zebra_consensus/

transaction.rs

//! Asynchronous verification of transactions.

use std::{
    collections::{HashMap, HashSet},
    future::Future,
    pin::Pin,
    sync::Arc,
    task::{Context, Poll},
    time::Duration,
};

use chrono::{DateTime, Utc};
use futures::{
    stream::{FuturesUnordered, StreamExt},
    FutureExt,
};
use tokio::sync::oneshot;
use tower::{
    buffer::Buffer,
    timeout::{error::Elapsed, Timeout},
    util::BoxService,
    Service, ServiceExt,
};
use tracing::Instrument;

use zcash_protocol::value::ZatBalance;

use zebra_chain::{
    amount::{Amount, NonNegative},
    block,
    parameters::{Network, NetworkUpgrade},
    primitives::Groth16Proof,
    serialization::DateTime32,
    transaction::{
        self, HashType, SigHash, Transaction, UnminedTx, UnminedTxId, VerifiedUnminedTx,
    },
    transparent,
};

use zebra_node_services::mempool;
use zebra_script::{CachedFfiTransaction, Sigops};
use zebra_state as zs;

use crate::{error::TransactionError, groth16::DescriptionWrapper, primitives, script, BoxError};

pub mod check;
#[cfg(test)]
mod tests;

/// A timeout applied to UTXO lookup requests.
///
/// The exact value is non-essential, but this should be long enough to allow
/// out-of-order verification of blocks (UTXOs are not required to be ready
/// immediately) while being short enough to:
///   * prune blocks that are too far in the future to be worth keeping in the
///     queue,
///   * fail blocks that reference invalid UTXOs, and
///   * fail blocks that reference UTXOs from blocks that have temporarily failed
///     to download, because a peer sent Zebra a bad list of block hashes. (The
///     UTXO verification failure will restart the sync, and re-download the
///     chain in the correct order.)
const UTXO_LOOKUP_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(6 * 60);

/// A timeout applied to output lookup requests sent to the mempool. This is shorter than the
/// timeout for the state UTXO lookups because a block is likely to be mined every 75 seconds
/// after Blossom is active, changing the best chain tip and requiring re-verification of transactions
/// in the mempool.
///
/// This is how long Zebra will wait for an output to be added to the mempool before verification
/// of the transaction that spends it will fail.
const MEMPOOL_OUTPUT_LOOKUP_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(60);

/// How long to wait after responding to a mempool request with a transaction that creates new
/// transparent outputs before polling the mempool service so that it will try adding the verified
/// transaction and responding to any potential `AwaitOutput` requests.
///
/// This should be long enough for the mempool service's `Downloads` to finish processing the
/// response from the transaction verifier.
const POLL_MEMPOOL_DELAY: std::time::Duration = Duration::from_millis(50);

/// Asynchronous transaction verification.
///
/// # Correctness
///
/// Transaction verification requests should be wrapped in a timeout, so that
/// out-of-order and invalid requests do not hang indefinitely. See the [`router`](`crate::router`)
/// module documentation for details.
pub struct Verifier<ZS, Mempool> {
    network: Network,
    state: Timeout<ZS>,
    // TODO: Use an enum so that this can either be Pending(oneshot::Receiver) or Initialized(MempoolService)
    mempool: Option<Timeout<Mempool>>,
    script_verifier: script::Verifier,
    mempool_setup_rx: oneshot::Receiver<Mempool>,
}

impl<ZS, Mempool> Verifier<ZS, Mempool>
where
    ZS: Service<zs::Request, Response = zs::Response, Error = BoxError> + Send + Clone + 'static,
    ZS::Future: Send + 'static,
    Mempool: Service<mempool::Request, Response = mempool::Response, Error = BoxError>
        + Send
        + Clone
        + 'static,
    Mempool::Future: Send + 'static,
{
    /// Create a new transaction verifier.
    pub fn new(network: &Network, state: ZS, mempool_setup_rx: oneshot::Receiver<Mempool>) -> Self {
        Self {
            network: network.clone(),
            state: Timeout::new(state, UTXO_LOOKUP_TIMEOUT),
            mempool: None,
            script_verifier: script::Verifier,
            mempool_setup_rx,
        }
    }
}

impl<ZS>
    Verifier<
        ZS,
        Buffer<BoxService<mempool::Request, mempool::Response, BoxError>, mempool::Request>,
    >
where
    ZS: Service<zs::Request, Response = zs::Response, Error = BoxError> + Send + Clone + 'static,
    ZS::Future: Send + 'static,
{
    /// Create a new transaction verifier with a closed channel receiver for mempool setup for tests.
    #[cfg(test)]
    pub fn new_for_tests(network: &Network, state: ZS) -> Self {
        Self {
            network: network.clone(),
            state: Timeout::new(state, UTXO_LOOKUP_TIMEOUT),
            mempool: None,
            script_verifier: script::Verifier,
            mempool_setup_rx: oneshot::channel().1,
        }
    }
}

/// Specifies whether a transaction should be verified as part of a block or as
/// part of the mempool.
///
/// Transaction verification has slightly different consensus rules, depending on
/// whether the transaction is to be included in a block on in the mempool.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum Request {
    /// Verify the supplied transaction as part of a block.
    Block {
        /// The transaction hash.
        transaction_hash: transaction::Hash,
        /// The transaction itself.
        transaction: Arc<Transaction>,
        /// Set of transaction hashes that create new transparent outputs.
        known_outpoint_hashes: Arc<HashSet<transaction::Hash>>,
        /// Additional UTXOs which are known at the time of verification.
        known_utxos: Arc<HashMap<transparent::OutPoint, transparent::OrderedUtxo>>,
        /// The height of the block containing this transaction.
        height: block::Height,
        /// The time that the block was mined.
        time: DateTime<Utc>,
    },
    /// Verify the supplied transaction as part of the mempool.
    ///
    /// Mempool transactions do not have any additional UTXOs.
    ///
    /// Note: coinbase transactions are invalid in the mempool
    Mempool {
        /// The transaction itself.
        transaction: UnminedTx,
        /// The height of the next block.
        ///
        /// The next block is the first block that could possibly contain a
        /// mempool transaction.
        height: block::Height,
    },
}

/// The response type for the transaction verifier service.
/// Responses identify the transaction that was verified.
#[derive(Clone, Debug, PartialEq)]
pub enum Response {
    /// A response to a block transaction verification request.
    Block {
        /// The witnessed transaction ID for this transaction.
        ///
        /// [`Response::Block`] responses can be uniquely identified by
        /// [`UnminedTxId::mined_id`], because the block's authorizing data root
        /// will be checked during contextual validation.
        tx_id: UnminedTxId,

        /// The miner fee for this transaction.
        ///
        /// `None` for coinbase transactions.
        ///
        /// # Consensus
        ///
        /// > The remaining value in the transparent transaction value pool
        /// > of a coinbase transaction is destroyed.
        ///
        /// <https://zips.z.cash/protocol/protocol.pdf#transactions>
        miner_fee: Option<Amount<NonNegative>>,

        /// The number of legacy signature operations in this transaction's
        /// transparent inputs and outputs.
        sigops: u32,
    },

    /// A response to a mempool transaction verification request.
    Mempool {
        /// The full content of the verified mempool transaction.
        /// Also contains the transaction fee and other associated fields.
        ///
        /// Mempool transactions always have a transaction fee,
        /// because coinbase transactions are rejected from the mempool.
        ///
        /// [`Response::Mempool`] responses are uniquely identified by the
        /// [`UnminedTxId`] variant for their transaction version.
        transaction: VerifiedUnminedTx,

        /// A list of spent [`transparent::OutPoint`]s that were found in
        /// the mempool's list of `created_outputs`.
        ///
        /// Used by the mempool to determine dependencies between transactions
        /// in the mempool and to avoid adding transactions with missing spends
        /// to its verified set.
        spent_mempool_outpoints: Vec<transparent::OutPoint>,
    },
}

#[cfg(any(test, feature = "proptest-impl"))]
impl From<VerifiedUnminedTx> for Response {
    fn from(transaction: VerifiedUnminedTx) -> Self {
        Response::Mempool {
            transaction,
            spent_mempool_outpoints: Vec::new(),
        }
    }
}

impl Request {
    /// The transaction to verify that's in this request.
    pub fn transaction(&self) -> Arc<Transaction> {
        match self {
            Request::Block { transaction, .. } => transaction.clone(),
            Request::Mempool { transaction, .. } => transaction.transaction.clone(),
        }
    }

    /// The unverified mempool transaction, if this is a mempool request.
    pub fn mempool_transaction(&self) -> Option<UnminedTx> {
        match self {
            Request::Block { .. } => None,
            Request::Mempool { transaction, .. } => Some(transaction.clone()),
        }
    }

    /// The unmined transaction ID for the transaction in this request.
    pub fn tx_id(&self) -> UnminedTxId {
        match self {
            // TODO: get the precalculated ID from the block verifier
            Request::Block { transaction, .. } => transaction.unmined_id(),
            Request::Mempool { transaction, .. } => transaction.id,
        }
    }

    /// The mined transaction ID for the transaction in this request.
    pub fn tx_mined_id(&self) -> transaction::Hash {
        match self {
            Request::Block {
                transaction_hash, ..
            } => *transaction_hash,
            Request::Mempool { transaction, .. } => transaction.id.mined_id(),
        }
    }

    /// The set of additional known unspent transaction outputs that's in this request.
    pub fn known_utxos(&self) -> Arc<HashMap<transparent::OutPoint, transparent::OrderedUtxo>> {
        match self {
            Request::Block { known_utxos, .. } => known_utxos.clone(),
            Request::Mempool { .. } => HashMap::new().into(),
        }
    }

    /// The set of additional known [`transparent::OutPoint`]s of unspent transaction outputs that's in this request.
    pub fn known_outpoint_hashes(&self) -> Arc<HashSet<transaction::Hash>> {
        match self {
            Request::Block {
                known_outpoint_hashes,
                ..
            } => known_outpoint_hashes.clone(),
            Request::Mempool { .. } => HashSet::new().into(),
        }
    }

    /// The height used to select the consensus rules for verifying this transaction.
    pub fn height(&self) -> block::Height {
        match self {
            Request::Block { height, .. } | Request::Mempool { height, .. } => *height,
        }
    }

    /// The block time used for lock time consensus rules validation.
    pub fn block_time(&self) -> Option<DateTime<Utc>> {
        match self {
            Request::Block { time, .. } => Some(*time),
            Request::Mempool { .. } => None,
        }
    }

    /// The network upgrade to consider for the verification.
    ///
    /// This is based on the block height from the request, and the supplied `network`.
    pub fn upgrade(&self, network: &Network) -> NetworkUpgrade {
        NetworkUpgrade::current(network, self.height())
    }

    /// Returns true if the request is a mempool request.
    pub fn is_mempool(&self) -> bool {
        matches!(self, Request::Mempool { .. })
    }
}

impl Response {
    /// The unmined transaction ID for the transaction in this response.
    pub fn tx_id(&self) -> UnminedTxId {
        match self {
            Response::Block { tx_id, .. } => *tx_id,
            Response::Mempool { transaction, .. } => transaction.transaction.id,
        }
    }

    /// The miner fee for the transaction in this response.
    ///
    /// Coinbase transactions do not have a miner fee,
    /// and they don't need UTXOs to calculate their value balance,
    /// because they don't spend any inputs.
    pub fn miner_fee(&self) -> Option<Amount<NonNegative>> {
        match self {
            Response::Block { miner_fee, .. } => *miner_fee,
            Response::Mempool { transaction, .. } => Some(transaction.miner_fee),
        }
    }

