Fix attribute's default namespace behavior

NOTE: It's a backward incompatible change. If we have any serious
problems with this change, we may revert this change.

The XML namespace specification says the default namespace doesn't
apply to attribute names but it does in REXML without this change:

https://www.w3.org/TR/xml-names/#uniqAttrs

> the default namespace does not apply to attribute names

REXML reports a parse error for the following XML that is described as
a valid XML in the XML nsmaspace specification without this change:

    <!-- http://www.w3.org is bound to n1 and is the default -->
    <x xmlns:n1="http://www.w3.org"
       xmlns="http://www.w3.org" >
      <good a="1"     b="2" />
      <good a="1"     n1:a="2" />
    </x>

If attribute doesn't have prefix, the attribute should return "" for
both #prefix and #namespace.

lib/rexml/attribute.rb

@@ -67,15 +67,11 @@ def initialize( first, second=nil, parent=nil )
     #  e.add_attribute( "nsa:a", "aval" )
     #  e.add_attribute( "b", "bval" )
     #  e.attributes.get_attribute( "a" ).prefix   # -> "nsa"
-    #  e.attributes.get_attribute( "b" ).prefix   # -> "elns"
+    #  e.attributes.get_attribute( "b" ).prefix   # -> ""
     #  a = Attribute.new( "x", "y" )
     #  a.prefix                                   # -> ""
     def prefix
-      pf = super
-      if pf == ""
-        pf = @element.prefix if @element
-      end
-      pf
+      super
     end
 
     # Returns the namespace URL, if defined, or nil otherwise
@@ -87,9 +83,8 @@ def prefix
     #  e.attribute("ns:a").namespace # => "http://url"
     #  e.attribute("nsx:a").namespace # => nil
     #
-    # TODO: This method should always return nil for no namespace
-    # attribute. Because the default namespace doesn't apply to
-    # attribute name.
+    # This method always returns "" for no namespace attribute. Because
+    # the default namespace doesn't apply to attribute names.
     #
     # From https://www.w3.org/TR/xml-names/#uniqAttrs
     #
@@ -99,10 +94,14 @@ def prefix
     #  e.add_namespace("", "http://example.com/")
     #  e.namespace # => "http://example.com/"
     #  e.add_attribute("a", "b")
-    #  e.attribute("a").namespace # => nil
+    #  e.attribute("a").namespace # => ""
     def namespace arg=nil
       arg = prefix if arg.nil?
-      @element.namespace arg
+      if arg == ""
+        ""
+      else
+        @element.namespace(arg)
+      end
     end
 
     # Returns true if other is an Attribute and has the same name and value,

lib/rexml/element.rb

@@ -1133,8 +1133,8 @@ def []=( name, value )
       elsif old_attr.prefix != value.prefix
         # Check for conflicting namespaces
         if value.prefix != "xmlns" and old_attr.prefix != "xmlns"
-          old_namespace = @element.namespace(old_attr.prefix)
-          new_namespace = @element.namespace(value.prefix)
+          old_namespace = old_attr.namespace
+          new_namespace = value.namespace
           if old_namespace == new_namespace
             raise ParseException.new(
                     "Namespace conflict in adding attribute \"#{value.name}\": "+

lib/rexml/xpath_parser.rb

@@ -493,17 +493,11 @@ def node_test(path_stack, nodesets, any_type: :element)
               if prefix.nil?
                 raw_node.name == name
               elsif prefix.empty?
-                # FIXME: This DOUBLES the time XPath searches take
-                raw_node.name == name and
-                  raw_node.namespace == raw_node.element.namespace
+                raw_node.name == name and raw_node.namespace == ""
               else
                 # FIXME: This DOUBLES the time XPath searches take
                 ns = get_namespace(raw_node.element, prefix)
-                if ns.empty?
-                  raw_node.name == name and raw_node.prefix.empty?
-                else
-                  raw_node.name == name and raw_node.namespace == ns
-                end
+                raw_node.name == name and raw_node.namespace == ns
               end
             else
               false

test/rexml/test_core.rb

@@ -1,4 +1,4 @@
-# coding: binary
+# -*- coding: utf-8 -*-
 # frozen_string_literal: false
 
 require_relative "rexml_test_utils"
@@ -116,6 +116,54 @@ def test_attribute
         name4='test4'/>).join(' '), e.to_s
     end
 
+    def test_attribute_namespace_conflict
+      # https://www.w3.org/TR/xml-names/#uniqAttrs
+      message = <<-MESSAGE
+Duplicate attribute "a"
+Line: 4
+Position: 140
+Last 80 unconsumed characters:
+      MESSAGE
+      assert_raise_with_message(REXML::ParseException, message) do
+        Document.new(<<-XML)
+<!-- http://www.w3.org is bound to n1 and n2 -->
+<x xmlns:n1="http://www.w3.org"
+   xmlns:n2="http://www.w3.org" >
+  <bad a="1"     a="2" />
+  <bad n1:a="1"  n2:a="2" />
+</x>
+        XML
+      end
+    end
+
+    def test_attribute_default_namespace
+      # https://www.w3.org/TR/xml-names/#uniqAttrs
+      document = Document.new(<<-XML)
+<!-- http://www.w3.org is bound to n1 and is the default -->
+<x xmlns:n1="http://www.w3.org"
+   xmlns="http://www.w3.org" >
+  <good a="1"     b="2" />
+  <good a="1"     n1:a="2" />
+</x>
+      XML
+      attributes = document.root.elements.collect do |element|
+        element.attributes.each_attribute.collect do |attribute|
+          [attribute.prefix, attribute.namespace, attribute.name]
+        end
+      end
+      assert_equal([
+                     [
+                       ["", "", "a"],
+                       ["", "", "b"],
+                     ],
+                     [
+                       ["", "", "a"],
+                       ["n1", "http://www.w3.org", "a"],
+                     ],
+                   ],
+                   attributes)
+    end
+
     def test_cdata
       test = "The quick brown fox jumped
         & < & < \" '