Mempool component and storage (#2651)

* First pass at a Mempool Service, incl. a storage layer underneath

* Fixed up Mempool service and storage

* allow dead code where needed

* clippy

* typo

* only drain if the mempool is full

* add a basic storage test

* remove space

* fix test for when MEMPOOL_SIZE change

* group some imports

* add a basic mempool service test

* add clippy suggestions

* remove not needed allow dead code

* Apply suggestions from code review

Co-authored-by: teor <teor@riseup.net>

Co-authored-by: Alfredo Garcia <oxarbitrage@gmail.com>
Co-authored-by: teor <teor@riseup.net>

zebrad/src/components/mempool.rs

@@ -1,8 +1,89 @@
 //! Zebra mempool.
 
-/// Mempool-related errors.
-pub mod error;
+use std::{
+    collections::HashSet,
+    future::Future,
+    pin::Pin,
+    task::{Context, Poll},
+};
+
+use futures::future::FutureExt;
+use tower::Service;
+
+use zebra_chain::{
+    parameters::Network,
+    transaction::{UnminedTx, UnminedTxId},
+};
+
+use crate::BoxError;
 
 mod crawler;
+mod error;
+mod storage;
+
+#[cfg(test)]
+mod tests;
 
 pub use self::crawler::Crawler;
+pub use self::error::MempoolError;
+
+#[derive(Debug)]
+#[allow(dead_code)]
+pub enum Request {
+    TransactionIds,
+    TransactionsById(HashSet<UnminedTxId>),
+}
+
+#[derive(Debug)]
+pub enum Response {
+    Transactions(Vec<UnminedTx>),
+    TransactionIds(Vec<UnminedTxId>),
+}
+
+/// Mempool async management and query service.
+///
+/// The mempool is the set of all verified transactions that this node is aware
+/// of that have yet to be confirmed by the Zcash network. A transaction is
+/// confirmed when it has been included in a block ('mined').
+#[derive(Clone)]
+pub struct Mempool {
+    /// The Mempool storage itself.
+    ///
+    /// ##: Correctness: only components internal to the [`Mempool`] struct are allowed to
+    /// inject transactions into `storage`, as transactions must be verified beforehand.
+    storage: storage::Storage,
+}
+
+impl Mempool {
+    #[allow(dead_code)]
+    pub(crate) fn new(_network: Network) -> Self {
+        Mempool {
+            storage: Default::default(),
+        }
+    }
+}
+
+impl Service<Request> for Mempool {
+    type Response = Response;
+    type Error = BoxError;
+    type Future =
+        Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>> + Send + 'static>>;
+
+    fn poll_ready(&mut self, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        Poll::Ready(Ok(()))
+    }
+
+    #[instrument(name = "mempool", skip(self, req))]
+    fn call(&mut self, req: Request) -> Self::Future {
+        match req {
+            Request::TransactionIds => {
+                let res = self.storage.clone().tx_ids();
+                async move { Ok(Response::TransactionIds(res)) }.boxed()
+            }
+            Request::TransactionsById(ids) => {
+                let rsp = Ok(self.storage.clone().transactions(ids)).map(Response::Transactions);
+                async move { rsp }.boxed()
+            }
+        }
+    }
+}

zebrad/src/components/mempool/error.rs

@@ -22,4 +22,7 @@ pub enum MempoolError {
 
     #[error("transaction was not found in mempool")]
     NotInMempool,
+
+    #[error("transaction evicted from the mempool due to size restrictions")]
+    Excess,
 }

