Fix performance issue caused by using repeated > characters after `…

…<!DOCTYPE name`

A `<` is treated as a string delimiter. 
In certain cases, if `<` is used in succession, read and match are
repeated, which slows down the process. Therefore, the following is used
to read ahead to a specific part of the string in advance.

lib/rexml/parsers/baseparser.rb

@@ -128,6 +128,7 @@ module Private
         INSTRUCTION_TERM = "?>"
         COMMENT_TERM = "-->"
         CDATA_TERM = "]]>"
+        DOCTYPE_TERM = "]>"
         TAG_PATTERN = /((?>#{QNAME_STR}))\s*/um
         CLOSE_PATTERN = /(#{QNAME_STR})\s*>/um
         ATTLISTDECL_END = /\s+#{NAME}(?:#{ATTDEF})*\s*>/um
@@ -384,7 +385,7 @@ def pull_event
               end
               return [ :comment, md[1] ] if md
             end
-          elsif match = @source.match(/(%.*?;)\s*/um, true)
+          elsif match = @source.match(/(%.*?;)\s*/um, true, term: Private::DOCTYPE_TERM)
             return [ :externalentity, match[1] ]
           elsif @source.match(/\]\s*>/um, true)
             @document_status = :after_doctype

test/parse/test_document_type_declaration.rb

@@ -1,9 +1,13 @@
 # frozen_string_literal: false
 require "test/unit"
+require "core_assertions"
+
 require "rexml/document"
 
 module REXMLTests
   class TestParseDocumentTypeDeclaration < Test::Unit::TestCase
+    include Test::Unit::CoreAssertions
+
     private
     def parse(doctype)
       REXML::Document.new(<<-XML).doctype
@@ -276,6 +280,16 @@ def test_notation_attlist
                      doctype.children.collect(&:class))
       end
 
+      def test_gt_linear_performance_malformed_entity
+        seq = [10000, 50000, 100000, 150000, 200000]
+        assert_linear_performance(seq, rehearsal: 10) do |n|
+          begin
+            REXML::Document.new('<!DOCTYPE root [' + "%>" * n + ']><test/>')
+          rescue
+          end
+        end
+      end
+
       private
       def parse(internal_subset)
         super(<<-DOCTYPE)