qianbkk · Sep 23, 2026 · Sep 24, 2026 · greptile-apps · Sep 23, 2026
@@ -0,0 +1,6 @@
+Fixed a regression where :py:meth:`~aiohttp.BodyPartReader.read_chunk` raised
+an ``AssertionError`` on a multipart body part with ``Content-Length: 0``
+instead of returning an empty chunk. The empty length is no longer routed
+through the stream-based reader; the length-based reader returns ``b""`` and
+the part is marked at EOF, matching the behaviour of any other
+zero-length :rfc:`7230` body -- by :user:`qianbkk`.
@@ -386,7 +386,7 @@ async def read_chunk(self, size: int = chunk_size) -> bytes:
         if carry:
             self._b64_carry = b""
             want = max(want, self._boundary_len)
-        if self._length:
+        if self._length is not None:
             fresh = await self._read_chunk_from_length(want)
         else:
             fresh = await self._read_chunk_from_stream(want)

@@ -205,6 +205,19 @@ async def test_read_chunk_without_content_length(self) -> None:
         assert c1 + c2 == b"Hello, world!"
         assert c3 == b""
 
+    async def test_read_chunk_with_content_length_zero(self) -> None:
+        """A part with Content-Length: 0 is legal (multipart/mixed, etc.) and
+        should yield an immediate empty chunk instead of routing through the
+        stream path and asserting on the requested chunk size (#13758).
+        """
+        with Stream(b"\r\n--:") as stream:
+            d = HeadersDictProxy(CIMultiDict({"Content-Length": "0"}))
+            obj = aiohttp.BodyPartReader(BOUNDARY, d, stream)
+            assert obj._length == 0
+            result = await obj.read_chunk(4)
+        assert result == b""
+        assert obj.at_eof()
+
     async def test_read_incomplete_chunk(self) -> None:
         with Stream(b"") as stream: