Remove deficit gossipsub scoring during topic transition (#4486)

## Issue Addressed

This PR closes https://github.com/sigp/lighthouse/issues/3237

## Proposed Changes

Remove topic weight of old topics when the fork happens.

## Additional Info

- Divided `NetworkService::start()` into `NetworkService::build()` and `NetworkService::start()` for ease of testing.

beacon_node/network/src/service/tests.rs

@@ -4,14 +4,26 @@ mod tests {
     use crate::persisted_dht::load_dht;
     use crate::{NetworkConfig, NetworkService};
     use beacon_chain::test_utils::BeaconChainHarness;
-    use beacon_processor::BeaconProcessorChannels;
-    use lighthouse_network::Enr;
+    use beacon_chain::BeaconChainTypes;
+    use beacon_processor::{BeaconProcessorChannels, BeaconProcessorConfig};
+    use futures::StreamExt;
+    use lighthouse_network::types::{GossipEncoding, GossipKind};
+    use lighthouse_network::{Enr, GossipTopic};
     use slog::{o, Drain, Level, Logger};
     use sloggers::{null::NullLoggerBuilder, Build};
     use std::str::FromStr;
     use std::sync::Arc;
     use tokio::runtime::Runtime;
-    use types::MinimalEthSpec;
+    use types::{Epoch, EthSpec, ForkName, MinimalEthSpec, SubnetId};
+
+    impl<T: BeaconChainTypes> NetworkService<T> {
+        fn get_topic_params(
+            &self,
+            topic: GossipTopic,
+        ) -> Option<&lighthouse_network::libp2p::gossipsub::TopicScoreParams> {
+            self.libp2p.get_topic_params(topic)
+        }
+    }
 
     fn get_logger(actual_log: bool) -> Logger {
         if actual_log {
@@ -102,4 +114,126 @@ mod tests {
             "should have persisted the second ENR to store"
         );
     }
+
+    // Test removing topic weight on old topics when a fork happens.
+    #[test]
+    fn test_removing_topic_weight_on_old_topics() {
+        let runtime = Arc::new(Runtime::new().unwrap());
+
+        // Capella spec
+        let mut spec = MinimalEthSpec::default_spec();
+        spec.altair_fork_epoch = Some(Epoch::new(0));
+        spec.bellatrix_fork_epoch = Some(Epoch::new(0));
+        spec.capella_fork_epoch = Some(Epoch::new(1));
+
+        // Build beacon chain.
+        let beacon_chain = BeaconChainHarness::builder(MinimalEthSpec)
+            .spec(spec.clone())
+            .deterministic_keypairs(8)
+            .fresh_ephemeral_store()
+            .mock_execution_layer()
+            .build()
+            .chain;
+        let (next_fork_name, _) = beacon_chain.duration_to_next_fork().expect("next fork");
+        assert_eq!(next_fork_name, ForkName::Capella);
+
+        // Build network service.
+        let (mut network_service, network_globals, _network_senders) = runtime.block_on(async {
+            let (_, exit) = exit_future::signal();
+            let (shutdown_tx, _) = futures::channel::mpsc::channel(1);
+            let executor = task_executor::TaskExecutor::new(
+                Arc::downgrade(&runtime),
+                exit,
+                get_logger(false),
+                shutdown_tx,
+            );
+
+            let mut config = NetworkConfig::default();
+            config.set_ipv4_listening_address(std::net::Ipv4Addr::UNSPECIFIED, 21214, 21214, 21215);
+            config.discv5_config.table_filter = |_| true; // Do not ignore local IPs
+            config.upnp_enabled = false;
+
+            let beacon_processor_channels =
+                BeaconProcessorChannels::new(&BeaconProcessorConfig::default());
+            NetworkService::build(
+                beacon_chain.clone(),
+                &config,
+                executor.clone(),
+                None,
+                beacon_processor_channels.beacon_processor_tx,
+                beacon_processor_channels.work_reprocessing_tx,
+            )
+            .await
+            .unwrap()
+        });
+
+        // Subscribe to the topics.
+        runtime.block_on(async {
+            while network_globals.gossipsub_subscriptions.read().len() < 2 {
+                if let Some(msg) = network_service.attestation_service.next().await {
+                    network_service.on_attestation_service_msg(msg);
+                }
+            }
+        });
+
+        // Make sure the service is subscribed to the topics.
+        let (old_topic1, old_topic2) = {
+            let mut subnets = SubnetId::compute_subnets_for_epoch::<MinimalEthSpec>(
+                network_globals.local_enr().node_id().raw().into(),
+                beacon_chain.epoch().unwrap(),
+                &spec,
+            )
+            .unwrap()
+            .0
+            .collect::<Vec<_>>();
+            assert_eq!(2, subnets.len());
+
+            let old_fork_digest = beacon_chain.enr_fork_id().fork_digest;
+            let old_topic1 = GossipTopic::new(
+                GossipKind::Attestation(subnets.pop().unwrap()),
+                GossipEncoding::SSZSnappy,
+                old_fork_digest,
+            );
+            let old_topic2 = GossipTopic::new(
+                GossipKind::Attestation(subnets.pop().unwrap()),
+                GossipEncoding::SSZSnappy,
+                old_fork_digest,
+            );
+
+            (old_topic1, old_topic2)
+        };
+        let subscriptions = network_globals.gossipsub_subscriptions.read().clone();
+        assert_eq!(2, subscriptions.len());
+        assert!(subscriptions.contains(&old_topic1));
+        assert!(subscriptions.contains(&old_topic2));
+        let old_topic_params1 = network_service
+            .get_topic_params(old_topic1.clone())
+            .expect("topic score params");
+        assert!(old_topic_params1.topic_weight > 0.0);
+        let old_topic_params2 = network_service
+            .get_topic_params(old_topic2.clone())
+            .expect("topic score params");
+        assert!(old_topic_params2.topic_weight > 0.0);
+
+        // Advance slot to the next fork
+        for _ in 0..MinimalEthSpec::slots_per_epoch() {
+            beacon_chain.slot_clock.advance_slot();
+        }
+
+        // Run `NetworkService::update_next_fork()`.
+        runtime.block_on(async {
+            network_service.update_next_fork();
+        });
+
+        // Check that topic_weight on the old topics has been zeroed.
+        let old_topic_params1 = network_service
+            .get_topic_params(old_topic1)
+            .expect("topic score params");
+        assert_eq!(0.0, old_topic_params1.topic_weight);
+
+        let old_topic_params2 = network_service
+            .get_topic_params(old_topic2)
+            .expect("topic score params");
+        assert_eq!(0.0, old_topic_params2.topic_weight);
+    }
 }