[3.7] Ensure zero byte files can be sent (#5125) (#5129)

Backports the following commits to 3.7:
 - Ensure zero byte files can be sent (#5125)

Co-authored-by: J. Nick Koston <nick@koston.org>

Author: aio-libs-github-bot[bot]

CHANGES/5124.bugfix

@@ -0,0 +1 @@
+Ensure sending a zero byte file does not throw an exception

aiohttp/web_fileresponse.py

@@ -98,7 +98,13 @@ async def sendfile(self) -> None:
         if hasattr(loop, "sendfile"):
             # Python 3.7+
             self.transport.write(data)
-            await loop.sendfile(self.transport, self._fobj, self._offset, self._count)
+            if self._count != 0:
+                await loop.sendfile(
+                    self.transport,
+                    self._fobj,
+                    self._offset,
+                    self._count
+                )
             await super().write_eof()
             return
 

tests/data.zero_bytes

@@ -45,6 +45,25 @@ async def handler(request):
     await resp.release()
 
 
+async def test_zero_bytes_file_ok(aiohttp_client, sender) -> None:
+    filepath = pathlib.Path(__file__).parent / "data.zero_bytes"
+
+    async def handler(request):
+        return sender(filepath)
+
+    app = web.Application()
+    app.router.add_get("/", handler)
+    client = await aiohttp_client(app)
+
+    resp = await client.get("/")
+    assert resp.status == 200
+    txt = await resp.text()
+    assert "" == txt.rstrip()
+    assert "application/octet-stream" == resp.headers["Content-Type"]
+    assert resp.headers.get("Content-Encoding") is None
+    await resp.release()
+
+
 async def test_static_file_ok_string_path(aiohttp_client, sender) -> None:
     filepath = pathlib.Path(__file__).parent / "data.unknown_mime_type"