Fix Element#attribute when a namespace URI is bound to multiple prefixes

Element#attribute used namespaces.key(namespace), which returns only the
first prefix bound to that URI, so an attribute using any other prefix for
the same URI was not found. Try every prefix bound to the URI instead,
keeping the unprefixed attribute preferred when the URI is the default
namespace.

Author: sjh9714

lib/rexml/element.rb

@@ -1269,20 +1269,22 @@ def [](name_or_index)
     #   document.root.attribute("x", "a") # => a:x='a:x'
     #
     def attribute( name, namespace=nil )
-      prefix = namespaces.key(namespace) if namespace
-      prefix = nil if prefix == 'xmlns'
+      prefixes = namespace ? namespaces.select {|_, uri| uri == namespace}.keys : []
 
-      ret_val =
-        attributes.get_attribute( prefix ? "#{prefix}:#{name}" : name )
-
-      return ret_val unless ret_val.nil?
-      return nil if prefix.nil?
+      # An unprefixed attribute is used for the default namespace.
+      if prefixes.empty? or prefixes.include?( 'xmlns' )
+        ret_val = attributes.get_attribute( name )
+        return ret_val unless ret_val.nil?
+      end
 
-      # now check that prefix'es namespace is not the same as the
-      # default namespace
-      return nil unless ( namespaces[ prefix ] == namespaces[ 'xmlns' ] )
+      # The same namespace URI may be bound to multiple prefixes.
+      prefixes.each do |prefix|
+        next if prefix == 'xmlns'
+        ret_val = attributes.get_attribute( "#{prefix}:#{name}" )
+        return ret_val unless ret_val.nil?
+      end
 
-      attributes.get_attribute( name )
+      nil
     end
 
     # :call-seq:

test/test_element.rb

@@ -11,5 +11,20 @@ def test_array_reference_symbol
       doc = REXML::Document.new("<language name='Ruby'/>")
       assert_equal("Ruby", doc.root[:name])
     end
+
+    def test_attribute_duplicated_namespace_url
+      doc = REXML::Document.new("<root xmlns='url1' xmlns:ns1='url1' " +
+                                "xmlns:ns2='url2' xmlns:ns3='url2' " +
+                                "a='' ns1:a='' ns1:b='' ns2:c='' ns3:d=''/>")
+      root = doc.root
+      attributes = [
+        root.attribute("a", "url1"),
+        root.attribute("b", "url1"),
+        root.attribute("c", "url2"),
+        root.attribute("d", "url2"),
+      ]
+      assert_equal(["a", "ns1:b", "ns2:c", "ns3:d"],
+                   attributes.collect {|attribute| attribute&.expanded_name})
+    end
   end
 end