zebrad/src/components/mempool/storage.rs

@@ -0,0 +1,106 @@
+use std::collections::{HashMap, HashSet, VecDeque};
+
+use zebra_chain::{
+    block,
+    transaction::{UnminedTx, UnminedTxId},
+};
+use zebra_consensus::error::TransactionError;
+
+use super::MempoolError;
+
+#[cfg(test)]
+pub mod tests;
+
+const MEMPOOL_SIZE: usize = 2;
+
+#[allow(dead_code)]
+#[derive(Clone, Debug)]
+pub enum State {
+    /// Rejected because verification failed.
+    Invalid(TransactionError),
+    /// An otherwise valid mempool transaction was mined into a block, therefore
+    /// no longer belongs in the mempool.
+    Confirmed(block::Hash),
+    /// Stayed in mempool for too long without being mined.
+    // TODO(2021-08-20): set expiration at 2 weeks? This is what Bitcoin does.
+    Expired,
+    /// Transaction fee is too low for the current mempool state.
+    LowFee,
+    /// Otherwise valid transaction removed from mempool, say because of FIFO
+    /// (first in, first out) policy.
+    Excess,
+}
+
+#[derive(Clone, Default)]
+pub struct Storage {
+    /// The set of verified transactions in the mempool. This is a
+    /// cache of size [`MEMPOOL_SIZE`].
+    verified: VecDeque<UnminedTx>,
+    /// The set of rejected transactions by id, and their rejection reasons.
+    rejected: HashMap<UnminedTxId, State>,
+}
+
+impl Storage {
+    /// Insert a [`UnminedTx`] into the mempool.
+    ///
+    /// If its insertion results in evicting other transactions, they will be tracked
+    /// as [`State::Excess`].
+    #[allow(dead_code)]
+    pub fn insert(&mut self, tx: UnminedTx) -> Result<UnminedTxId, MempoolError> {
+        let tx_id = tx.id;
+
+        // First, check if we should reject this transaction.
+        if self.rejected.contains_key(&tx.id) {
+            return Err(match self.rejected.get(&tx.id).unwrap() {
+                State::Invalid(e) => MempoolError::Invalid(e.clone()),
+                State::Expired => MempoolError::Expired,
+                State::Confirmed(block_hash) => MempoolError::InBlock(*block_hash),
+                State::Excess => MempoolError::Excess,
+                State::LowFee => MempoolError::LowFee,
+            });
+        }
+
+        // If `tx` is already in the mempool, we don't change anything.
+        //
+        // Security: transactions must not get refreshed by new queries,
+        // because that allows malicious peers to keep transactions live forever.
+        if self.verified.contains(&tx) {
+            return Err(MempoolError::InMempool);
+        }
+
+        // Then, we insert into the pool.
+        self.verified.push_front(tx);
+
+        // Once inserted, we evict transactions over the pool size limit in FIFO
+        // order.
+        if self.verified.len() > MEMPOOL_SIZE {
+            for evicted_tx in self.verified.drain(MEMPOOL_SIZE..) {
+                let _ = self.rejected.insert(evicted_tx.id, State::Excess);
+            }
+
+            assert_eq!(self.verified.len(), MEMPOOL_SIZE);
+        }
+
+        Ok(tx_id)
+    }
+
+    /// Returns `true` if a [`UnminedTx`] matching an [`UnminedTxId`] is in
+    /// the mempool.
+    #[allow(dead_code)]
+    pub fn contains(self, txid: &UnminedTxId) -> bool {
+        self.verified.iter().any(|tx| &tx.id == txid)
+    }
+
+    /// Returns the set of [`UnminedTxId`]s in the mempool.
+    pub fn tx_ids(self) -> Vec<UnminedTxId> {
+        self.verified.iter().map(|tx| tx.id).collect()
+    }
+
+    /// Returns the set of [`Transaction`]s matching ids in the mempool.
+    pub fn transactions(self, tx_ids: HashSet<UnminedTxId>) -> Vec<UnminedTx> {
+        self.verified
+            .into_iter()
+            .filter(|tx| tx_ids.contains(&tx.id))
+            .collect()
+    }
+}

