Add a note that peer_disconnected impls must be idempotent

It appears our code is already correct here, but its also nice to
add a quick safety check in `channel.rs` which ensures we will
remain idempotent.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -3550,6 +3550,12 @@ impl<Signer: Sign> Channel<Signer> {
 			return;
 		}
 
+		if self.channel_state & (ChannelState::PeerDisconnected as u32) == (ChannelState::PeerDisconnected as u32) {
+			// While the below code should be idempotent, it's simpler to just return early, as
+			// redundant disconnect events can fire, though they should be rare.
+			return;
+		}
+
 		if self.announcement_sigs_state == AnnouncementSigsState::MessageSent || self.announcement_sigs_state == AnnouncementSigsState::Committed {
 			self.announcement_sigs_state = AnnouncementSigsState::NotSent;
 		}

lightning/src/ln/msgs.rs

@@ -883,6 +883,9 @@ pub trait ChannelMessageHandler : MessageSendEventsProvider {
 	/// is believed to be possible in the future (eg they're sending us messages we don't
 	/// understand or indicate they require unknown feature bits), no_connection_possible is set
 	/// and any outstanding channels should be failed.
+	///
+	/// Note that in some rare cases this may be called without a corresponding
+	/// [`Self::peer_connected`].
 	fn peer_disconnected(&self, their_node_id: &PublicKey, no_connection_possible: bool);
 
 	/// Handle a peer reconnecting, possibly generating channel_reestablish message(s).
@@ -979,6 +982,9 @@ pub trait OnionMessageHandler : OnionMessageProvider {
 	fn peer_connected(&self, their_node_id: &PublicKey, init: &Init);
 	/// Indicates a connection to the peer failed/an existing connection was lost. Allows handlers to
 	/// drop and refuse to forward onion messages to this peer.
+	///
+	/// Note that in some rare cases this may be called without a corresponding
+	/// [`Self::peer_connected`].
 	fn peer_disconnected(&self, their_node_id: &PublicKey, no_connection_possible: bool);
 
 	// Handler information: