don't search boundary twice

aiohttp/multipart.py

@@ -303,21 +303,17 @@ def _read_chunk_from_stream(self, size):
 
         :rtype: bytearray
         """
-        assert size >= len(self._boundary), \
-            'Chunk size must be greater or equal than boundary length'
+        assert size >= len(self._boundary) + 2, \
+            'Chunk size must be greater or equal than boundary length + 2'
         if self._prev_chunk is None:
             self._prev_chunk = yield from self._content.read(size)
 
         chunk = yield from self._content.read(size)
 
         window = self._prev_chunk + chunk
-
-        try:
-            idx = window.index(b'\r\n' + self._boundary)
-        except ValueError:
-            # boundary not found
-            pass
-        else:
+        sub = b'\r\n' + self._boundary
+        idx = window.find(sub, len(self._prev_chunk) - len(sub))
+        if idx >= 0:
             # pushing boundary back to content
             self._content.unread_data(window[idx:])
             if size > idx:

tests/test_multipart.py

@@ -598,10 +598,10 @@ def test_read_chunk_from_stream_doesnt_breaks_reader(self):
             {CONTENT_TYPE: 'multipart/related;boundary=":"'},
             Stream(b'--:\r\n'
                    b'\r\n'
-                   b'test'
+                   b'chunk'
                    b'\r\n--:\r\n'
                    b'\r\n'
-                   b'passed'
+                   b'two_chunks'
                    b'\r\n--:--'))
         body_parts = []
         while True:
@@ -610,9 +610,11 @@ def test_read_chunk_from_stream_doesnt_breaks_reader(self):
             if part is None:
                 break
             while not part.at_eof():
-                read_part += yield from part.read_chunk(4)
+                chunk = yield from part.read_chunk(5)
+                self.assertTrue(chunk)
+                read_part += chunk
             body_parts.append(read_part)
-        self.assertListEqual(body_parts, [b'test', b'passed'])
+        self.assertListEqual(body_parts, [b'chunk', b'two_chunks'])
 
 
 class BodyPartWriterTestCase(unittest.TestCase):