Cleaner read_bytes and a test case

No longer reads via buffer for unbuffered bytes

msgpack/_msgpack.pyx

@@ -458,13 +458,11 @@ cdef class Unpacker(object):
     def read_bytes(self, Py_ssize_t nbytes):
         """read a specified number of raw bytes from the stream"""
         cdef size_t nread
-        ret = ''
-        while len(ret) < nbytes and self.file_like is not None:
-            if self.buf_head == self.buf_tail:
-                self.fill_buffer()
-            nread = min(self.buf_tail - self.buf_head, nbytes - len(ret))
-            ret += PyBytes_FromStringAndSize(self.buf + self.buf_head, nread)
-            self.buf_head += nread
+        nread = min(self.buf_tail - self.buf_head, nbytes)
+        ret = PyBytes_FromStringAndSize(self.buf + self.buf_head, nread)
+        self.buf_head += nread
+        if len(ret) < nbytes and self.file_like is not None:
+            ret += self.file_like.read(nbytes - len(ret))
         return ret
 
     def __iter__(self):

test/test_sequnpack.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 # coding: utf-8
 
+import six
 from msgpack import Unpacker, BufferFull
 import nose
 
@@ -42,5 +43,20 @@ def test_maxbuffersize():
     assert ord('b') == next(unpacker)
 
 
+def test_readbytes():
+    unpacker = Unpacker(read_size=3)
+    unpacker.feed(b'foobar')
+    assert unpacker.unpack() == ord(b'f')
+    assert unpacker.read_bytes(3) == b'oob'
+    assert unpacker.unpack() == ord(b'a')
+    assert unpacker.unpack() == ord(b'r')
+
+    # Test buffer refill
+    unpacker = Unpacker(six.BytesIO(b'foobar'), read_size=3)
+    assert unpacker.unpack() == ord(b'f')
+    assert unpacker.read_bytes(3) == b'oob'
+    assert unpacker.unpack() == ord(b'a')
+    assert unpacker.unpack() == ord(b'r')
+
 if __name__ == '__main__':
     nose.main()