    /// The number of legacy transparent signature operations in this transaction's
    /// inputs and outputs.
    pub fn sigops(&self) -> u32 {
        match self {
            Response::Block { sigops, .. } => *sigops,
            Response::Mempool { transaction, .. } => transaction.sigops,
        }
    }

    /// Returns true if the request is a mempool request.
    pub fn is_mempool(&self) -> bool {
        match self {
            Response::Block { .. } => false,
            Response::Mempool { .. } => true,
        }
    }
}

impl<ZS, Mempool> Service<Request> for Verifier<ZS, Mempool>
where
    ZS: Service<zs::Request, Response = zs::Response, Error = BoxError> + Send + Clone + 'static,
    ZS::Future: Send + 'static,
    Mempool: Service<mempool::Request, Response = mempool::Response, Error = BoxError>
        + Send
        + Clone
        + 'static,
    Mempool::Future: Send + 'static,
{
    type Response = Response;
    type Error = TransactionError;
    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>> {
        // Note: The block verifier expects the transaction verifier to always be ready.

        if self.mempool.is_none() {
            if let Ok(mempool) = self.mempool_setup_rx.try_recv() {
                self.mempool = Some(Timeout::new(mempool, MEMPOOL_OUTPUT_LOOKUP_TIMEOUT));
            }
        }

        Poll::Ready(Ok(()))
    }

    // TODO: break up each chunk into its own method
    fn call(&mut self, req: Request) -> Self::Future {
        let script_verifier = self.script_verifier;
        let network = self.network.clone();
        let state = self.state.clone();
        let mempool = self.mempool.clone();

        let tx = req.transaction();
        let tx_id = req.tx_id();
        let span = tracing::debug_span!("tx", ?tx_id);

        async move {
            tracing::trace!(?tx_id, ?req, "got tx verify request");

            if let Some(result) = Self::find_verified_unmined_tx(&req, mempool.clone(), state.clone()).await {
                let verified_tx = result?;

                return Ok(Response::Block {
                    tx_id,
                    miner_fee: Some(verified_tx.miner_fee),
                    sigops: verified_tx.sigops
                });
            }

            // Do quick checks first
            check::has_inputs_and_outputs(&tx)?;
            check::has_enough_orchard_flags(&tx)?;
            check::consensus_branch_id(&tx, req.height(), &network)?;

            // Validate the coinbase input consensus rules
            if req.is_mempool() && tx.is_coinbase() {
                return Err(TransactionError::CoinbaseInMempool);
            }

            if tx.is_coinbase() {
                check::coinbase_tx_no_prevout_joinsplit_spend(&tx)?;
            } else if !tx.is_valid_non_coinbase() {
                return Err(TransactionError::NonCoinbaseHasCoinbaseInput);
            }

            // Validate `nExpiryHeight` consensus rules
            if tx.is_coinbase() {
                check::coinbase_expiry_height(&req.height(), &tx, &network)?;
            } else {
                check::non_coinbase_expiry_height(&req.height(), &tx)?;
            }

            // Consensus rule:
            //
            // > Either v_{pub}^{old} or v_{pub}^{new} MUST be zero.
            //
            // https://zips.z.cash/protocol/protocol.pdf#joinsplitdesc
            check::joinsplit_has_vpub_zero(&tx)?;

            // [Canopy onward]: `vpub_old` MUST be zero.
            // https://zips.z.cash/protocol/protocol.pdf#joinsplitdesc
            check::disabled_add_to_sprout_pool(&tx, req.height(), &network)?;

            check::spend_conflicts(&tx)?;

            tracing::trace!(?tx_id, "passed quick checks");

            if let Some(block_time) = req.block_time() {
                check::lock_time_has_passed(&tx, req.height(), block_time)?;
            } else {
                // Skip the state query if we don't need the time for this check.
                let next_median_time_past = if tx.lock_time_is_time() {
                    // This state query is much faster than loading UTXOs from the database,
                    // so it doesn't need to be executed in parallel
                    let state = state.clone();
                    Some(Self::mempool_best_chain_next_median_time_past(state).await?.to_chrono())
                } else {
                    None
                };

                // This consensus check makes sure Zebra produces valid block templates.
                check::lock_time_has_passed(&tx, req.height(), next_median_time_past)?;
            }

            // "The consensus rules applied to valueBalance, vShieldedOutput, and bindingSig
            // in non-coinbase transactions MUST also be applied to coinbase transactions."
            //
            // This rule is implicitly implemented during Sapling and Orchard verification,
            // because they do not distinguish between coinbase and non-coinbase transactions.
            //
            // Note: this rule originally applied to Sapling, but we assume it also applies to Orchard.
            //
            // https://zips.z.cash/zip-0213#specification

            // Load spent UTXOs from state.
            // The UTXOs are required for almost all the async checks.
            let load_spent_utxos_fut =
                Self::spent_utxos(tx.clone(), req.clone(), state.clone(), mempool.clone(),);
            let (spent_utxos, spent_outputs, spent_mempool_outpoints) = load_spent_utxos_fut.await?;