zebrad/src/components/mempool/storage/tests.rs

@@ -0,0 +1,79 @@
+use super::*;
+
+use zebra_chain::{
+    block::Block, parameters::Network, serialization::ZcashDeserializeInto, transaction::UnminedTx,
+};
+
+use color_eyre::eyre::Result;
+
+#[test]
+fn mempool_storage_basic() -> Result<()> {
+    zebra_test::init();
+
+    mempool_storage_basic_for_network(Network::Mainnet)?;
+    mempool_storage_basic_for_network(Network::Testnet)?;
+
+    Ok(())
+}
+
+fn mempool_storage_basic_for_network(network: Network) -> Result<()> {
+    // Create an empty storage
+    let mut storage: Storage = Default::default();
+
+    // Get transactions from the first 10 blocks of the Zcash blockchain
+    let (total_transactions, unmined_transactions) = unmined_transactions_in_blocks(10, network);
+
+    // Insert them all to the storage
+    for unmined_transaction in unmined_transactions.clone() {
+        storage.insert(unmined_transaction)?;
+    }
+
+    // Only MEMPOOL_SIZE should land in verified
+    assert_eq!(storage.verified.len(), MEMPOOL_SIZE);
+
+    // The rest of the transactions will be in rejected
+    assert_eq!(storage.rejected.len(), total_transactions - MEMPOOL_SIZE);
+
+    // Make sure the last MEMPOOL_SIZE transactions we sent are in the verified
+    for tx in unmined_transactions.iter().rev().take(MEMPOOL_SIZE) {
+        assert!(storage.clone().contains(&tx.id));
+    }
+
+    // Anything greater should not be in the verified
+    for tx in unmined_transactions
+        .iter()
+        .take(unmined_transactions.len() - MEMPOOL_SIZE)
+    {
+        assert!(!storage.clone().contains(&tx.id));
+    }
+
+    Ok(())
+}
+
+pub fn unmined_transactions_in_blocks(
+    last_block_height: u32,
+    network: Network,
+) -> (usize, Vec<UnminedTx>) {
+    let mut transactions = vec![];
+    let mut total = 0;
+
+    let block_iter = match network {
+        Network::Mainnet => zebra_test::vectors::MAINNET_BLOCKS.iter(),
+        Network::Testnet => zebra_test::vectors::TESTNET_BLOCKS.iter(),
+    };
+
+    for (&height, block) in block_iter {
+        if height <= last_block_height {
+            let block = block
+                .zcash_deserialize_into::<Block>()
+                .expect("block is structurally valid");
+
+            for transaction in block.transactions.iter() {
+                transactions.push(UnminedTx::from(transaction));
+                total += 1;
+            }
+        }
+    }
+
+    (total, transactions)
+}

zebrad/src/components/mempool/tests.rs

@@ -0,0 +1,46 @@
+use super::*;
+use color_eyre::Report;
+use std::collections::HashSet;
+use storage::tests::unmined_transactions_in_blocks;
+use tower::ServiceExt;
+
+#[tokio::test]
+async fn mempool_service_basic() -> Result<(), Report> {
+    // Using the mainnet for now
+    let network = Network::Mainnet;
+
+    // get the genesis block transactions from the Zcash blockchain.
+    let genesis_transactions = unmined_transactions_in_blocks(0, network);
+    // Start the mempool service
+    let mut service = Mempool::new(network);
+    // Insert the genesis block coinbase transaction into the mempool storage.
+    service.storage.insert(genesis_transactions.1[0].clone())?;
+
+    // Test `Request::TransactionIds`
+    let response = service
+        .clone()
+        .oneshot(Request::TransactionIds)
+        .await
+        .unwrap();
+    let transaction_ids = match response {
+        Response::TransactionIds(ids) => ids,
+        _ => unreachable!("will never happen in this test"),
+    };
+
+    // Test `Request::TransactionsById`
+    let hash_set = transaction_ids.iter().copied().collect::<HashSet<_>>();
+    let response = service
+        .oneshot(Request::TransactionsById(hash_set))
+        .await
+        .unwrap();
+    let transactions = match response {
+        Response::Transactions(transactions) => transactions,
+        _ => unreachable!("will never happen in this test"),
+    };
+
+    // Make sure the transaction from the blockchain test vector is the same as the
+    // response of `Request::TransactionsById`
+    assert_eq!(genesis_transactions.1[0], transactions[0]);
+
+    Ok(())
+}