Makes sure the sub_dir is only checked once

lib/wpscan/target/platform/wordpress/custom_directories.rb

@@ -99,9 +99,11 @@ module WPScan
 
         # @return [ String, False ] String of the sub_dir found, false otherwise
         # @note: nil can not be returned here, otherwise if there is no sub_dir
-        #        the check would be done each time
+        #        the check would be done each time, which would make enumeration of
+        #        long list of items very slow to generate
         def sub_dir
-          unless @sub_dir
+          return @sub_dir unless @sub_dir.nil?
+
           # url_pattern is from CMSScanner::Target
           pattern = %r{#{url_pattern}(.+?)/(?:xmlrpc\.php|wp\-includes/)}i
 
@@ -112,9 +114,6 @@ module WPScan
           @sub_dir = false
         end
 
-          @sub_dir
-        end
-
         # Override of the WebSite#url to consider the custom WP directories
         #
         # @param [ String ] path Optional path to merge with the uri

spec/shared_examples/target/platform/wordpress/custom_directories.rb

@@ -138,6 +138,17 @@ shared_examples 'WordPress::CustomDirectories' do
         expect(target.sub_dir).to eql expected
       end
     end
+
+    context 'when no sub_dir detected' do
+      it 'only checks the in_scope_uris once' do
+        stub_request(:get, target.url).to_return(body: File.read(File.join(fixtures, 'default.html')))
+
+        expect(target.sub_dir).to eql false
+
+        expect(target).to_not receive(:in_scope_uris)
+        expect(target.sub_dir).to eql false
+      end
+    end
   end
 
   describe '#url' do