            // WONTFIX: Return an error for Request::Block as well to replace this check in
            //       the state once #2336 has been implemented?
            if req.is_mempool() {
                Self::check_maturity_height(&network, &req, &spent_utxos)?;
            }

            let nu = req.upgrade(&network);
            let cached_ffi_transaction =
                Arc::new(CachedFfiTransaction::new(tx.clone(), Arc::new(spent_outputs), nu).map_err(|_| TransactionError::UnsupportedByNetworkUpgrade(tx.version(), nu))?);

            tracing::trace!(?tx_id, "got state UTXOs");

            let mut async_checks = match tx.as_ref() {
                Transaction::V1 { .. } | Transaction::V2 { .. } | Transaction::V3 { .. } => {
                    tracing::debug!(?tx, "got transaction with wrong version");
                    return Err(TransactionError::WrongVersion);
                }
                Transaction::V4 {
                    joinsplit_data,
                    ..
                } => Self::verify_v4_transaction(
                    &req,
                    &network,
                    script_verifier,
                    cached_ffi_transaction.clone(),
                    joinsplit_data,
                )?,
                Transaction::V5 {
                    ..
                } => Self::verify_v5_transaction(
                    &req,
                    &network,
                    script_verifier,
                    cached_ffi_transaction.clone(),
                )?,
                #[cfg(feature="tx_v6")]
                Transaction::V6 {
                    ..
                } => Self::verify_v6_transaction(
                    &req,
                    &network,
                    script_verifier,
                    cached_ffi_transaction.clone(),
                )?,
            };

            if let Some(unmined_tx) = req.mempool_transaction() {
                let check_anchors_and_revealed_nullifiers_query = state
                    .clone()
                    .oneshot(zs::Request::CheckBestChainTipNullifiersAndAnchors(
                        unmined_tx,
                    ))
                    .map(|res| {
                        assert!(
                            res? == zs::Response::ValidBestChainTipNullifiersAndAnchors,
                            "unexpected response to CheckBestChainTipNullifiersAndAnchors request"
                        );
                        Ok(())
                    }
                );

                async_checks.push(check_anchors_and_revealed_nullifiers_query);
            }

            tracing::trace!(?tx_id, "awaiting async checks...");

            async_checks.check().await?;

            tracing::trace!(?tx_id, "finished async checks");

            // Get the `value_balance` to calculate the transaction fee.
            let value_balance = tx.value_balance(&spent_utxos);

            // Calculate the fee only for non-coinbase transactions.
            let mut miner_fee = None;
            if !tx.is_coinbase() {
                // TODO: deduplicate this code with remaining_transaction_value()?
                miner_fee = match value_balance {
                    Ok(vb) => match vb.remaining_transaction_value() {
                        Ok(tx_rtv) => Some(tx_rtv),
                        Err(_) => return Err(TransactionError::IncorrectFee),
                    },
                    Err(_) => return Err(TransactionError::IncorrectFee),
                };
            }

            let sigops = tx.sigops().map_err(zebra_script::Error::from)?;

            let rsp = match req {
                Request::Block { .. } => Response::Block {
                    tx_id,
                    miner_fee,
                    sigops,
                },
                Request::Mempool { transaction: tx, .. } => {
                    let transaction = VerifiedUnminedTx::new(
                        tx,
                        miner_fee.expect("fee should have been checked earlier"),
                        sigops,
                    )?;

                    if let Some(mut mempool) = mempool {
                        tokio::spawn(async move {
                            // Best-effort poll of the mempool to provide a timely response to
                            // `sendrawtransaction` RPC calls or `AwaitOutput` mempool calls.
                            tokio::time::sleep(POLL_MEMPOOL_DELAY).await;
                            let _ = mempool
                                .ready()
                                .await
                                .expect("mempool poll_ready() method should not return an error")
                                .call(mempool::Request::CheckForVerifiedTransactions)
                                .await;
                        });
                    }

                    Response::Mempool { transaction, spent_mempool_outpoints }
                },
            };

            Ok(rsp)
        }
        .inspect(move |result| {
            // Hide the transaction data to avoid filling the logs
            tracing::trace!(?tx_id, result = ?result.as_ref().map(|_tx| ()), "got tx verify result");
        })
        .instrument(span)
        .boxed()
    }
}

impl<ZS, Mempool> Verifier<ZS, Mempool>
where
    ZS: Service<zs::Request, Response = zs::Response, Error = BoxError> + Send + Clone + 'static,
    ZS::Future: Send + 'static,
    Mempool: Service<mempool::Request, Response = mempool::Response, Error = BoxError>
        + Send
        + Clone
        + 'static,
    Mempool::Future: Send + 'static,
{
    /// Fetches the median-time-past of the *next* block after the best state tip.
    ///
    /// This is used to verify that the lock times of mempool transactions
    /// can be included in any valid next block.
    async fn mempool_best_chain_next_median_time_past(
        state: Timeout<ZS>,
    ) -> Result<DateTime32, TransactionError> {
        let query = state
            .clone()
            .oneshot(zs::Request::BestChainNextMedianTimePast);

        if let zebra_state::Response::BestChainNextMedianTimePast(median_time_past) = query
            .await
            .map_err(|e| TransactionError::ValidateMempoolLockTimeError(e.to_string()))?
        {
            Ok(median_time_past)
        } else {
            unreachable!("Request::BestChainNextMedianTimePast always responds with BestChainNextMedianTimePast")
        }
    }

    /// Attempts to find a transaction in the mempool by its transaction hash and checks
    /// that all of its dependencies are available in the block or in the state.  Waits
    /// for UTXOs being spent by the given transaction to arrive in the state if they're
    /// not found elsewhere.
    ///
    /// Returns [`Some(Ok(VerifiedUnminedTx))`](VerifiedUnminedTx) if successful,
    /// None if the transaction id was not found in the mempool,
    /// or `Some(Err(TransparentInputNotFound))` if the transaction was found, but some of its
    /// dependencies were not found in the block or state after a timeout.
    async fn find_verified_unmined_tx(
        req: &Request,
        mempool: Option<Timeout<Mempool>>,
        state: Timeout<ZS>,
    ) -> Option<Result<VerifiedUnminedTx, TransactionError>> {
        let tx = req.transaction();

        if req.is_mempool() || tx.is_coinbase() {
            return None;
        }

        let mempool = mempool?;
        let known_outpoint_hashes = req.known_outpoint_hashes();
        let tx_id = req.tx_mined_id();

        let mempool::Response::TransactionWithDeps {
            transaction: verified_tx,
            dependencies,
        } = mempool
            .oneshot(mempool::Request::TransactionWithDepsByMinedId(tx_id))
            .await
            .ok()?
        else {
            panic!("unexpected response to TransactionWithDepsByMinedId request");
        };

