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

io.c

@@ -1518,8 +1518,8 @@ io_binwritev(struct iovec *iov, int iovcnt, rb_io_t *fptr)
     }
 
     if (fptr->wbuf.ptr && fptr->wbuf.len) {
-	if (fptr->wbuf.off + fptr->wbuf.len + total <= fptr->wbuf.capa) {
-	    long offset = fptr->wbuf.off;
+	long offset = fptr->wbuf.off + fptr->wbuf.len;
+	if (offset + total <= fptr->wbuf.capa) {
 	    for (i = 1; i < iovcnt; i++) {
 		memcpy(fptr->wbuf.ptr+offset, iov[i].iov_base, iov[i].iov_len);
 		offset += iov[i].iov_len;

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|