zakura-consensus 5.0.0

//! Async Sapling batch verifier service

use core::fmt;
use std::{
    future::Future,
    mem,
    pin::Pin,
    task::{Context, Poll},
};

use futures::{future::BoxFuture, FutureExt};
use once_cell::sync::Lazy;
use rand::thread_rng;
use tokio::sync::watch;
use tower::{util::ServiceFn, Service};
use tower_batch_control::{Batch, BatchControl, RequestWeight};
use tower_fallback::Fallback;

use sapling_crypto::{bundle::Authorized, BatchValidator, Bundle};
use zakura_chain::transaction::SigHash;
use zcash_proofs::prover::LocalTxProver;
use zcash_protocol::value::ZatBalance;

use crate::{error::TransactionError, BoxError};

/// Sapling prover containing spend and output params for the Sapling circuit.
///
/// Used to:
///
/// - construct Sapling outputs in coinbase txs, and
/// - verify Sapling shielded data in the tx verifier.
static SAPLING: Lazy<LocalTxProver> = Lazy::new(LocalTxProver::bundled);

/// Returns the process-wide Sapling prover for constructing Sapling proofs, initializing it on
/// first use.
///
/// The bundled Sapling spend and output proving parameters are parsed once, then the same prover is
/// reused for proof construction and verification for the lifetime of the process.
pub fn sapling_prover() -> &'static LocalTxProver {
    Lazy::force(&SAPLING)
}

#[derive(Clone)]
pub struct Item {
    /// The bundle containing the Sapling shielded data to verify.
    bundle: Bundle<Authorized, ZatBalance>,
    /// The sighash of the transaction that contains the Sapling shielded data.
    sighash: SigHash,
}

impl Item {
    /// Creates a new [`Item`] from a Sapling bundle and sighash.
    pub fn new(bundle: Bundle<Authorized, ZatBalance>, sighash: SigHash) -> Self {
        Self { bundle, sighash }
    }
}

impl RequestWeight for Item {
    fn request_weight(&self) -> usize {
        self.bundle
            .shielded_spends()
            .len()
            .saturating_add(self.bundle.shielded_outputs().len())
    }
}

/// A service that verifies Sapling shielded data in batches.
///
/// Handles batching incoming requests, driving batches to completion, and reporting results.
#[derive(Default)]
pub struct Verifier {
    /// A batch verifier for Sapling shielded data.
    batch: BatchValidator,

    /// A channel for broadcasting the verification result of the batch.
    ///
    /// Each batch gets a newly created channel, so there is only ever one result sent per channel.
    /// Tokio doesn't have a oneshot multi-consumer channel, so we use a watch channel.
    tx: watch::Sender<Option<bool>>,
}

impl fmt::Debug for Verifier {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Verifier")
            .field("batch", &"..")
            .field("tx", &self.tx)
            .finish()
    }
}

impl Drop for Verifier {
    // Flush the current batch in case there are still any pending futures.
    //
    // Flushing the batch means we need to validate it. This function fires off the validation and
    // returns immediately, usually before the validation finishes.
    fn drop(&mut self) {
        let batch = mem::take(&mut self.batch);
        let tx = mem::take(&mut self.tx);

        // The validation is CPU-intensive; do it on a dedicated thread so it does not block.
        rayon::spawn_fifo(move || {
            let (spend_vk, output_vk) = SAPLING.verifying_keys();

            // Validate the batch and send the result through the channel.
            let res = batch.validate(&spend_vk, &output_vk, thread_rng());
            let _ = tx.send(Some(res));
        });
    }
}

impl Service<BatchControl<Item>> for Verifier {
    type Response = ();
    type Error = Box<dyn std::error::Error + Send + Sync>;
    type Future = Pin<Box<dyn Future<Output = Result<(), Self::Error>> + Send>>;

    fn poll_ready(&mut self, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        Poll::Ready(Ok(()))
    }

    fn call(&mut self, req: BatchControl<Item>) -> Self::Future {
        match req {
            BatchControl::Item(item) => {
                let mut rx = self.tx.subscribe();

                let bundle_check = self
                    .batch
                    .check_bundle(item.bundle, item.sighash.into())
                    .then_some(())
                    .ok_or(TransactionError::SaplingVerificationFailed);

                async move {
                    bundle_check.map_err(BoxError::from)?;

                    rx.changed()
                        .await
                        .map_err(|_| BoxError::from("verifier was dropped without flushing"))?;

