Fail RGS data processing early if there is a chain hash mismatch

No point in doing any extra processing if we don't even have a match
for the chain hash.

lightning-background-processor/src/lib.rs

@@ -1124,7 +1124,7 @@ mod tests {
 	fn create_nodes(num_nodes: usize, persist_dir: &str) -> (String, Vec<Node>) {
 		let persist_temp_path = env::temp_dir().join(persist_dir);
 		let persist_dir = persist_temp_path.to_string_lossy().to_string();
-		let network = Network::Testnet;
+		let network = Network::Bitcoin;
 		let mut nodes = Vec::new();
 		for i in 0..num_nodes {
 			let tx_broadcaster = Arc::new(test_utils::TestBroadcaster::new(network));
@@ -1135,7 +1135,7 @@ mod tests {
 			let scorer = Arc::new(Mutex::new(TestScorer::new()));
 			let seed = [i as u8; 32];
 			let router = Arc::new(DefaultRouter::new(network_graph.clone(), logger.clone(), seed, scorer.clone(), ()));
-			let chain_source = Arc::new(test_utils::TestChainSource::new(Network::Testnet));
+			let chain_source = Arc::new(test_utils::TestChainSource::new(Network::Bitcoin));
 			let persister = Arc::new(FilesystemPersister::new(format!("{}_persister_{}", &persist_dir, i)));
 			let now = Duration::from_secs(genesis_block.header.time as u64);
 			let keys_manager = Arc::new(KeysManager::new(&seed, now.as_secs(), now.subsec_nanos()));

lightning-rapid-gossip-sync/src/processing.rs

@@ -68,6 +68,16 @@ impl<NG: Deref<Target=NetworkGraph<L>>, L: Deref> RapidGossipSync<NG, L> where L
 		}
 
 		let chain_hash: BlockHash = Readable::read(read_cursor)?;
+		let ng_genesis_hash = self.network_graph.get_genesis_hash();
+		if chain_hash != ng_genesis_hash {
+			return Err(
+				LightningError {
+					err: "Rapid Gossip Sync data's chain hash does not match the network graph's".to_owned(),
+					action: ErrorAction::IgnoreError,
+				}.into()
+			);
+		}
+
 		let latest_seen_timestamp: u32 = Readable::read(read_cursor)?;
 
 		if let Some(time) = current_time_unix {
@@ -667,4 +677,22 @@ mod tests {
 			panic!("Unexpected update result: {:?}", update_result)
 		}
 	}
+
+	#[test]
+	fn fails_early_on_chain_hash_mismatch() {
+		let logger = TestLogger::new();
+		// Set to testnet so that the VALID_RGS_BINARY chain hash of mainnet does not match.
+		let network_graph = NetworkGraph::new(Network::Testnet, &logger);
+
+		assert_eq!(network_graph.read_only().channels().len(), 0);
+
+		let rapid_sync = RapidGossipSync::new(&network_graph, &logger);
+		let update_result = rapid_sync.update_network_graph_no_std(&VALID_RGS_BINARY, Some(0));
+		assert!(update_result.is_err());
+		if let Err(GraphSyncError::LightningError(err)) = update_result {
+			assert_eq!(err.err, "Rapid Gossip Sync data's chain hash does not match the network graph's");
+		} else {
+			panic!("Unexpected update result: {:?}", update_result)
+		}
+	}
 }

lightning/src/routing/gossip.rs

@@ -366,6 +366,11 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 			},
 		}
 	}
+
+	/// Gets the genesis hash for this network graph.
+	pub fn get_genesis_hash(&self) -> BlockHash {
+		self.genesis_hash
+	}
 }
 
 macro_rules! secp_verify_sig {