        // Note: This does not verify that the spends are in order, the spend order
        //       should be verified during contextual validation in zebra-state.
        let missing_deps: HashSet<_> = dependencies
            .into_iter()
            .filter(|dependency_id| !known_outpoint_hashes.contains(dependency_id))
            .collect();

        if missing_deps.is_empty() {
            return Some(Ok(verified_tx));
        }

        let missing_outpoints = tx.inputs().iter().filter_map(|input| {
            if let transparent::Input::PrevOut { outpoint, .. } = input {
                missing_deps.contains(&outpoint.hash).then_some(outpoint)
            } else {
                None
            }
        });

        for missing_outpoint in missing_outpoints {
            let query = state
                .clone()
                .oneshot(zebra_state::Request::AwaitUtxo(*missing_outpoint));
            match query.await {
                Ok(zebra_state::Response::Utxo(_)) => {}
                Err(_) => return Some(Err(TransactionError::TransparentInputNotFound)),
                _ => unreachable!("AwaitUtxo always responds with Utxo"),
            };
        }

        Some(Ok(verified_tx))
    }

    /// Wait for the UTXOs that are being spent by the given transaction.
    ///
    /// Looks up UTXOs that are being spent by the given transaction in the state or waits
    /// for them to be added to the mempool for [`Mempool`](Request::Mempool) requests.
    ///
    /// Returns a triple containing:
    /// - `OutPoint` -> `Utxo` map,
    /// - vec of `Output`s in the same order as the matching inputs in the `tx`,
    /// - vec of `Outpoint`s spent by a mempool `tx` that were not found in the best chain's utxo set.
    async fn spent_utxos(
        tx: Arc<Transaction>,
        req: Request,
        state: Timeout<ZS>,
        mempool: Option<Timeout<Mempool>>,
    ) -> Result<
        (
            HashMap<transparent::OutPoint, transparent::Utxo>,
            Vec<transparent::Output>,
            Vec<transparent::OutPoint>,
        ),
        TransactionError,
    > {
        let is_mempool = req.is_mempool();
        // Additional UTXOs known at the time of validation,
        // i.e., from previous transactions in the block.
        let known_utxos = req.known_utxos();

        let inputs = tx.inputs();
        let mut spent_utxos = HashMap::new();
        let mut spent_outputs = Vec::new();
        let mut spent_mempool_outpoints = Vec::new();

        for input in inputs {
            if let transparent::Input::PrevOut { outpoint, .. } = input {
                tracing::trace!("awaiting outpoint lookup");
                let utxo = if let Some(output) = known_utxos.get(outpoint) {
                    tracing::trace!("UXTO in known_utxos, discarding query");
                    output.utxo.clone()
                } else if is_mempool {
                    let query = state
                        .clone()
                        .oneshot(zs::Request::UnspentBestChainUtxo(*outpoint));

                    let zebra_state::Response::UnspentBestChainUtxo(utxo) = query
                        .await
                        .map_err(|_| TransactionError::TransparentInputNotFound)?
                    else {
                        unreachable!("UnspentBestChainUtxo always responds with Option<Utxo>")
                    };

                    let Some(utxo) = utxo else {
                        spent_mempool_outpoints.push(*outpoint);
                        continue;
                    };

                    utxo
                } else {
                    let query = state
                        .clone()
                        .oneshot(zebra_state::Request::AwaitUtxo(*outpoint));
                    if let zebra_state::Response::Utxo(utxo) = query.await? {
                        utxo
                    } else {
                        unreachable!("AwaitUtxo always responds with Utxo")
                    }
                };
                tracing::trace!(?utxo, "got UTXO");
                spent_outputs.push(utxo.output.clone());
                spent_utxos.insert(*outpoint, utxo);
            } else {
                continue;
            }
        }

        if let Some(mempool) = mempool {
            for &spent_mempool_outpoint in &spent_mempool_outpoints {
                let query = mempool
                    .clone()
                    .oneshot(mempool::Request::AwaitOutput(spent_mempool_outpoint));

                let output = match query.await {
                    Ok(mempool::Response::UnspentOutput(output)) => output,
                    Ok(_) => unreachable!("UnspentOutput always responds with UnspentOutput"),
                    Err(err) => {
                        return match err.downcast::<Elapsed>() {
                            Ok(_) => Err(TransactionError::TransparentInputNotFound),
                            Err(err) => Err(err.into()),
                        };
                    }
                };

                spent_outputs.push(output.clone());
                spent_utxos.insert(
                    spent_mempool_outpoint,
                    // Assume the Utxo height will be next height after the best chain tip height
                    //
                    // # Correctness
                    //
                    // If the tip height changes while an umined transaction is being verified,
                    // the transaction must be re-verified before being added to the mempool.
                    transparent::Utxo::new(output, req.height(), false),
                );
            }
        } else if !spent_mempool_outpoints.is_empty() {
            return Err(TransactionError::TransparentInputNotFound);
        }

