Fix read_chunk() on body parts with a zero Content-Length (aio-libs#13760)

Author: istoolsfox

CHANGES/13758.bugfix.rst

@@ -0,0 +1,6 @@
+Fixed a crash in :meth:`~aiohttp.BodyPartReader.read_chunk` on a body part
+with an explicit ``Content-Length: 0``: the part fell through to the
+streaming read strategy, whose minimum chunk size assertion then failed for
+chunk sizes below the boundary length. Such parts now yield an immediate
+empty chunk, like any other part with a known length
+-- by :user:`istoolsfox`.

CHANGES/13760.bugfix.rst

@@ -0,0 +1 @@
+13758.bugfix.rst

CONTRIBUTORS.txt

@@ -188,6 +188,7 @@ Illia Volochii
 Ilya Chichak
 Ilya Gruzinov
 Ingmar Steen
+istoolsfox
 Ivan Lakovic
 Ivan Larin
 J. Nick Koston

aiohttp/multipart.py

@@ -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)

tests/test_multipart.py

@@ -206,6 +206,14 @@ 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_zero_content_length(self) -> None:
+        with Stream(b"\r\n--:--\r\n") as stream:
+            d = HeadersDictProxy(CIMultiDict({"Content-Length": "0"}))
+            obj = aiohttp.BodyPartReader(BOUNDARY, d, stream)
+            result = await obj.read_chunk(4)
+            assert obj.at_eof()
+        assert b"" == result
+
     async def test_read_incomplete_chunk(self) -> None:
         with Stream(b"") as stream: