gh-123720: When closing an asyncio server, stop the handlers

[gvanrossum]

Since 3.12, when server.wait_closed() was fixed, cancelling a serve_forever() call would wait for the last handler to exit, which may be never if it is waiting to read from a connection that is never closed by the client. To fix this, we insert a close_clients() call in the exit sequence (between close() and wait_closed()).

The repro shows that in 3.11 you need only a single ^C to stop the server; in 3.12 you need two. The fix indeed undoes that regression.

Some philosophical question remain:

• Should we call close_clients() in close(), so everyone who closes a server gets the same benefit?
• Should we use abort_clients() instead? (I think it's better to allow handlers to catch the cancellation and e.g. send a "goodbye" message to the client.)
• Is this a bugfix (backportable) or a new feature? I'm inclined to call it a bugfix, given the regressions in 3.12 and beyond.

Note that there's still a difference with 3.11: in 3.11, the handler receives a CancelledError; in main with this PR added, the handler receives an empty string, indicating that the connection was closed (by close_clients()). Maybe I should do something that actually cancels the handlers? I will look into the feasibility of this.

• Issue: Unable to cancel Server.serve_forever() if a reader is blocked while reading (3.12 regression) #123720

[gvanrossum]

The only failing test is the check for a news blurb. Once I've got time for that I'll move forward.

[danpascu]

I think this fix is incomplete. I believe the __aexit__ method of AbstractServer at

cpython/Lib/asyncio/events.py

Lines 253 to 255 in ef06508

	async def __aexit__(self, *exc):
	self.close()
	await self.wait_closed()

also needs to be modified to include a call to self.close_clients().

[kumaraditya303]

The only failing test is the check for a news blurb. Once I've got time for that I'll move forward.

@gvanrossum Do you plan to complete this PR and merge it?

[gvanrossum]

Sorry I have lost context. Is anyone waiting for it?

[kumaraditya303]

Sorry I have lost context. Is anyone waiting for it?

No, was just checking if this could be completed before #131009