        Ok((spent_utxos, spent_outputs, spent_mempool_outpoints))
    }

    /// Accepts `request`, a transaction verifier [`&Request`](Request),
    /// and `spent_utxos`, a HashMap of UTXOs in the chain that are spent by this transaction.
    ///
    /// Gets the `transaction`, `height`, and `known_utxos` for the request and checks calls
    /// [`check::tx_transparent_coinbase_spends_maturity`] to verify that every transparent
    /// coinbase output spent by the transaction will have matured by `height`.
    ///
    /// Returns `Ok(())` if every transparent coinbase output spent by the transaction is
    /// mature and valid for the request height, or a [`TransactionError`] if the transaction
    /// spends transparent coinbase outputs that are immature and invalid for the request height.
    pub fn check_maturity_height(
        network: &Network,
        request: &Request,
        spent_utxos: &HashMap<transparent::OutPoint, transparent::Utxo>,
    ) -> Result<(), TransactionError> {
        check::tx_transparent_coinbase_spends_maturity(
            network,
            request.transaction(),
            request.height(),
            request.known_utxos(),
            spent_utxos,
        )
    }

    /// Verify a V4 transaction.
    ///
    /// Returns a set of asynchronous checks that must all succeed for the transaction to be
    /// considered valid. These checks include:
    ///
    /// - transparent transfers
    /// - sprout shielded data
    /// - sapling shielded data
    ///
    /// The parameters of this method are:
    ///
    /// - the `request` to verify (that contains the transaction and other metadata, see [`Request`]
    ///   for more information)
    /// - the `network` to consider when verifying
    /// - the `script_verifier` to use for verifying the transparent transfers
    /// - the prepared `cached_ffi_transaction` used by the script verifier
    /// - the Sprout `joinsplit_data` shielded data in the transaction
    /// - the `sapling_shielded_data` in the transaction
    #[allow(clippy::unwrap_in_result)]
    fn verify_v4_transaction(
        request: &Request,
        network: &Network,
        script_verifier: script::Verifier,
        cached_ffi_transaction: Arc<CachedFfiTransaction>,
        joinsplit_data: &Option<transaction::JoinSplitData<Groth16Proof>>,
    ) -> Result<AsyncChecks, TransactionError> {
        let tx = request.transaction();
        let nu = request.upgrade(network);

        Self::verify_v4_transaction_network_upgrade(&tx, nu)?;

        let sapling_bundle = cached_ffi_transaction.sighasher().sapling_bundle();

        let sighash = cached_ffi_transaction
            .sighasher()
            .sighash(HashType::ALL, None);

        Ok(Self::verify_transparent_inputs_and_outputs(
            request,
            script_verifier,
            cached_ffi_transaction,
        )?
        .and(Self::verify_sprout_shielded_data(joinsplit_data, &sighash)?)
        .and(Self::verify_sapling_bundle(sapling_bundle, &sighash)))
    }

    /// Verifies if a V4 `transaction` is supported by `network_upgrade`.
    fn verify_v4_transaction_network_upgrade(
        transaction: &Transaction,
        network_upgrade: NetworkUpgrade,
    ) -> Result<(), TransactionError> {
        match network_upgrade {
            // Supports V4 transactions
            //
            // # Consensus
            //
            // > [Sapling to Canopy inclusive, pre-NU5] The transaction version number MUST be 4,
            // > and the version group ID MUST be 0x892F2085.
            //
            // > [NU5 onward] The transaction version number MUST be 4 or 5.
            // > If the transaction version number is 4 then the version group ID MUST be 0x892F2085.
            // > If the transaction version number is 5 then the version group ID MUST be 0x26A7270A.
            //
            // https://zips.z.cash/protocol/protocol.pdf#txnconsensus
            //
            // Note: Here we verify the transaction version number of the above two rules, the group
            // id is checked in zebra-chain crate, in the transaction serialize.
            NetworkUpgrade::Sapling
            | NetworkUpgrade::Blossom
            | NetworkUpgrade::Heartwood
            | NetworkUpgrade::Canopy
            | NetworkUpgrade::Nu5
            | NetworkUpgrade::Nu6
            | NetworkUpgrade::Nu6_1 => Ok(()),

            #[cfg(zcash_unstable = "zfuture")]
            NetworkUpgrade::ZFuture => Ok(()),

            // Does not support V4 transactions
            NetworkUpgrade::Genesis
            | NetworkUpgrade::BeforeOverwinter
            | NetworkUpgrade::Overwinter
            | NetworkUpgrade::Nu7 => Err(TransactionError::UnsupportedByNetworkUpgrade(
                transaction.version(),
                network_upgrade,
            )),
        }
    }

    /// Verify a V5 transaction.
    ///
    /// Returns a set of asynchronous checks that must all succeed for the transaction to be
    /// considered valid. These checks include:
    ///
    /// - transaction support by the considered network upgrade (see [`Request::upgrade`])
    /// - transparent transfers
    /// - sapling shielded data (TODO)
    /// - orchard shielded data (TODO)
    ///
    /// The parameters of this method are:
    ///
    /// - the `request` to verify (that contains the transaction and other metadata, see [`Request`]
    ///   for more information)
    /// - the `network` to consider when verifying
    /// - the `script_verifier` to use for verifying the transparent transfers
    /// - the prepared `cached_ffi_transaction` used by the script verifier
    /// - the sapling shielded data of the transaction, if any
    /// - the orchard shielded data of the transaction, if any
    #[allow(clippy::unwrap_in_result)]
    fn verify_v5_transaction(
        request: &Request,
        network: &Network,
        script_verifier: script::Verifier,
        cached_ffi_transaction: Arc<CachedFfiTransaction>,
    ) -> Result<AsyncChecks, TransactionError> {
        let transaction = request.transaction();
        let nu = request.upgrade(network);

        Self::verify_v5_transaction_network_upgrade(&transaction, nu)?;

        let sapling_bundle = cached_ffi_transaction.sighasher().sapling_bundle();
        let orchard_bundle = cached_ffi_transaction.sighasher().orchard_bundle();

        let sighash = cached_ffi_transaction
            .sighasher()
            .sighash(HashType::ALL, None);

