Prevent double import of blocks (#2647)

## Issue Addressed

Resolves #2611 

## Proposed Changes

Adds a duplicate block root cache to the `BeaconProcessor`. Adds the block root to the cache before calling `process_gossip_block` and `process_rpc_block`. Since `process_rpc_block` is called only for single block lookups, we don't have to worry about batched block imports.

The block is imported from the source(gossip/rpc) that arrives first. The block that arrives second is not imported to avoid the db access issue.
There are 2 cases:
1. Block that arrives second is from rpc: In this case, we return an optimistic `BlockError::BlockIsAlreadyKnown` to sync.
2. Block that arrives second is from gossip: In this case, we only do gossip verification and forwarding but don't import the block into the the beacon chain.

## Additional info
Splits up `process_gossip_block` function to `process_gossip_unverified_block` and `process_gossip_verified_block`.

beacon_node/network/src/beacon_processor/mod.rs

@@ -39,6 +39,7 @@
 //! task.
 
 use crate::{metrics, service::NetworkMessage, sync::SyncMessage};
+use beacon_chain::parking_lot::Mutex;
 use beacon_chain::{BeaconChain, BeaconChainTypes, BlockError, GossipVerifiedBlock};
 use futures::stream::{Stream, StreamExt};
 use futures::task::Poll;
@@ -47,13 +48,13 @@ use lighthouse_network::{
     Client, MessageId, NetworkGlobals, PeerId, PeerRequestId,
 };
 use slog::{crit, debug, error, trace, warn, Logger};
-use std::cmp;
 use std::collections::VecDeque;
 use std::fmt;
 use std::pin::Pin;
 use std::sync::{Arc, Weak};
 use std::task::Context;
 use std::time::{Duration, Instant};
+use std::{cmp, collections::HashSet};
 use task_executor::TaskExecutor;
 use tokio::sync::{mpsc, oneshot};
 use types::{
@@ -284,6 +285,55 @@ impl<T> LifoQueue<T> {
     }
 }
 
+/// A handle that sends a message on the provided channel to a receiver when it gets dropped.
+///
+/// The receiver task is responsible for removing the provided `entry` from the `DuplicateCache`
+/// and perform any other necessary cleanup.
+pub struct DuplicateCacheHandle {
+    entry: Hash256,
+    cache: DuplicateCache,
+}
+
+impl Drop for DuplicateCacheHandle {
+    fn drop(&mut self) {
+        self.cache.remove(&self.entry);
+    }
+}
+
+/// A simple  cache for detecting duplicate block roots across multiple threads.
+#[derive(Clone, Default)]
+pub struct DuplicateCache {
+    inner: Arc<Mutex<HashSet<Hash256>>>,
+}
+
+impl DuplicateCache {
+    /// Checks if the given block_root exists and inserts it into the cache if
+    /// it doesn't exist.
+    ///
+    /// Returns a `Some(DuplicateCacheHandle)` if the block_root was successfully
+    /// inserted and `None` if the block root already existed in the cache.
+    ///
+    /// The handle removes the entry from the cache when it is dropped. This ensures that any unclean
+    /// shutdowns in the worker tasks does not leave inconsistent state in the cache.
+    pub fn check_and_insert(&self, block_root: Hash256) -> Option<DuplicateCacheHandle> {
+        let mut inner = self.inner.lock();
+        if inner.insert(block_root) {
+            Some(DuplicateCacheHandle {
+                entry: block_root,
+                cache: self.clone(),
+            })
+        } else {
+            None
+        }
+    }
+
+    /// Remove the given block_root from the cache.
+    pub fn remove(&self, block_root: &Hash256) {
+        let mut inner = self.inner.lock();
+        inner.remove(block_root);
+    }
+}
+
 /// An event to be processed by the manager task.
 pub struct WorkEvent<T: BeaconChainTypes> {
     drop_during_sync: bool,
@@ -787,6 +837,7 @@ pub struct BeaconProcessor<T: BeaconChainTypes> {
     pub executor: TaskExecutor,
     pub max_workers: usize,
     pub current_workers: usize,
+    pub importing_blocks: DuplicateCache,
     pub log: Logger,
 }
 
@@ -1302,6 +1353,8 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
             log: self.log.clone(),
         };
 
