generator.c: fix infinite recursion

* ext/json/generator/generator.c (generate_json): get rid of
  unnecessary recursive calls which can cause infinite recursion.
  T_STRING may not have rb_cString.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49562 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,9 @@
+Wed Feb 11 11:01:33 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/json/generator/generator.c (generate_json): get rid of
+	  unnecessary recursive calls which can cause infinite recursion.
+	  T_STRING may not have rb_cString.
+
 Wed Feb 11 07:53:35 2015  Masaki Suketa <masaki.suketa@nifty.ne.jp>
 
 	* test/win32ole/test_word.rb: use skip method to skip test.

ext/json/generator/generator.c

@@ -880,7 +880,7 @@ static void generate_json(FBuffer *buffer, VALUE Vstate, JSON_Generator_State *s
     } else {
         tmp = rb_funcall(obj, i_to_s, 0);
         Check_Type(tmp, T_STRING);
-        generate_json(buffer, Vstate, state, tmp);
+        generate_json_string(buffer, Vstate, state, tmp);
     }
 }
 

test/json/test_json_generate.rb

@@ -334,4 +334,14 @@ EOT
       assert_equal true, JSON.generate(["\xea"])
     end
   end
+
+  def test_string_subclass
+    s = Class.new(String) do
+      def to_s; self; end
+      undef to_json
+    end
+    assert_nothing_raised(SystemStackError) do
+      assert_equal '[""]', JSON.generate([s.new])
+    end
+  end
 end