network/sync: Extra justification response might be lost when peer disconnects

[dmitry-markin]

In justification_requests.rs active_requests are moved to pending_requests when sync peer disconnects:

polkadot-sdk/substrate/client/network/sync/src/justification_requests.rs

Lines 117 to 121 in 946afaa

	pub(crate) fn peer_disconnected(&mut self, who: &PeerId) {
	if let Some(request) = self.active_requests.remove(who) {
	self.pending_requests.push_front(request);
	}
	}

When receiving a response the code relies on the request being present in active_requests:

polkadot-sdk/substrate/client/network/sync/src/justification_requests.rs

Lines 132 to 140 in 946afaa

	if let Some(request) = self.active_requests.remove(&who) {
	if let Some(r) = resp {
	trace!(target: LOG_TARGET,
	"Queuing import of {} from {:?} for {:?}",
	self.request_type_name, who, request,
	);
	self.importing_requests.insert(request);
	return Some((who, request.0, request.1, r))

We can hit a race condition if the response from the request-response protocol is delivered after the sync peer disconnect event from the notifications protocol.