 
   describe 'GET #show' do