parser tree: improve namespace conflicted attribute check performance

It was slow for deep element.

Reported by l33thaxor. Thanks!!!

lib/rexml/element.rb

@@ -2384,17 +2384,6 @@ def []=( name, value )
       elsif old_attr.kind_of? Hash
         old_attr[value.prefix] = value
       elsif old_attr.prefix != value.prefix
-        # Check for conflicting namespaces
-        if value.prefix != "xmlns" and old_attr.prefix != "xmlns"
-          old_namespace = old_attr.namespace
-          new_namespace = value.namespace
-          if old_namespace == new_namespace
-            raise ParseException.new(
-                    "Namespace conflict in adding attribute \"#{value.name}\": "+
-                    "Prefix \"#{old_attr.prefix}\" = \"#{old_namespace}\" and "+
-                    "prefix \"#{value.prefix}\" = \"#{new_namespace}\"")
-          end
-        end
         store value.name, {old_attr.prefix => old_attr,
                            value.prefix    => value}
       else

lib/rexml/parsers/baseparser.rb

@@ -754,6 +754,7 @@ def process_instruction
 
       def parse_attributes(prefixes)
         attributes = {}
+        expanded_names = {}
         closed = false
         while true
           if @source.match(">", true)
@@ -805,6 +806,20 @@ def parse_attributes(prefixes)
               raise REXML::ParseException.new(msg, @source, self)
             end
 
+            unless prefix == "xmlns"
+              uri = @namespaces[prefix]
+              expanded_name = [uri, local_part]
+              existing_prefix = expanded_names[expanded_name]
+              if existing_prefix
+                message = "Namespace conflict in adding attribute " +
+                          "\"#{local_part}\": " +
+                          "Prefix \"#{existing_prefix}\" = \"#{uri}\" and " +
+                          "prefix \"#{prefix}\" = \"#{uri}\""
+                raise REXML::ParseException.new(message, @source, self)
+              end
+              expanded_names[expanded_name] = prefix
+            end
+
             attributes[name] = value
           else
             message = "Invalid attribute name: <#{@source.buffer.split(%r{[/>\s]}).first}>"

test/parse/test_element.rb

@@ -131,5 +131,19 @@ def test_linear_performance_attribute_value_gt
         REXML::Document.new('<test testing="' + ">" * n + '"></test>')
       end
     end
+
+    def test_linear_performance_deep_same_name_attributes
+      seq = [100, 500, 1000, 1500, 2000]
+      assert_linear_performance(seq, rehearsal: 10) do |n|
+        xml = <<-XML
+<?xml version="1.0"?>
+<root xmlns:ns="ns-uri">
+#{"<x ns:name='ns-value' name='value'>\n" * n}
+#{"</x>\n" * n}
+</root>
+        XML
+        REXML::Document.new(xml)
+      end
+    end
   end
 end

test/test_core.rb

@@ -136,6 +136,10 @@ def test_attribute_namespace_conflict
       # https://www.w3.org/TR/xml-names/#uniqAttrs
       message = <<-MESSAGE.chomp
 Namespace conflict in adding attribute "a": Prefix "n1" = "http://www.w3.org" and prefix "n2" = "http://www.w3.org"
+Line: 4
+Position: 140
+Last 80 unconsumed characters:
+/>
       MESSAGE
       assert_raise(REXML::ParseException.new(message)) do
         Document.new(<<-XML)