+        let duplicate_cache = self.importing_blocks.clone();
+
         trace!(
             self.log,
             "Spawning beacon processor worker";
@@ -1373,7 +1426,8 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                         peer_id,
                         peer_client,
                         *block,
-                        work_reprocessing_tx,
+                        work_reprocessing_tx.clone(),
+                        duplicate_cache,
                         seen_timestamp,
                     ),
                     /*
@@ -1455,7 +1509,12 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                      * Verification for beacon blocks received during syncing via RPC.
                      */
                     Work::RpcBlock { block, result_tx } => {
-                        worker.process_rpc_block(*block, result_tx, work_reprocessing_tx)
+                        worker.process_rpc_block(
+                            *block,
+                            result_tx,
+                            work_reprocessing_tx.clone(),
+                            duplicate_cache,
+                        );
                     }
                     /*
                      * Verification for a chain segment (multiple blocks).

beacon_node/network/src/beacon_processor/tests.rs

@@ -201,6 +201,7 @@ impl TestRig {
             executor,
             max_workers: cmp::max(1, num_cpus::get()),
             current_workers: 0,
+            importing_blocks: Default::default(),
             log: log.clone(),
         }
         .spawn_manager(beacon_processor_rx, Some(work_journal_tx));

beacon_node/network/src/beacon_processor/worker/gossip_methods.rs

@@ -25,6 +25,7 @@ use super::{
     },
     Worker,
 };
+use crate::beacon_processor::DuplicateCache;
 
 /// An attestation that has been validated by the `BeaconChain`.
 ///
@@ -190,7 +191,7 @@ impl<T: BeaconChainTypes> Worker<T> {
     /// Creates a log if there is an internal error.
     /// Propagates the result of the validation for the given message to the network. If the result
     /// is valid the message gets forwarded to other peers.
-    fn propagate_validation_result(
+    pub(crate) fn propagate_validation_result(
         &self,
         message_id: MessageId,
         propagation_source: PeerId,
@@ -618,6 +619,7 @@ impl<T: BeaconChainTypes> Worker<T> {
     ///   be downloaded.
     ///
     /// Raises a log if there are errors.
+    #[allow(clippy::too_many_arguments)]
     pub fn process_gossip_block(
         self,
         message_id: MessageId,
@@ -625,8 +627,50 @@ impl<T: BeaconChainTypes> Worker<T> {
         peer_client: Client,
         block: SignedBeaconBlock<T::EthSpec>,
         reprocess_tx: mpsc::Sender<ReprocessQueueMessage<T>>,
+        duplicate_cache: DuplicateCache,
         seen_duration: Duration,
     ) {
+        if let Some(gossip_verified_block) = self.process_gossip_unverified_block(
+            message_id,
+            peer_id,
+            peer_client,
+            block,
+            reprocess_tx.clone(),
+            seen_duration,
+        ) {
+            let block_root = gossip_verified_block.block_root;
+            if let Some(handle) = duplicate_cache.check_and_insert(block_root) {
+                self.process_gossip_verified_block(
+                    peer_id,
+                    gossip_verified_block,
+                    reprocess_tx,
+                    seen_duration,
+                );
+                // Drop the handle to remove the entry from the cache
+                drop(handle);
+            } else {
+                debug!(
+                    self.log,
+                    "RPC block is being imported";
+                    "block_root" => %block_root,
+                );
+            }
+        }
+    }
+
+    /// Process the beacon block received from the gossip network and
+    /// if it passes gossip propagation criteria, tell the network thread to forward it.
+    ///
+    /// Returns the `GossipVerifiedBlock` if verification passes and raises a log if there are errors.
+    pub fn process_gossip_unverified_block(
+        &self,
+        message_id: MessageId,
+        peer_id: PeerId,
+        peer_client: Client,
+        block: SignedBeaconBlock<T::EthSpec>,
+        reprocess_tx: mpsc::Sender<ReprocessQueueMessage<T>>,
+        seen_duration: Duration,
+    ) -> Option<GossipVerifiedBlock<T>> {
         let block_delay =
             get_block_delay_ms(seen_duration, block.message(), &self.chain.slot_clock);
         // Log metrics to track delay from other nodes on the network.
@@ -687,7 +731,7 @@ impl<T: BeaconChainTypes> Worker<T> {
                     "root" => ?block.canonical_root()
                 );
                 self.send_sync_message(SyncMessage::UnknownBlock(peer_id, block));
-                return;
+                return None;
             }
             Err(e @ BlockError::FutureSlot { .. })
             | Err(e @ BlockError::WouldRevertFinalizedSlot { .. })
@@ -700,7 +744,7 @@ impl<T: BeaconChainTypes> Worker<T> {
                 // Prevent recurring behaviour by penalizing the peer slightly.
                 self.gossip_penalize_peer(peer_id, PeerAction::HighToleranceError);
                 self.propagate_validation_result(message_id, peer_id, MessageAcceptance::Ignore);
-                return;
+                return None;
             }
             Err(e @ BlockError::StateRootMismatch { .. })
             | Err(e @ BlockError::IncorrectBlockProposer { .. })
@@ -720,7 +764,7 @@ impl<T: BeaconChainTypes> Worker<T> {
                             "error" => %e);
                 self.propagate_validation_result(message_id, peer_id, MessageAcceptance::Reject);
                 self.gossip_penalize_peer(peer_id, PeerAction::LowToleranceError);
-                return;
+                return None;
             }
         };
 
