607 lines
25 KiB
Rust
607 lines
25 KiB
Rust
use std::{collections::HashSet, pin::Pin, sync::Arc, time::Duration};
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use color_eyre::eyre::{eyre, Report};
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use futures::{
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future::FutureExt,
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stream::{FuturesUnordered, StreamExt},
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};
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use tokio::time::delay_for;
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use tower::{
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builder::ServiceBuilder, hedge::Hedge, limit::ConcurrencyLimit, retry::Retry, timeout::Timeout,
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Service, ServiceExt,
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};
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use zebra_chain::{
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block::{self, Block},
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parameters::genesis_hash,
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};
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use zebra_network as zn;
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use zebra_state as zs;
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use crate::config::ZebradConfig;
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mod downloads;
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use downloads::{AlwaysHedge, Downloads};
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/// Controls the number of peers used for each ObtainTips and ExtendTips request.
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const FANOUT: usize = 4;
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/// Controls how many times we will retry each block download.
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///
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/// Failing block downloads is important because it defends against peers who
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/// feed us bad hashes. But spurious failures of valid blocks cause the syncer to
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/// restart from the previous checkpoint, potentially re-downloading blocks.
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///
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/// We also hedge requests, so we may retry up to twice this many times.
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const BLOCK_DOWNLOAD_RETRY_LIMIT: usize = 2;
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/// Controls how far ahead of the chain tip the syncer tries to download before
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/// waiting for queued verifications to complete.
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///
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/// Increasing this limit increases the buffer size, so it reduces the impact of
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/// missing a block on the critical path. The block size limit is 2MB, so in
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/// theory, this could represent multiple gigabytes of data, if we downloaded
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/// arbitrary blocks. However, because we randomly load balance outbound
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/// requests, and separate block download from obtaining block hashes, an
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/// adversary would have to control a significant fraction of our peers to lead
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/// us astray.
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const LOOKAHEAD_LIMIT: usize = zebra_consensus::MAX_CHECKPOINT_HEIGHT_GAP * 2;
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/// Controls how long we wait for a tips response to return.
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const TIPS_RESPONSE_TIMEOUT: Duration = Duration::from_secs(6);
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/// Controls how long we wait for a block download request to complete.
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const BLOCK_DOWNLOAD_TIMEOUT: Duration = Duration::from_secs(20);
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/// The maximum amount of time that Zebra should take to download a checkpoint
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/// full of blocks. Based on the current `MAX_CHECKPOINT_BYTE_SIZE`.
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///
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/// We assume that Zebra nodes have at least 10 Mbps bandwidth, and allow some
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/// extra time for request latency.
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const MAX_CHECKPOINT_DOWNLOAD_SECONDS: u64 = 300;
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/// Controls how long we wait for a block verify task to complete.
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///
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/// This timeout makes sure that the syncer and verifiers do not deadlock.
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/// When the `LOOKAHEAD_LIMIT` is reached, the syncer waits for blocks to verify
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/// (or fail). If the verifiers are also waiting for more blocks from the syncer,
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/// then without a timeout, Zebra would deadlock.
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const BLOCK_VERIFY_TIMEOUT: Duration = Duration::from_secs(MAX_CHECKPOINT_DOWNLOAD_SECONDS);
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/// Controls how long we wait to restart syncing after finishing a sync run.
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///
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/// This timeout should be long enough to:
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/// - allow zcashd peers to process pending requests. If the node only has a
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/// few peers, we want to clear as much peer state as possible. In
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/// particular, zcashd sends "next block range" hints, based on zcashd's
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/// internal model of our sync progress. But we want to discard these hints,
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/// so they don't get confused with ObtainTips and ExtendTips responses.
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///
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/// This timeout is particularly important on instances with slow or unreliable
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/// networks, and on testnet, which has a small number of slow peers.
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const SYNC_RESTART_TIMEOUT: Duration = Duration::from_secs(45);
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type BoxError = Box<dyn std::error::Error + Send + Sync + 'static>;
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/// Helps work around defects in the bitcoin protocol by checking whether
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/// the returned hashes actually extend a chain tip.
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#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
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struct CheckedTip {
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tip: block::Hash,
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expected_next: block::Hash,
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}
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pub struct ChainSync<ZN, ZS, ZV>
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where
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ZN: Service<zn::Request, Response = zn::Response, Error = BoxError> + Send + Clone + 'static,
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ZN::Future: Send,
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ZS: Service<zs::Request, Response = zs::Response, Error = BoxError> + Send + Clone + 'static,
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ZS::Future: Send,
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ZV: Service<Arc<Block>, Response = block::Hash, Error = BoxError> + Send + Clone + 'static,
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ZV::Future: Send,
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{
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/// Used to perform ObtainTips and ExtendTips requests, with no retry logic
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/// (failover is handled using fanout).
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tip_network: Timeout<ZN>,
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state: ZS,
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prospective_tips: HashSet<CheckedTip>,
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genesis_hash: block::Hash,
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downloads: Pin<
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Box<
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Downloads<
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Hedge<ConcurrencyLimit<Retry<zn::RetryLimit, Timeout<ZN>>>, AlwaysHedge>,
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Timeout<ZV>,
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>,
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>,
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>,
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}
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/// Polls the network to determine whether further blocks are available and
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/// downloads them.
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///
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/// This component is used for initial block sync, but the `Inbound` service is
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/// responsible for participating in the gossip protocols used for block
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/// diffusion.
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impl<ZN, ZS, ZV> ChainSync<ZN, ZS, ZV>
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where
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ZN: Service<zn::Request, Response = zn::Response, Error = BoxError> + Send + Clone + 'static,
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ZN::Future: Send,
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ZS: Service<zs::Request, Response = zs::Response, Error = BoxError> + Send + Clone + 'static,
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ZS::Future: Send,
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ZV: Service<Arc<Block>, Response = block::Hash, Error = BoxError> + Send + Clone + 'static,
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ZV::Future: Send,
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{
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/// Returns a new syncer instance, using:
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/// - chain: the zebra-chain `Network` to download (Mainnet or Testnet)
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/// - peers: the zebra-network peers to contact for downloads
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/// - state: the zebra-state that stores the chain
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/// - verifier: the zebra-consensus verifier that checks the chain
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pub fn new(config: &ZebradConfig, peers: ZN, state: ZS, verifier: ZV) -> Self {
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let tip_network = Timeout::new(peers.clone(), TIPS_RESPONSE_TIMEOUT);
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// The Hedge middleware is the outermost layer, hedging requests
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// between two retry-wrapped networks. The innermost timeout
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// layer is relatively unimportant, because slow requests will
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// probably be pre-emptively hedged.
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//
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// XXX add ServiceBuilder::hedge() so this becomes
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// ServiceBuilder::new().hedge(...).retry(...)...
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let block_network = Hedge::new(
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ServiceBuilder::new()
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.concurrency_limit(config.sync.max_concurrent_block_requests)
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.retry(zn::RetryLimit::new(BLOCK_DOWNLOAD_RETRY_LIMIT))
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.timeout(BLOCK_DOWNLOAD_TIMEOUT)
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.service(peers),
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AlwaysHedge,
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20,
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0.95,
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2 * SYNC_RESTART_TIMEOUT,
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);
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Self {
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tip_network,
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state,
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downloads: Box::pin(Downloads::new(
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block_network,
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Timeout::new(verifier, BLOCK_VERIFY_TIMEOUT),
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)),
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prospective_tips: HashSet::new(),
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genesis_hash: genesis_hash(config.network.network),
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}
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}
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#[instrument(skip(self))]
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pub async fn sync(&mut self) -> Result<(), Report> {
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// We can't download the genesis block using our normal algorithm,
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// due to protocol limitations
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self.request_genesis().await?;
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// Distinguishes a restart from a start, so we don't delay when starting
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// the sync process, but we can keep restart logic in one place.
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let mut started_once = false;
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'sync: loop {
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if started_once {
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tracing::info!(timeout = ?SYNC_RESTART_TIMEOUT, "waiting to restart sync");
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self.prospective_tips = HashSet::new();
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self.downloads.cancel_all();
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self.update_metrics();
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delay_for(SYNC_RESTART_TIMEOUT).await;
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} else {
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started_once = true;
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}
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tracing::info!("starting sync, obtaining new tips");
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if let Err(e) = self.obtain_tips().await {
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tracing::warn!(?e);
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continue 'sync;
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}
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self.update_metrics();
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while !self.prospective_tips.is_empty() {
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// Check whether any block tasks are currently ready:
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while let Some(Some(rsp)) = self.downloads.next().now_or_never() {
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match rsp {
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Ok(hash) => {
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tracing::trace!(?hash, "verified and committed block to state");
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}
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Err(e) => {
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tracing::warn!(?e);
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continue 'sync;
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}
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}
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}
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self.update_metrics();
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// If we have too many pending tasks, wait for some to finish.
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//
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// Starting to wait is interesting, but logging each wait can be
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// very verbose.
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if self.downloads.in_flight() > LOOKAHEAD_LIMIT {
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tracing::info!(
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tips.len = self.prospective_tips.len(),
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in_flight = self.downloads.in_flight(),
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lookahead_limit = LOOKAHEAD_LIMIT,
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"waiting for pending blocks",
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);
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}
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while self.downloads.in_flight() > LOOKAHEAD_LIMIT {
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tracing::trace!(
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tips.len = self.prospective_tips.len(),
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in_flight = self.downloads.in_flight(),
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lookahead_limit = LOOKAHEAD_LIMIT,
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"waiting for pending blocks",
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);
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match self.downloads.next().await.expect("downloads is nonempty") {
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Ok(hash) => {
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tracing::trace!(?hash, "verified and committed block to state");
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}
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Err(e) => {
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tracing::warn!(?e);
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continue 'sync;
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}
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}
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self.update_metrics();
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}
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// Once we're below the lookahead limit, we can keep extending the tips.
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tracing::info!(
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tips.len = self.prospective_tips.len(),
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in_flight = self.downloads.in_flight(),
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lookahead_limit = LOOKAHEAD_LIMIT,
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"extending tips",
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);
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if let Err(e) = self.extend_tips().await {
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tracing::warn!(?e);
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continue 'sync;
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}
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self.update_metrics();
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}
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tracing::info!("exhausted prospective tip set");
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}
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}
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/// Given a block_locator list fan out request for subsequent hashes to
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/// multiple peers
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#[instrument(skip(self))]
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async fn obtain_tips(&mut self) -> Result<(), Report> {
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let block_locator = self
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.state
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.ready_and()
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.await
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.map_err(|e| eyre!(e))?
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.call(zebra_state::Request::BlockLocator)
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.await
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.map(|response| match response {
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zebra_state::Response::BlockLocator(block_locator) => block_locator,
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_ => unreachable!(
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"GetBlockLocator request can only result in Response::BlockLocator"
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),
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})
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.map_err(|e| eyre!(e))?;
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tracing::debug!(?block_locator, "trying to obtain new chain tips");
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let mut requests = FuturesUnordered::new();
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for _ in 0..FANOUT {
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requests.push(
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self.tip_network
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.ready_and()
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.await
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.map_err(|e| eyre!(e))?
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.call(zn::Request::FindBlocks {
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known_blocks: block_locator.clone(),
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stop: None,
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}),
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);
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}
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let mut download_set = HashSet::new();
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while let Some(res) = requests.next().await {
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match res.map_err::<Report, _>(|e| eyre!(e)) {
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Ok(zn::Response::BlockHashes(hashes)) => {
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tracing::trace!(?hashes);
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// zcashd sometimes appends an unrelated hash at the start
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// or end of its response.
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//
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// We can't discard the first hash, because it might be a
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// block we want to download. So we just accept any
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// out-of-order first hashes.
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// We use the last hash for the tip, and we want to avoid bad
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// tips. So we discard the last hash. (We don't need to worry
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// about missed downloads, because we will pick them up again
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// in ExtendTips.)
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let hashes = match hashes.as_slice() {
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[] => continue,
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[rest @ .., _last] => rest,
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};
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let mut first_unknown = None;
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for (i, &hash) in hashes.iter().enumerate() {
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if !self.state_contains(hash).await? {
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first_unknown = Some(i);
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break;
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}
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}
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tracing::debug!(hashes.len = ?hashes.len(), ?first_unknown);
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let unknown_hashes = if let Some(index) = first_unknown {
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&hashes[index..]
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} else {
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continue;
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};
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tracing::trace!(?unknown_hashes);
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let new_tip = if let Some(end) = unknown_hashes.rchunks_exact(2).next() {
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CheckedTip {
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tip: end[0],
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expected_next: end[1],
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}
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} else {
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tracing::debug!("discarding response that extends only one block");
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continue;
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};
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// Make sure we get the same tips, regardless of the
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// order of peer responses
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if !download_set.contains(&new_tip.expected_next) {
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tracing::debug!(?new_tip,
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"adding new prospective tip, and removing existing tips in the new block hash list");
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self.prospective_tips
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.retain(|t| !unknown_hashes.contains(&t.expected_next));
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self.prospective_tips.insert(new_tip);
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} else {
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tracing::debug!(
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?new_tip,
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"discarding prospective tip: already in download set"
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);
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}
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let prev_download_len = download_set.len();
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download_set.extend(unknown_hashes);
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let new_download_len = download_set.len();
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tracing::debug!(
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new_hashes = new_download_len - prev_download_len,
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"added hashes to download set"
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);
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}
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Ok(_) => unreachable!("network returned wrong response"),
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// We ignore this error because we made multiple fanout requests.
