RUBY-432

Do not encode strings to utf-8 after checking that utf-8 is valid.

Before this fix, an encoding operation could turn valid utf-8 into
invalid utf-8 if an encoding that requires conversion is forced on a
valid string.

ext/cbson/cbson.c

@@ -109,10 +109,8 @@ static int max_bson_size;
         }                                                               \
         _str;                                                           \
     })
-#define TO_UTF8(string) rb_str_export_to_enc((string), rb_utf8_encoding())
 #else
 #define STR_NEW(p,n) rb_str_new((p), (n))
-#define TO_UTF8(string) (string)
 #endif
 
 static void write_utf8(bson_buffer_t buffer, VALUE string, char check_null) {
@@ -125,7 +123,6 @@ static void write_utf8(bson_buffer_t buffer, VALUE string, char check_null) {
         bson_buffer_free(buffer);
         rb_raise(InvalidStringEncoding, "String not valid UTF-8");
     }
-    string = TO_UTF8(string);
     SAFE_WRITE(buffer, RSTRING_PTR(string), (int)RSTRING_LEN(string));
 }
 

lib/bson/bson_ruby.rb

@@ -62,7 +62,7 @@ module BSON
         rescue
           raise InvalidStringEncoding, "String not valid utf-8: #{str.inspect}"
         end
-        str.encode(UTF8_ENCODING).force_encoding(BINARY_ENCODING)
+        str.dup.force_encoding(BINARY_ENCODING)
       end
     else
       NULL_BYTE = "\0"

test/bson/bson_test.rb

@@ -141,6 +141,13 @@ class BSONTest < Test::Unit::TestCase
       end
     end
 
+    def test_forced_encoding_with_valid_utf8
+      doc = {'doc' => "\xC3\xB6".force_encoding("ISO-8859-1")}
+      serialized = @encoder.serialize(doc)
+      deserialized = @encoder.deserialize(serialized)
+      assert_equal(doc['doc'], deserialized['doc'].force_encoding("ISO-8859-1"))
+    end
+
     # Based on a test from sqlite3-ruby
     def test_default_internal_is_honored
       before_enc = Encoding.default_internal