Avoid querying the chain for outputs for channels we already have

If we receive a ChannelAnnouncement message but we already have the
channel, there's no reason to do a chain lookup. Instead of
immediately calling the user-provided `chain::Access` when handling
a ChannelAnnouncement, we first check if we have the corresponding
channel in the graph.

Note that if we do have the corresponding channel but it was not
previously checked against the blockchain, we should still check
with the `chain::Access` and update if necessary.

Author: TheBlueMatt

lightning/src/routing/gossip.rs

@@ -1433,6 +1433,22 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 			return Err(LightningError{err: "Channel announcement node had a channel with itself".to_owned(), action: ErrorAction::IgnoreError});
 		}
 
+		{
+			let mut channels = self.channels.read().unwrap();
+
+			if let Some(chan) = channels.get(&msg.short_channel_id) {
+				if chan.capacity_sats.is_some() {
+					// If we'd previously looked up the channel on-chain and checked the script
+					// against what appears on-chain, ignore the duplicate announcement.
+					return Err(LightningError{err: "".to_owned(), action: ErrorAction::IgnoreDuplicateGossip});
+				} else if chain_access.is_none() {
+					// Similarly, if we can't check the chain right now anyway, ignore the
+					// duplicate announcement without bothering to take the channels write lock.
+					return Err(LightningError{err: "".to_owned(), action: ErrorAction::IgnoreDuplicateGossip});
+				}
+			}
+		}
+
 		let utxo_value = match &chain_access {
 			&None => {
 				// Tentatively accept, potentially exposing us to DoS attacks