Fix HEAD requests for static content (#4813)

* Fix HEAD requests for static content

FileResponse directly injects the response into the socket with
sendfile() (if it can), which bypasses the checks in Response that
prevent any content being sent for HEAD requests and for 204 (No
Content) and 304 (Not Modified) responses. I've duplicated that logic
into FileResponse. I'm not sure if the status checks are actually
applicable (since StaticResource already has its own handling for Not
Modified) and I don't currently have any tests for them.

Closes #4809.

CHANGES/4809.bugfix

@@ -0,0 +1 @@
+Fix HEAD requests for static content.

CONTRIBUTORS.txt

@@ -52,6 +52,7 @@ Boyi Chen
 Brett Cannon
 Brian C. Lane
 Brian Muller
+Bruce Merry
 Bryan Kok
 Bryce Drennan
 Carl George

aiohttp/web_fileresponse.py

@@ -360,6 +360,9 @@ async def prepare(
             self.headers[hdrs.CONTENT_RANGE] = 'bytes {0}-{1}/{2}'.format(
                 real_start, real_start + count - 1, file_size)
 
+        if request.method == hdrs.METH_HEAD or self.status in [204, 304]:
+            return await super().prepare(request)
+
         fobj = await loop.run_in_executor(None, filepath.open, 'rb')
         if start:  # be aware that start could be None or int=0 here.
             offset = start

tests/test_web_urldispatcher.py

@@ -1,3 +1,4 @@
+import asyncio
 import pathlib
 from unittest import mock
 from unittest.mock import MagicMock
@@ -238,6 +239,33 @@ async def test_access_special_resource(tmp_path, aiohttp_client) -> None:
         assert r.status == 403
 
 
+async def test_static_head(tmp_path, aiohttp_client) -> None:
+    # Test HEAD on static route
+    my_file_path = tmp_path / 'test.txt'
+    with my_file_path.open('wb') as fw:
+        fw.write(b'should_not_see_this\n')
+
+    app = web.Application()
+    app.router.add_static('/', str(tmp_path))
+    client = await aiohttp_client(app)
+
+    r = await client.head('/test.txt')
+    assert r.status == 200
+
+    # Check that there is no content sent (see #4809). This can't easily be
+    # done with aiohttp_client because the buffering can consume the content.
+    reader, writer = await asyncio.open_connection(client.host, client.port)
+    writer.write(b'HEAD /test.txt HTTP/1.1\r\n')
+    writer.write(b'Host: localhost\r\n')
+    writer.write(b'Connection: close\r\n')
+    writer.write(b'\r\n')
+    while await reader.readline() != b'\r\n':
+        pass
+    content = await reader.read()
+    writer.close()
+    assert content == b''
+
+
 def test_system_route() -> None:
     route = SystemRoute(web.HTTPCreated(reason='test'))
     with pytest.raises(RuntimeError):