swarm: Introduce an idle_connection_timeout config option

[thomaseizinger]

Description

Original discussion thread: #3844

We want an idle_connection_timeout config on the SwarmBuilder that triggers once a connection reports KeepAlive::No. The current logic is here:

rust-libp2p/swarm/src/connection.rs

Lines 339 to 356 in 524bfc2

	let keep_alive = handler.connection_keep_alive();
	match (&mut *shutdown, keep_alive) {
	(Shutdown::Later(timer, deadline), KeepAlive::Until(t)) => {
	if *deadline != t {
	*deadline = t;
	if let Some(dur) = deadline.checked_duration_since(Instant::now()) {
	timer.reset(dur)
	}
	}
	}
	(_, KeepAlive::Until(t)) => {
	if let Some(dur) = t.checked_duration_since(Instant::now()) {
	*shutdown = Shutdown::Later(Delay::new(dur), t)
	}
	}
	(_, KeepAlive::No) => *shutdown = Shutdown::Asap,
	(_, KeepAlive::Yes) => *shutdown = Shutdown::None,
	};

I think what we can do here is utilize the already existing Shutdown::Later variant and simply set it to the configured timeout in the KeepAlive::No branch.

By default, the timeout should be 0 seconds.

Motivation

Several usecases like tests, examples, benchmarks and more client-server style applications often want to keep connections alive even if there currently isn't any work happening on the connection.

This will unblock #4029. At the same time with introducing this feature, we should deprecate keep_alive::Behaviour and recommend users to simply set the new configuration option to:

• u64::MAX if they want to keep the connection alive "forever"
• a value that makes sense in their context

For tests for example, we would probably be fine to set the value to something like 10 seconds. We can do that here: https://github.com/libp2p/rust-libp2p/blob/master/swarm-test/src/lib.rs#L221

As a result, we can likely remove the usage of keep_alive::Behaviour from several tests.

Are you planning to do it yourself in a pull request?

Maybe but would appreciate help.