        Ok(Self::verify_transparent_inputs_and_outputs(
            request,
            script_verifier,
            cached_ffi_transaction,
        )?
        .and(Self::verify_sapling_bundle(sapling_bundle, &sighash))
        .and(Self::verify_orchard_bundle(orchard_bundle, &sighash)))
    }

    /// Verifies if a V5 `transaction` is supported by `network_upgrade`.
    fn verify_v5_transaction_network_upgrade(
        transaction: &Transaction,
        network_upgrade: NetworkUpgrade,
    ) -> Result<(), TransactionError> {
        match network_upgrade {
            // Supports V5 transactions
            //
            // # Consensus
            //
            // > [NU5 onward] The transaction version number MUST be 4 or 5.
            // > If the transaction version number is 4 then the version group ID MUST be 0x892F2085.
            // > If the transaction version number is 5 then the version group ID MUST be 0x26A7270A.
            //
            // https://zips.z.cash/protocol/protocol.pdf#txnconsensus
            //
            // Note: Here we verify the transaction version number of the above rule, the group
            // id is checked in zebra-chain crate, in the transaction serialize.
            NetworkUpgrade::Nu5
            | NetworkUpgrade::Nu6
            | NetworkUpgrade::Nu6_1
            | NetworkUpgrade::Nu7 => Ok(()),

            #[cfg(zcash_unstable = "zfuture")]
            NetworkUpgrade::ZFuture => Ok(()),

            // Does not support V5 transactions
            NetworkUpgrade::Genesis
            | NetworkUpgrade::BeforeOverwinter
            | NetworkUpgrade::Overwinter
            | NetworkUpgrade::Sapling
            | NetworkUpgrade::Blossom
            | NetworkUpgrade::Heartwood
            | NetworkUpgrade::Canopy => Err(TransactionError::UnsupportedByNetworkUpgrade(
                transaction.version(),
                network_upgrade,
            )),
        }
    }

    /// Passthrough to verify_v5_transaction, but for V6 transactions.
    #[cfg(feature = "tx_v6")]
    fn verify_v6_transaction(
        request: &Request,
        network: &Network,
        script_verifier: script::Verifier,
        cached_ffi_transaction: Arc<CachedFfiTransaction>,
    ) -> Result<AsyncChecks, TransactionError> {
        Self::verify_v5_transaction(request, network, script_verifier, cached_ffi_transaction)
    }

    /// Verifies if a transaction's transparent inputs are valid using the provided
    /// `script_verifier` and `cached_ffi_transaction`.
    ///
    /// Returns script verification responses via the `utxo_sender`.
    fn verify_transparent_inputs_and_outputs(
        request: &Request,
        script_verifier: script::Verifier,
        cached_ffi_transaction: Arc<CachedFfiTransaction>,
    ) -> Result<AsyncChecks, TransactionError> {
        let transaction = request.transaction();

        if transaction.is_coinbase() {
            // The script verifier only verifies PrevOut inputs and their corresponding UTXOs.
            // Coinbase transactions don't have any PrevOut inputs.
            Ok(AsyncChecks::new())
        } else {
            // feed all of the inputs to the script verifier
            let inputs = transaction.inputs();

            let script_checks = (0..inputs.len())
                .map(move |input_index| {
                    let request = script::Request {
                        cached_ffi_transaction: cached_ffi_transaction.clone(),
                        input_index,
                    };

                    script_verifier.oneshot(request)
                })
                .collect();

            Ok(script_checks)
        }
    }

    /// Verifies a transaction's Sprout shielded join split data.
    fn verify_sprout_shielded_data(
        joinsplit_data: &Option<transaction::JoinSplitData<Groth16Proof>>,
        shielded_sighash: &SigHash,
    ) -> Result<AsyncChecks, TransactionError> {
        let mut checks = AsyncChecks::new();

        if let Some(joinsplit_data) = joinsplit_data {
            for joinsplit in joinsplit_data.joinsplits() {
                // # Consensus
                //
                // > The proof π_ZKJoinSplit MUST be valid given a
                // > primary input formed from the relevant other fields and h_{Sig}
                //
                // https://zips.z.cash/protocol/protocol.pdf#joinsplitdesc
                //
                // Queue the verification of the Groth16 spend proof
                // for each JoinSplit description while adding the
                // resulting future to our collection of async
                // checks that (at a minimum) must pass for the
                // transaction to verify.
                checks.push(primitives::groth16::JOINSPLIT_VERIFIER.oneshot(
                    DescriptionWrapper(&(joinsplit, &joinsplit_data.pub_key)).try_into()?,
                ));
            }

            // # Consensus
            //
            // > If effectiveVersion ≥ 2 and nJoinSplit > 0, then:
            // > - joinSplitPubKey MUST be a valid encoding of an Ed25519 validating key
            // > - joinSplitSig MUST represent a valid signature under
            //     joinSplitPubKey of dataToBeSigned, as defined in § 4.11
            //
            // https://zips.z.cash/protocol/protocol.pdf#txnconsensus
            //
            // The `if` part is indirectly enforced, since the `joinsplit_data`
            // is only parsed if those conditions apply in
            // [`Transaction::zcash_deserialize`].
            //
            // The valid encoding is defined in
            //
            // > A valid Ed25519 validating key is defined as a sequence of 32
            // > bytes encoding a point on the Ed25519 curve
            //
            // https://zips.z.cash/protocol/protocol.pdf#concreteed25519
            //
            // which is enforced during signature verification, in both batched
            // and single verification, when decompressing the encoded point.
            //
            // Queue the validation of the JoinSplit signature while
            // adding the resulting future to our collection of
            // async checks that (at a minimum) must pass for the
            // transaction to verify.
            //
            // https://zips.z.cash/protocol/protocol.pdf#sproutnonmalleability
            // https://zips.z.cash/protocol/protocol.pdf#txnencodingandconsensus
            let ed25519_verifier = primitives::ed25519::VERIFIER.clone();
            let ed25519_item =
                (joinsplit_data.pub_key, joinsplit_data.sig, shielded_sighash).into();

            checks.push(ed25519_verifier.oneshot(ed25519_item));
        }

        Ok(checks)
    }

    /// Verifies a transaction's Sapling shielded data.
    fn verify_sapling_bundle(
        bundle: Option<sapling_crypto::Bundle<sapling_crypto::bundle::Authorized, ZatBalance>>,
        sighash: &SigHash,
    ) -> AsyncChecks {
        let mut async_checks = AsyncChecks::new();

