Stop enqueueing messages for disconnected peers

If a channel operation occurs while a peer is disconnected, we'll
still have a `PeerState` for that peer. Historically, we haven't
bothered to check if a peer is actually connected before we push
new messages onto the `PeerState::pending_msg_events` field,
leaving us sending messages into the void.

This is generally not an issue as
`ChannelManager::get_and_clear_pending_msg_events` should be called
very regularly, removing these messages and then dropping them as
`PeerManager` won't have anything to do with them.

Still, there is a race condition here - if a peer manages to
connect between the generation of one such message and when
`get_and_clear_pending_msg_events` is called, we may enqueue a
message to the peer which makes no sense and could lead to a
spurious channel closure (especially in the case of an async
`ChannelMonitorUpdate` completion or async signing operation, which
often lead to normal channel message generation).

Further, if a peer is slow to send their `channel_reestablish`
message after connection this race could be substantially more
likely, as such normal channel messages may be nonsense until we've
completed the reestablish dance (i.e. the later reestablish dance
may lead us to re-send the same messages again immediately).

Here we remove most of the cases where we enqueue messages for
disconnected peers.

Note that we differentiate between two different checks for
connected-ness - for cases where we're sending an `error` or gossip
messages, we allow the messages to be enqueued if the peer is
connected at all. For most other cases, we only allow messages to
be enqueued if the peer is connected *and* the channel has
completed its reestablish dance (if required, i.e. the channel is
"connected").

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -3883,6 +3883,12 @@ where
 		self.is_usable() && !self.channel_state.is_peer_disconnected()
 	}
 
+	/// Returns true if the peer for this channel is currently connected and we're not waiting on
+	/// `channel_reestablish` messages to re-init the channel.
+	pub fn is_connected(&self) -> bool {
+		!self.channel_state.is_peer_disconnected()
+	}
+
 	/// Returns false if our last broadcasted channel_update message has the "channel disabled" bit set
 	pub fn is_enabled(&self) -> bool {
 		self.is_usable()