fix: Extra content at the end of the document

## Why?

XML with additional content at the end of the document is invalid.

https://www.w3.org/TR/2006/REC-xml11-20060816/#document

```
[1]   document   ::=   ( prolog element Misc* ) - ( Char* RestrictedChar Char* )
```

https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-Misc

```
[27]   	Misc	   ::=   	Comment | PI | S
```

https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-PI

```
[16]   	PI	   ::=   	'<?' PITarget (S (Char* - (Char* '?>' Char*)))? '?>'
```

https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-PITarget

```
[17]   	PITarget	   ::=   	Name - (('X' | 'x') ('M' | 'm') ('L' | 'l'))
```

lib/rexml/parsers/baseparser.rb

@@ -146,6 +146,7 @@ def initialize( source )
         self.stream = source
         @listeners = []
         @prefixes = Set.new
+        @root_tag = nil
       end
 
       def add_listener( listener )
@@ -460,15 +461,24 @@ def pull_event
                 @closed = tag
                 @nsstack.shift
               else
+                if @tags.size.zero? && !@root_tag.nil?
+                  raise ParseException.new("Malformed XML: Extra tag at the end of the document (got '<#{tag}')", @source)
+                end
                 @tags.push( tag )
               end
+              @root_tag ||= tag
               return [ :start_element, tag, attributes ]
             end
           else
             text = @source.read_until("<")
             if text.chomp!("<")
               @source.position -= "<".bytesize
             end
+            if @tags.size.zero? && !@root_tag.nil?
+              if text.strip != ""
+                raise ParseException.new("Malformed XML: Extra content at the end of the document (got '#{text}')", @source)
+              end
+            end
             return [ :text, text ]
           end
         rescue REXML::UndefinedNamespaceException
@@ -635,6 +645,9 @@ def process_instruction(start_position)
           @source.position = start_position
           raise REXML::ParseException.new(message, @source)
         end
+        if @tags.size.zero? && !@root_tag.nil? && match_data[1] == "xml"
+          raise ParseException.new("Malformed XML: Extra XML declaration at the end of the document (got '<?#{match_data[1]}')", @source)
+        end
         if @document_status.nil? and match_data[1] == "xml"
           content = match_data[2]
           version = VERSION.match(content)

test/parser/test_base_parser.rb

@@ -23,5 +23,80 @@ def test_large_xml
         parser.position < xml.bytesize
       end
     end
+
+    def test_parse_exception_for_multiple_root_elements
+      exception = assert_raise(REXML::ParseException) do
+        parser = REXML::Parsers::BaseParser.new('<a></a><b>')
+        while parser.has_next?
+          parser.pull
+        end
+      end
+
+      assert_equal(<<-DETAIL.chomp, exception.to_s)
+Malformed XML: Extra tag at the end of the document (got '<b')
+Line: 1
+Position: 10
+Last 80 unconsumed characters:
+
+      DETAIL
+    end
+
+    def test_parse_exception_for_extra_content_at_the_end_of_the_document
+      exception = assert_raise(REXML::ParseException) do
+        parser = REXML::Parsers::BaseParser.new('<a></a>c')
+        while parser.has_next?
+          parser.pull
+        end
+      end
+
+      assert_equal(<<-DETAIL.chomp, exception.to_s)
+Malformed XML: Extra content at the end of the document (got 'c')
+Line: 1
+Position: 8
+Last 80 unconsumed characters:
+
+      DETAIL
+    end
+
+    def test_parse_exception_for_extra_xml_declaration_at_the_end_of_the_document
+      exception = assert_raise(REXML::ParseException) do
+        parser = REXML::Parsers::BaseParser.new('<a></a><?xml version="1.0" ?>')
+        while parser.has_next?
+          parser.pull
+        end
+      end
+
+      assert_equal(<<-DETAIL.chomp, exception.to_s)
+Malformed XML: Extra XML declaration at the end of the document (got '<?xml')
+Line: 1
+Position: 29
+Last 80 unconsumed characters:
+
+      DETAIL
+    end
+
+    def test_extra_processing_instruction_node_at_the_end_of_the_document
+      parser = REXML::Parsers::BaseParser.new('<a></a><?abc version="1.0" ?>')
+
+      events = {}
+      while parser.has_next?
+        event = parser.pull
+        events[event[0]] = event[1]
+      end
+
+      assert_equal("abc", events[:processing_instruction])
+    end
+
+    def test_extra_comments_at_the_end_of_the_document
+      parser = REXML::Parsers::BaseParser.new('<a></a><!-- ok comment -->')
+
+      events = {}
+      while parser.has_next?
+        event = parser.pull
+        events[event[0]] = event[1]
+      end
+
+      assert_equal(" ok comment ", events[:comment])
+    end
   end
 end