From 88439641b2d7de9c088a060c327caec467f548f1 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Sat, 24 Oct 2020 12:41:41 -1000
Subject: [PATCH] Ensure zero byte files can be sent

---
 CHANGES/5124.bugfix                   |  1 +
 aiohttp/web_fileresponse.py           |  8 +++++++-
 tests/data.zero_bytes                 |  0
 tests/test_web_sendfile_functional.py | 19 +++++++++++++++++++
 4 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 CHANGES/5124.bugfix
 create mode 100644 tests/data.zero_bytes

diff --git a/CHANGES/5124.bugfix b/CHANGES/5124.bugfix
new file mode 100644
index 00000000000..7726a6bbe3a
--- /dev/null
+++ b/CHANGES/5124.bugfix
@@ -0,0 +1 @@
+Ensure sending a zero byte file does not throw an exception
diff --git a/aiohttp/web_fileresponse.py b/aiohttp/web_fileresponse.py
index bb84c4eca33..6b98c565788 100644
--- a/aiohttp/web_fileresponse.py
+++ b/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
 
diff --git a/tests/data.zero_bytes b/tests/data.zero_bytes
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/test_web_sendfile_functional.py b/tests/test_web_sendfile_functional.py
index 64b24a01bdf..f7f81001e84 100644
--- a/tests/test_web_sendfile_functional.py
+++ b/tests/test_web_sendfile_functional.py
@@ -44,6 +44,25 @@ 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"