        // The Sapling batch verifier checks the following consensus rules:
        //
        // # Consensus
        //
        // > The proof π_ZKSpend MUST be valid given a primary input formed from the other fields
        // > except spendAuthSig.
        //
        // > The spend authorization signature MUST be a valid SpendAuthSig signature over SigHash
        // > using rk as the validating key.
        //
        // > [NU5 onward] As specified in § 5.4.7 ‘RedDSA, RedJubjub, and RedPallas’ on p. 88, the
        // > validation of the 𝑅 component of the signature changes to prohibit non-canonical
        // > encodings.
        //
        // https://zips.z.cash/protocol/protocol.pdf#spenddesc
        //
        // # Consensus
        //
        // > The proof π_ZKOutput MUST be valid given a primary input formed from the other fields
        // > except C^enc and C^out.
        //
        // https://zips.z.cash/protocol/protocol.pdf#outputdesc
        //
        // # Consensus
        //
        // > The Spend transfers and Action transfers of a transaction MUST be consistent with its
        // > vbalanceSapling value as specified in § 4.13 ‘Balance and Binding Signature (Sapling)’.
        //
        // https://zips.z.cash/protocol/protocol.pdf#spendsandoutputs
        //
        // # Consensus
        //
        // > [Sapling onward] If effectiveVersion ≥ 4 and nSpendsSapling + nOutputsSapling > 0,
        // > then:
        // >
        // > – let bvk^{Sapling} and SigHash be as defined in § 4.13;
        // > – bindingSigSapling MUST represent a valid signature under the transaction binding
        // >   validating key bvk Sapling of SigHash — i.e.
        // >   BindingSig^{Sapling}.Validate_{bvk^{Sapling}}(SigHash, bindingSigSapling ) = 1.
        //
        // Note that the `if` part is indirectly enforced, since the `sapling_shielded_data` is only
        // parsed if those conditions apply in [`Transaction::zcash_deserialize`].
        //
        // > [NU5 onward] As specified in § 5.4.7, the validation of the 𝑅 component of the
        // > signature changes to prohibit non-canonical encodings.
        //
        // https://zips.z.cash/protocol/protocol.pdf#txnconsensus
        if let Some(bundle) = bundle {
            async_checks.push(
                primitives::sapling::VERIFIER
                    .clone()
                    .oneshot(primitives::sapling::Item::new(bundle, *sighash)),
            );
        }

        async_checks
    }

    /// Verifies a transaction's Orchard shielded data.
    fn verify_orchard_bundle(
        bundle: Option<::orchard::bundle::Bundle<::orchard::bundle::Authorized, ZatBalance>>,
        sighash: &SigHash,
    ) -> AsyncChecks {
        let mut async_checks = AsyncChecks::new();

        if let Some(bundle) = bundle {
            // # Consensus
            //
            // > The proof 𝜋 MUST be valid given a primary input (cv, rt^{Orchard},
            // > nf, rk, cm_x, enableSpends, enableOutputs)
            //
            // https://zips.z.cash/protocol/protocol.pdf#actiondesc
            //
            // Unlike Sapling, Orchard shielded transactions have a single
            // aggregated Halo2 proof per transaction, even with multiple
            // Actions in one transaction. So we queue it for verification
            // only once instead of queuing it up for every Action description.
            async_checks.push(
                primitives::halo2::VERIFIER
                    .clone()
                    .oneshot(primitives::halo2::Item::new(bundle, *sighash)),
            );
        }

        async_checks
    }
}

/// A set of unordered asynchronous checks that should succeed.
///
/// A wrapper around [`FuturesUnordered`] with some auxiliary methods.
struct AsyncChecks(FuturesUnordered<Pin<Box<dyn Future<Output = Result<(), BoxError>> + Send>>>);

impl AsyncChecks {
    /// Create an empty set of unordered asynchronous checks.
    pub fn new() -> Self {
        AsyncChecks(FuturesUnordered::new())
    }

    /// Push a check into the set.
    pub fn push(&mut self, check: impl Future<Output = Result<(), BoxError>> + Send + 'static) {
        self.0.push(check.boxed());
    }

    /// Push a set of checks into the set.
    ///
    /// This method can be daisy-chained.
    pub fn and(mut self, checks: AsyncChecks) -> Self {
        self.0.extend(checks.0);
        self
    }

    /// Wait until all checks in the set finish.
    ///
    /// If any of the checks fail, this method immediately returns the error and cancels all other
    /// checks by dropping them.
    async fn check(mut self) -> Result<(), BoxError> {
        // Wait for all asynchronous checks to complete
        // successfully, or fail verification if they error.
        while let Some(check) = self.0.next().await {
            tracing::trace!(?check, remaining = self.0.len());
            check?;
        }

        Ok(())
    }
}

impl<F> FromIterator<F> for AsyncChecks
where
    F: Future<Output = Result<(), BoxError>> + Send + 'static,
{
    fn from_iter<I>(iterator: I) -> Self
    where
        I: IntoIterator<Item = F>,
    {
        AsyncChecks(iterator.into_iter().map(FutureExt::boxed).collect())
    }
}