io.c: fix buffered output

* io.c (io_binwritev): append to buffered data, not overwriting.
  [Feature #9323]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60372 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 14 insertions, 0 deletions

diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 122e63da3a..1fa9dbb443 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -1225,6 +1225,20 @@ class TestIO < Test::Unit::TestCase
     end)
   end
 
+  def test_write_with_multiple_arguments_and_buffer
+    mkcdtmpdir do
+      line = "x"*9+"\n"
+      file = "test.out"
+      open(file, "wb") do |w|
+        w.write(line)
+        assert_equal(11, w.write(line, "\n"))
+      end
+      open(file, "rb") do |r|
+        assert_equal([line, line, "\n"], r.readlines)
+      end
+    end
+  end
+
   def test_write_with_many_arguments
     [1023, 1024].each do |n|
       pipe(proc do |w|