Fix FileResponse fallback code

In case the FileResponse is using _sendfile_fallback and the requested
range is smaller then the chunk size, we need to only read and send
count bytes.

CHANGES/6726.bugfix

@@ -0,0 +1 @@
+Fixed a bug in ``FileResponse`` fallback code when the requested range is smaller than the chunk size.

CONTRIBUTORS.txt

@@ -247,6 +247,7 @@ Martin Sucha
 Mathias Fröjdman
 Mathieu Dugré
 Matt VanEseltine
+Matthias Dellweg
 Matthias Marquardt
 Matthieu Hauglustaine
 Matthieu Rigal

aiohttp/web_fileresponse.py

@@ -114,7 +114,7 @@ async def _sendfile_fallback(
         chunk_size = self._chunk_size
         loop = asyncio.get_event_loop()
         chunk = await loop.run_in_executor(
-            None, self._seek_and_read, fobj, offset, chunk_size
+            None, self._seek_and_read, fobj, offset, min(chunk_size, count)
         )
         while chunk:
             await writer.write(chunk)

tests/test_web_sendfile_functional.py

@@ -1164,3 +1164,30 @@ async def handler(request: web.Request) -> web.FileResponse:
 
     resp.release()
     await client.close()
+
+
+@pytest.mark.parametrize("chunk_size", (15, 31))
+async def test_a_ranged_request_serves_no_additional_bytes(
+    aiohttp_client: AiohttpClient, sender: _Sender, chunk_size: int
+) -> None:
+    filepath = pathlib.Path(__file__).parent / "aiohttp.jpg"
+
+    async def handler(request: web.Request) -> web.FileResponse:
+        return sender(filepath, chunk_size=chunk_size)
+
+    app = web.Application()
+    app.router.add_get("/", handler)
+    client = await aiohttp_client(app)
+
+    resp = await client.get("/", headers={"Range": "bytes=20-40"})
+    assert resp.status == 206
+    body = await resp.read()
+
+    # 21 bytes, because http range headers are including both fenceposts.
+    assert len(body) == 21
+
+    # Additionally verify that we are served the right bytes from that file.
+    with filepath.open("rb") as f:
+        f.seek(20)
+        content = f.read(21)
+        assert content == body