feat(CRuby): XML::Reader#encoding returns detected enc when not set

Previously this always and only returned the encoding passed to the
constructor. Now, if no encoding is passed to the constructor, then
Reader#encoding will return the encoding detected by the parser.

Note that this has not been implemented for JRuby.

CHANGELOG.md

@@ -35,6 +35,7 @@ A related discussion about Trust exists at [#2357](https://github.com/sparklemot
 
 ### Improved
 
+* [CRuby] XML::Reader#encoding will return the encoding detected by the parser when it's not passed to the constructor. [[#980](https://github.com/sparklemotion/nokogiri/issues/980)]
 * [CRuby] Handle abruptly-closed HTML comments as WHATWG recommends for browsers. (Thanks to HackerOne user [tehryanx](https://hackerone.com/tehryanx?type=user) for reporting this!)
 * [CRuby] `Node#line` is no longer capped at 65535. libxml v2.9.0 and later support a new parse option, exposed as `Nokogiri::XML::ParseOptions::PARSE_BIG_LINES` and set in `ParseOptions::DEFAULT_XML`, `::DEFAULT_XSLT`, `::DEFAULT_HTML`, and `::DEFAULT_SCHEMA`. (Note that JRuby never had this problem.) [[#1764](https://github.com/sparklemotion/nokogiri/issues/1764), [#1493](https://github.com/sparklemotion/nokogiri/issues/1493), [#1617](https://github.com/sparklemotion/nokogiri/issues/1617), [#1505](https://github.com/sparklemotion/nokogiri/issues/1505), [#1003](https://github.com/sparklemotion/nokogiri/issues/1003), [#533](https://github.com/sparklemotion/nokogiri/issues/533)]
 * [CRuby] If a cycle is introduced when reparenting a node (i.e., the node becomes its own ancestor), a `RuntimeError` is raised. libxml2 does no checking for this, which means cycles would otherwise result in infinite loops on subsequent operations. (Note: JRuby/Xerces already does this.) [[#1912](https://github.com/sparklemotion/nokogiri/issues/1912)]

ext/java/nokogiri/XmlReader.java

@@ -184,6 +184,18 @@ public class XmlReader extends RubyObject
     return RubyBoolean.newBoolean(context.getRuntime(), !readerNode.hasChildren);
   }
 
+  @JRubyMethod
+  public IRubyObject
+  encoding(ThreadContext context)
+  {
+    IRubyObject constructor_encoding = getInstanceVariable("@encoding");
+    if (!constructor_encoding.isNil()) {
+      return constructor_encoding;
+    }
+    // TODO: get the parser's detected encoding
+    return context.getRuntime().getNil();
+  }
+
   @JRubyMethod(meta = true, rest = true)
   public static IRubyObject
   from_io(ThreadContext context, IRubyObject cls, IRubyObject args[])

ext/nokogiri/xml_reader.c

@@ -659,6 +659,24 @@ empty_element_p(VALUE self)
   return Qfalse;
 }
 
+static VALUE
+rb_xml_reader_encoding(VALUE rb_reader)
+{
+  xmlTextReaderPtr c_reader;
+  const char *parser_encoding;
+  VALUE constructor_encoding;
+
+  constructor_encoding = rb_iv_get(rb_reader, "@encoding");
+  if (RTEST(constructor_encoding)) {
+    return constructor_encoding;
+  }
+
+  Data_Get_Struct(rb_reader, xmlTextReader, c_reader);
+  parser_encoding = (const char *)xmlTextReaderConstEncoding(c_reader);
+  if (parser_encoding == NULL) { return Qnil; }
+  return NOKOGIRI_STR_NEW2(parser_encoding);
+}
+
 void
 noko_init_xml_reader()
 {
@@ -683,6 +701,7 @@ noko_init_xml_reader()
   rb_define_method(cNokogiriXmlReader, "default?", default_eh, 0);
   rb_define_method(cNokogiriXmlReader, "depth", depth, 0);
   rb_define_method(cNokogiriXmlReader, "empty_element?", empty_element_p, 0);
+  rb_define_method(cNokogiriXmlReader, "encoding", rb_xml_reader_encoding, 0);
   rb_define_method(cNokogiriXmlReader, "inner_xml", inner_xml, 0);
   rb_define_method(cNokogiriXmlReader, "lang", lang, 0);
   rb_define_method(cNokogiriXmlReader, "local_name", local_name, 0);

lib/nokogiri/xml/reader.rb

@@ -71,9 +71,6 @@ class Reader
       # A list of errors encountered while parsing
       attr_accessor :errors
 
-      # The encoding for the document
-      attr_reader :encoding
-
       # The XML source
       attr_reader :source
 

test/xml/test_reader_encoding.rb

@@ -13,6 +13,18 @@ def setup
           XML_FILE,
           "UTF-8"
         )
+        @reader_internal_encoding = Nokogiri::XML::Reader(<<~XML)
+          <?xml version="1.0" encoding="ISO-8859-1"?>
+          <root attr="foo"><employee /></root>
+        XML
+      end
+
+      def test_libxml2_detects_encoding_correctly
+        skip_unless_libxml2("This feature wasn't implemented for JRuby")
+        assert_nil(@reader_internal_encoding.encoding)
+        @reader_internal_encoding.each do
+          assert_equal("ISO-8859-1", @reader_internal_encoding.encoding)
+        end
       end
 
       def test_attribute_at