@@ -786,13 +830,9 @@ impl<T: BeaconChainTypes> Worker<T> {
                         "location" => "block gossip"
                     )
                 }
+                None
             }
-            Ok(_) => self.process_gossip_verified_block(
+            Ok(_) => Some(verified_block),
-                peer_id,
-                verified_block,
-                reprocess_tx,
-                seen_duration,
-            ),
             Err(e) => {
                 error!(
                     self.log,
@@ -801,7 +841,8 @@ impl<T: BeaconChainTypes> Worker<T> {
                     "block_slot" => %block_slot,
                     "block_root" => ?block_root,
                     "location" => "block gossip"
-                )
+                );
+                None
             }
         }
     }

beacon_node/network/src/beacon_processor/worker/sync_methods.rs

@@ -1,6 +1,6 @@
 use super::{super::work_reprocessing_queue::ReprocessQueueMessage, Worker};
 use crate::beacon_processor::worker::FUTURE_SLOT_TOLERANCE;
-use crate::beacon_processor::BlockResultSender;
+use crate::beacon_processor::{BlockResultSender, DuplicateCache};
 use crate::metrics;
 use crate::sync::manager::{SyncMessage, SyncRequestType};
 use crate::sync::{BatchProcessResult, ChainId};
@@ -33,7 +33,11 @@ impl<T: BeaconChainTypes> Worker<T> {
         block: SignedBeaconBlock<T::EthSpec>,
         result_tx: BlockResultSender<T::EthSpec>,
         reprocess_tx: mpsc::Sender<ReprocessQueueMessage<T>>,
+        duplicate_cache: DuplicateCache,
     ) {
+        let block_root = block.canonical_root();
+        // Checks if the block is already being imported through another source
+        if let Some(handle) = duplicate_cache.check_and_insert(block_root) {
             let slot = block.slot();
             let block_result = self.chain.process_block(block);
 
@@ -63,6 +67,28 @@ impl<T: BeaconChainTypes> Worker<T> {
             if result_tx.send(block_result).is_err() {
                 crit!(self.log, "Failed return sync block result");
             }
+            // Drop the handle to remove the entry from the cache
+            drop(handle);
+        } else {
+            debug!(
+                self.log,
+                "Gossip block is being imported";
+                "block_root" => %block_root,
+            );
+            // The gossip block that is being imported should eventually
+            // trigger reprocessing of queued attestations once it is imported.
+            // If the gossip block fails import, then it will be downscored
+            // appropriately in `process_gossip_block`.
+
+            // Here, we assume that the block will eventually be imported and
+            // send a `BlockIsAlreadyKnown` message to sync.
+            if result_tx
+                .send(Err(BlockError::BlockIsAlreadyKnown))
+                .is_err()
+            {
+                crit!(self.log, "Failed return sync block result");
+            }
+        }
     }
 
     /// Attempt to import the chain segment (`blocks`) to the beacon chain, informing the sync

beacon_node/network/src/router/processor.rs

@@ -65,6 +65,7 @@ impl<T: BeaconChainTypes> Processor<T> {
             executor,
             max_workers: cmp::max(1, num_cpus::get()),
             current_workers: 0,
+            importing_blocks: Default::default(),
             log: log.clone(),
         }
         .spawn_manager(beacon_processor_receive, None);