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

@@ -460,15 +460,24 @@ def pull_event
                 @closed = tag
                 @nsstack.shift
               else
+                if @tags.empty? and @have_root
+                  raise ParseException.new("Malformed XML: Extra tag at the end of the document (got '<#{tag}')", @source)
+                end
                 @tags.push( tag )
               end
+              @have_root = true
               return [ :start_element, tag, attributes ]
             end
           else
             text = @source.read_until("<")
             if text.chomp!("<")
               @source.position -= "<".bytesize
             end
+            if @tags.empty? and @have_root
+              unless /\A\s*\z/.match?(text)
+                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

test/parse/test_comment.rb

@@ -105,5 +105,17 @@ def test_after_doctype_malformed_comment_end
         DETAIL
       end
     end
+
+    def test_after_root
+      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

test/parse/test_element.rb

@@ -85,6 +85,40 @@ def test_garbage_less_than_slash_before_end_tag_at_line_start
 </ </x>
         DETAIL
       end
+
+      def test_after_root
+        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_after_empty_element_tag_root
+        exception = assert_raise(REXML::ParseException) do
+          parser = REXML::Parsers::BaseParser.new('<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: 7
+          Last 80 unconsumed characters:
+
+        DETAIL
+      end
     end
   end
 end

test/parse/test_processing_instruction.rb

@@ -40,5 +40,17 @@ def test_garbage_text
                      ])
       end
     end
+
+    def test_after_root
+      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
   end
 end

test/parse/test_text.rb

@@ -0,0 +1,25 @@
+require "test/unit"
+require 'rexml/parsers/baseparser'
+
+module REXMLTests
+  class TestParseText < Test::Unit::TestCase
+    class TestInvalid < self
+      def test_after_root
+        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
+    end
+  end
+end