Send mined transaction IDs to the download/verify task for cancellation (#2786)

* Send mined transaction IDs to the download and verify task for cancellation

* Pass a HashSet of transaction hashes to be cancelled

* Add mempool_cancel_mined() test

* Fix starvation in test

* Fix typo in comment

zebrad/src/components/mempool.rs

@@ -83,7 +83,7 @@ enum ActiveState {
         /// ##: Correctness: only components internal to the [`Mempool`] struct are allowed to
         /// inject transactions into `storage`, as transactions must be verified beforehand.
         storage: storage::Storage,
-        /// The transaction dowload and verify stream.
+        /// The transaction download and verify stream.
         tx_downloads: Pin<Box<InboundTxDownloads>>,
     },
 }
@@ -246,9 +246,19 @@ impl Service<Request> for Mempool {
                 storage,
                 tx_downloads,
             } => {
-                // Clear the mempool if there has been a chain tip reset.
-                if let Some(TipAction::Reset { .. }) = self.chain_tip_change.last_tip_change() {
-                    storage.clear();
+                if let Some(tip_action) = self.chain_tip_change.last_tip_change() {
+                    match tip_action {
+                        // Clear the mempool if there has been a chain tip reset.
+                        TipAction::Reset { .. } => {
+                            storage.clear();
+                        }
+                        // Cancel downloads/verifications of transactions with the same
+                        // IDs as recently mined transactions.
+                        TipAction::Grow { block } => {
+                            let txid_set = block.transaction_hashes.iter().collect();
+                            tx_downloads.cancel(txid_set);
+                        }
+                    }
                 }
 
                 // Clean up completed download tasks and add to mempool if successful.
@@ -268,7 +278,7 @@ impl Service<Request> for Mempool {
             ActiveState::Disabled => {
                 // When the mempool is disabled we still return that the service is ready.
                 // Otherwise, callers could block waiting for the mempool to be enabled,
-                // which may not be the desired behaviour.
+                // which may not be the desired behavior.
             }
         }
 

zebrad/src/components/mempool/downloads.rs

@@ -1,5 +1,5 @@
 use std::{
-    collections::HashMap,
+    collections::{HashMap, HashSet},
     pin::Pin,
     task::{Context, Poll},
     time::Duration,
@@ -16,7 +16,7 @@ use tokio::{sync::oneshot, task::JoinHandle};
 use tower::{Service, ServiceExt};
 use tracing_futures::Instrument;
 
-use zebra_chain::transaction::{UnminedTx, UnminedTxId};
+use zebra_chain::transaction::{self, UnminedTx, UnminedTxId};
 use zebra_consensus::transaction as tx;
 use zebra_network as zn;
 use zebra_state as zs;
@@ -315,6 +315,25 @@ where
         Ok(())
     }
 
+    /// Cancel download/verification tasks of transactions with the
+    /// given transaction hash (see [`UnminedTxId::mined_id`]).
+    pub fn cancel(&mut self, mined_ids: HashSet<&transaction::Hash>) {
+        // TODO: this can be simplified with [`HashMap::drain_filter`] which
+        // is currently nightly-only experimental API.
+        let removed_txids: Vec<UnminedTxId> = self
+            .cancel_handles
+            .keys()
+            .filter(|txid| mined_ids.contains(&txid.mined_id()))
+            .cloned()
+            .collect();
+
+        for txid in removed_txids {
+            if let Some(handle) = self.cancel_handles.remove(&txid) {
+                let _ = handle.send(());
+            }
+        }
+    }
+
     /// Get the number of currently in-flight download tasks.
     // Note: copied from zebrad/src/components/sync/downloads.rs
     #[allow(dead_code)]

zebrad/src/components/mempool/tests.rs

@@ -1,9 +1,12 @@
 use super::*;
 use color_eyre::Report;
-use std::collections::HashSet;
+use std::{collections::HashSet, sync::Arc};
 use storage::tests::unmined_transactions_in_blocks;
+use tokio::time;
 use tower::{ServiceBuilder, ServiceExt};
 
+use zebra_chain::block::Block;
+use zebra_chain::serialization::ZcashDeserializeInto;
 use zebra_consensus::Config as ConsensusConfig;
 use zebra_state::Config as StateConfig;
 use zebra_test::mock_service::MockService;
@@ -348,3 +351,133 @@ async fn mempool_service_disabled() -> Result<(), Report> {
 
