I'm wrapping the function in PyGObject.  It apparently doesn't return line with newlines included as it claims in the docs: "Returns: a string with the line that was read in (including the newlines). Set length to a gsize to get the length of the read line. Returns NULL on an error."

If this can help, here is a test case in Python:

class TestDataInputStream(unittest.TestCase):
    def setUp(self):
        self.base_stream = gio.MemoryInputStream()
        self.data_stream = gio.DataInputStream(self.base_stream)

    def test_read_line(self):
        self.base_stream.add_data('foo\nbar\n\nbaz')
        self.assertEquals('foo\n', self.data_stream.read_line())
        self.assertEquals('bar\n', self.data_stream.read_line())
        self.assertEquals('\n', self.data_stream.read_line())
        self.assertEquals('baz', self.data_stream.read_line())

As I see, it returns 'foo', 'bar', '', 'baz' instead of expected values.