                    // We use a new channel for each batch, so we always get the correct
                    // batch result here.
                    let is_valid = *rx.borrow().as_ref().ok_or_else(|| {
                        Box::<dyn std::error::Error + Send + Sync>::from(
                            "threadpool unexpectedly dropped channel sender",
                        )
                    })?;

                    if is_valid {
                        metrics::counter!("proofs.sapling.verified").increment(1);
                        Ok(())
                    } else {
                        metrics::counter!("proofs.sapling.invalid").increment(1);
                        Err(BoxError::from(TransactionError::SaplingVerificationFailed))
                    }
                }
                .boxed()
            }

            BatchControl::Flush => {
                let batch = mem::take(&mut self.batch);
                let tx = mem::take(&mut self.tx);

                async move {
                    let start = std::time::Instant::now();
                    let spawn_result = tokio::task::spawn_blocking(move || {
                        let (spend_vk, output_vk) = SAPLING.verifying_keys();
                        batch.validate(&spend_vk, &output_vk, thread_rng())
                    })
                    .await;
                    let duration = start.elapsed().as_secs_f64();

                    let result_label = match &spawn_result {
                        Ok(true) => "success",
                        _ => "failure",
                    };
                    metrics::histogram!(
                        "zakura.consensus.batch.duration_seconds",
                        "verifier" => "groth16_sapling",
                        "result" => result_label
                    )
                    .record(duration);

                    // Extract the value before consuming spawn_result
                    let is_valid = spawn_result.as_ref().ok().copied();
                    let _ = tx.send(is_valid);
                    spawn_result.map(|_| ()).map_err(Self::Error::from)
                }
                .boxed()
            }
        }
    }
}

/// Verifies a single [`Item`].
pub fn verify_single(
    item: Item,
) -> Pin<Box<dyn Future<Output = Result<(), Box<dyn std::error::Error + Send + Sync>>> + Send>> {
    async move {
        let mut verifier = Verifier::default();

        let check = verifier
            .batch
            .check_bundle(item.bundle, item.sighash.into())
            .then_some(())
            .ok_or(TransactionError::SaplingVerificationFailed);
        check.map_err(BoxError::from)?;

        let is_valid = tokio::task::spawn_blocking(move || {
            let (spend_vk, output_vk) = SAPLING.verifying_keys();

            mem::take(&mut verifier.batch).validate(&spend_vk, &output_vk, thread_rng())
        })
        .await
        .map_err(|_| BoxError::from("Sapling bundle validation thread panicked"))?;

        if is_valid {
            Ok(())
        } else {
            Err(BoxError::from(TransactionError::SaplingVerificationFailed))
        }
    }
    .boxed()
}

/// Global batch verification context for Sapling shielded data.
pub static VERIFIER: Lazy<
    Fallback<
        Batch<Verifier, Item>,
        ServiceFn<
            fn(Item) -> BoxFuture<'static, Result<(), Box<dyn std::error::Error + Send + Sync>>>,
        >,
    >,
> = Lazy::new(|| {
    Fallback::new(
        Batch::new(
            Verifier::default(),
            super::MAX_BATCH_SIZE,
            None,
            super::MAX_BATCH_LATENCY,
        ),
        tower::service_fn(verify_single),
    )
});

#[cfg(test)]
mod tests {
    use super::sapling_prover;

    #[test]
    fn sapling_prover_is_reused() {
        assert!(std::ptr::eq(sapling_prover(), sapling_prover()));
    }
}