     Ok(())
 }
+
+#[tokio::test]
+async fn mempool_cancel_mined() -> Result<(), Report> {
+    let block1: Arc<Block> = zebra_test::vectors::BLOCK_MAINNET_1_BYTES
+        .zcash_deserialize_into()
+        .unwrap();
+    let block2: Arc<Block> = zebra_test::vectors::BLOCK_MAINNET_2_BYTES
+        .zcash_deserialize_into()
+        .unwrap();
+
+    // Using the mainnet for now
+    let network = Network::Mainnet;
+    let consensus_config = ConsensusConfig::default();
+    let state_config = StateConfig::ephemeral();
+    let peer_set = MockService::build().for_unit_tests();
+    let (sync_status, mut recent_syncs) = SyncStatus::new();
+    let (state, latest_chain_tip, chain_tip_change) =
+        zebra_state::init(state_config.clone(), network);
+
+    let mut state_service = ServiceBuilder::new().buffer(1).service(state);
+    let (_chain_verifier, tx_verifier) =
+        zebra_consensus::chain::init(consensus_config.clone(), network, state_service.clone())
+            .await;
+
+    time::pause();
+
+    // Start the mempool service
+    let mut mempool = Mempool::new(
+        network,
+        Buffer::new(BoxService::new(peer_set), 1),
+        state_service.clone(),
+        tx_verifier,
+        sync_status,
+        latest_chain_tip,
+        chain_tip_change,
+    );
+
+    // Enable the mempool
+    let _ = mempool.enable(&mut recent_syncs).await;
+    assert!(mempool.is_enabled());
+
+    // Push the genesis block to the state
+    let genesis_block: Arc<Block> = zebra_test::vectors::BLOCK_MAINNET_GENESIS_BYTES
+        .zcash_deserialize_into()
+        .unwrap();
+    state_service
+        .ready_and()
+        .await
+        .unwrap()
+        .call(zebra_state::Request::CommitFinalizedBlock(
+            genesis_block.clone().into(),
+        ))
+        .await
+        .unwrap();
+
+    // Queue transaction from block 2 for download
+    let txid = block2.transactions[0].unmined_id();
+    let response = mempool
+        .ready_and()
+        .await
+        .unwrap()
+        .call(Request::Queue(vec![txid.into()]))
+        .await
+        .unwrap();
+    let queued_responses = match response {
+        Response::Queued(queue_responses) => queue_responses,
+        _ => unreachable!("will never happen in this test"),
+    };
+    assert_eq!(queued_responses.len(), 1);
+    assert!(queued_responses[0].is_ok());
+    assert_eq!(mempool.tx_downloads().in_flight(), 1);
+
+    // Query the mempool to make it poll chain_tip_change
+    let _response = mempool
+        .ready_and()
+        .await
+        .unwrap()
+        .call(Request::TransactionIds)
+        .await
+        .unwrap();
+
+    // Push block 1 to the state
+    state_service
+        .ready_and()
+        .await
+        .unwrap()
+        .call(zebra_state::Request::CommitFinalizedBlock(
+            block1.clone().into(),
+        ))
+        .await
+        .unwrap();
+
+    // Query the mempool to make it poll chain_tip_change
+    let _response = mempool
+        .ready_and()
+        .await
+        .unwrap()
+        .call(Request::TransactionIds)
+        .await
+        .unwrap();
+
+    // Push block 2 to the state
+    state_service
+        .oneshot(zebra_state::Request::CommitFinalizedBlock(
+            block2.clone().into(),
+        ))
+        .await
+        .unwrap();
+
+    // This is done twice because after the first query the cancellation
+    // is picked up by select!, and after the second the mempool gets the
+    // result and the download future is removed.
+    for _ in 0..2 {
+        // Query the mempool just to poll it and make it cancel the download.
+        let _response = mempool
+            .ready_and()
+            .await
+            .unwrap()
+            .call(Request::TransactionIds)
+            .await
+            .unwrap();
+        // Sleep to avoid starvation and make sure the cancellation is picked up.
+        time::sleep(time::Duration::from_millis(100)).await;
+    }
+
+    // Check if download was cancelled.
+    assert_eq!(mempool.tx_downloads().in_flight(), 0);
+
+    Ok(())
+}