Raise an exception on request body reading after sending response

[asvetlov]

See #2893 for the issue motivation

[hubo1016]

Should it be "after respond ends"?

[asvetlov]

I don't see a big difference but "after respond ends" sounds good

[hubo1016]

If the POST data is not read when respond ends, what should be done? Read the entire post data to make the connection ready for the next request, or close it forcely? The post data might be very large, but forcely close the connection may break the "Connection: keep-alive" in the previous response.

A mark could be set after that to stop further reads.

[hubo1016]

It should be "read the entire post data" and "do nothing". The connection cannot be forcely closed, or the response might be lost due to connection reset (client write to a closed connection resets the connection)

[asvetlov]

From my perspective we have two cases:

1. POST data is not consumed by user's code but placed into StreamReader.
   Nothing to do here but forbid a fetching from StreamReader after response sending to fix a logical error.
2. The data is not in StreamReader buffer because the data size is too big. In this case we cannot gracefully release the connection but should force-close it. Because we cannot read the whole huge body (it can take an infinite amount of time).

[hubo1016]

@asvetlov after reading the code, I found it already implemented

aiohttp/aiohttp/web_protocol.py

Lines 422 to 434 in c5d3999

	with suppress(
	asyncio.TimeoutError, asyncio.CancelledError):
	while not payload.is_eof() and now < end_t:
	timeout = min(end_t - now, lingering_time)
	with CeilTimeout(timeout, loop=loop):
	# read and ignore
	await payload.readany()
	now = loop.time()
	# if payload still uncompleted
	if not payload.is_eof() and not self._force_close:
	self.log_debug('Uncompleted request.')
	self.close()

Seems that it reads for lingering_time (by default 10s) and forcely close the connection after that. This is a reasonable solution.

[asvetlov]

Yes. The only thing is left: close payload as well. Now it is possible to read payload (StreamReader) data after sending response, which is weird and error prone. I thing request.content.set_exception() should be enough.

[asvetlov]

Fixed by #2927