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swarm/src/lib: Continue polling network when behaviour is blocked (#2304
) With #2248 a connection task `await`s sending an event to the behaviour before polling for new events from the behaviour [1]. When `Swarm::poll` is unable to deliver an event to a connection task it returns `Poll::Pending` even though (a) polling `Swarm::network` might be able to make progress (`network_not_ready` being `false`) and (b) it does not register a waker to be woken up [2]. In combination this can lead to a deadlock where a connection task waits to send an event to the behaviour and `Swarm::poll` returns `Poll::Pending` failing to send an event to the connection task, not registering a waiker in order to be polled again. With this commit `Swarm::poll` will only return `Poll::Pending`, when failing to deliver an event to a connection task, if the network is unable to make progress (i.e. `network_not_ready` being `true`). In the long-run `Swarm::poll` should likely be redesigned, prioritizing the behaviour over the network, given the former is the control plane and the latter potentially yields new work from the outside. [1]: https://github.com/libp2p/rust-libp2p/blob/ca1b7cf043b4264c69b19fe75de488330a7a1f2f/core/src/connection/pool/task.rs#L224-L232 [2]: https://github.com/libp2p/rust-libp2p/blob/ca1b7cf043b4264c69b19fe75de488330a7a1f2f/swarm/src/lib.rs#L756-L783
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