add support for connection closed listeners

[ioistired]

#421 (comment)

• I'm not sure that the API is what I want. It's called a "cleanup listener" internally and a "close listener" externally.
• I don't know how to add tests for this.

Other than that, how does this look? The following code demonstrates the functionality. It prints True after a postgres server restart, then exits:

#!/usr/bin/env python3

import asyncpg
import asyncio

async def amain():
	conn = await asyncpg.connect()
	cleanup_ran = False
	def close_listener(_):
		nonlocal cleanup_ran
		cleanup_ran = True
	conn.add_close_listener(close_listener)
	while await asyncio.sleep(1, True):
		if conn.is_closed():
			print(cleanup_ran)
			break

if __name__ == '__main__':
	asyncio.run(amain())

[elprans]

I'm not sure that the API is what I want. It's called a "cleanup listener" internally and a "close listener" externally.

close_listener is better.

I don't know how to add tests for this.

Look at test_adversity.py for an example. There is a special proxy to simulate connection issues. You would need to add a method to drop the connection to it (_testbase/fuzzer.py)

[ioistired]

Please clarify.

• Which class needs a "drop connection" method? The TCPFuzzingProxy class already has a trigger_connectivity_loss() method. What should mine do differently?
• Should my test cases be in test_adversity.py as well? If so, should they be in a separate class that also extends tb.ProxiedClusterTestCase, or should they be added as methods to TestConnectionLoss?

[ioistired]

I am un-drafting this PR because I don't know how to write proper tests for this code. Maintainers are free to commit to my PR, and if anyone else wants to pick up the PR, feel free to make a new one based on mine.

[ioistired]

Can any maintainer please take a look at this and try to write up some test cases?

[stalkerg]

it's a very important feature, can somebody check it?

[ioistired]

Can you elaborate on the name "termination listener"? How does "close listener" read worse?

[elprans]

In the phrase "close listener" the word "close" reads like a distance adjective, i.e. "a near listener". "Termination listener" is simply better English.

[ioistired]

Thank you so much!