[PR #8283/54e13b0a backport][3.9] Fix blocking I/O in the event loop while processing files in a post request

CHANGES/8283.bugfix.rst

@@ -0,0 +1,2 @@
+Fixed blocking I/O in the event loop while processing files in a POST request
+-- by :user:`bdraco`.

aiohttp/test_utils.py

@@ -594,8 +594,15 @@ def make_mocked_request(
     """
     task = mock.Mock()
     if loop is ...:
-        loop = mock.Mock()
-        loop.create_future.return_value = ()
+        # no loop passed, try to get the current one if
+        # its is running as we need a real loop to create
+        # executor jobs to be able to do testing
+        # with a real executor
+        try:
+            loop = asyncio.get_running_loop()
+        except RuntimeError:
+            loop = mock.Mock()
+            loop.create_future.return_value = ()
 
     if version < HttpVersion(1, 1):
         closing = True

aiohttp/web_request.py

@@ -726,19 +726,21 @@ async def post(self) -> "MultiDictProxy[Union[str, bytes, FileField]]":
                     # https://tools.ietf.org/html/rfc7578#section-4.4
                     if field.filename:
                         # store file in temp file
-                        tmp = tempfile.TemporaryFile()
+                        tmp = await self._loop.run_in_executor(
+                            None, tempfile.TemporaryFile
+                        )
                         chunk = await field.read_chunk(size=2**16)
                         while chunk:
                             chunk = field.decode(chunk)
-                            tmp.write(chunk)
+                            await self._loop.run_in_executor(None, tmp.write, chunk)
                             size += len(chunk)
                             if 0 < max_size < size:
-                                tmp.close()
+                                await self._loop.run_in_executor(None, tmp.close)
                                 raise HTTPRequestEntityTooLarge(
                                     max_size=max_size, actual_size=size
                                 )
                             chunk = await field.read_chunk(size=2**16)
-                        tmp.seek(0)
+                        await self._loop.run_in_executor(None, tmp.seek, 0)
 
                         if field_ct is None:
                             field_ct = "application/octet-stream"