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Err(e) => tracing::debug!(?e),
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}
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}
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tracing::debug!(?self.prospective_tips);
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// Check that the new tips we got are actually unknown.
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for hash in &download_set {
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tracing::debug!(?hash, "checking if state contains hash");
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if self.state_contains(*hash).await? {
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return Err(eyre!("queued download of hash behind our chain tip"));
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}
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}
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self.request_blocks(download_set).await?;
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Ok(())
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}
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#[instrument(skip(self))]
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async fn extend_tips(&mut self) -> Result<(), Report> {
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let tips = std::mem::take(&mut self.prospective_tips);
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let mut download_set = HashSet::new();
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for tip in tips {
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tracing::debug!(?tip, "extending tip");
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let mut responses = FuturesUnordered::new();
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for _ in 0..FANOUT {
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responses.push(
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self.tip_network
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.ready_and()
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.await
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.map_err(|e| eyre!(e))?
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.call(zn::Request::FindBlocks {
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known_blocks: vec![tip.tip],
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stop: None,
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}),
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);
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}
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while let Some(res) = responses.next().await {
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match res.map_err::<Report, _>(|e| eyre!(e)) {
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Ok(zn::Response::BlockHashes(hashes)) => {
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tracing::debug!(first = ?hashes.first(), len = ?hashes.len());
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tracing::trace!(?hashes);
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// zcashd sometimes appends an unrelated hash at the
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// start or end of its response. Check the first hash
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// against the previous response, and discard mismatches.
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let unknown_hashes = match hashes.as_slice() {
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[expected_hash, rest @ ..] if expected_hash == &tip.expected_next => {
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rest
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}
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// If the first hash doesn't match, retry with the second.
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[first_hash, expected_hash, rest @ ..]
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if expected_hash == &tip.expected_next =>
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{
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tracing::debug!(?first_hash,
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?tip.expected_next,
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?tip.tip,
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"unexpected first hash, but the second matches: using the hashes after the match");
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rest
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}
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// We ignore these responses
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[] => continue,
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[single_hash] => {
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tracing::debug!(?single_hash,
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?tip.expected_next,
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?tip.tip,
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"discarding response containing a single unexpected hash");
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continue;
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}
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[first_hash, second_hash, rest @ ..] => {
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tracing::debug!(?first_hash,
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?second_hash,
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rest_len = ?rest.len(),
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?tip.expected_next,
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?tip.tip,
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"discarding response that starts with two unexpected hashes");
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continue;
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}
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};
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// We use the last hash for the tip, and we want to avoid
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// bad tips. So we discard the last hash. (We don't need
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// to worry about missed downloads, because we will pick
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// them up again in the next ExtendTips.)
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let unknown_hashes = match unknown_hashes {
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[] => continue,
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[rest @ .., _last] => rest,
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};
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let new_tip = if let Some(end) = unknown_hashes.rchunks_exact(2).next() {
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CheckedTip {
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tip: end[0],
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expected_next: end[1],
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}
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} else {
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tracing::debug!("discarding response that extends only one block");
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continue;
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};
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tracing::trace!(?unknown_hashes);
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// Make sure we get the same tips, regardless of the
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// order of peer responses
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if !download_set.contains(&new_tip.expected_next) {
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tracing::debug!(?new_tip,
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"adding new prospective tip, and removing any existing tips in the new block hash list");
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self.prospective_tips
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.retain(|t| !unknown_hashes.contains(&t.expected_next));
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self.prospective_tips.insert(new_tip);
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} else {
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tracing::debug!(
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?new_tip,
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"discarding prospective tip: already in download set"
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);
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}
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let prev_download_len = download_set.len();
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download_set.extend(unknown_hashes);
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let new_download_len = download_set.len();
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tracing::debug!(
|
|
new_hashes = new_download_len - prev_download_len,
|
|
"added hashes to download set"
|
|
);
|
|
}
|
|
Ok(_) => unreachable!("network returned wrong response"),
|
|
// We ignore this error because we made multiple fanout requests.
|
|
Err(e) => tracing::debug!(?e),
|
|
}
|
|
}
|
|
}
|
|
|
|
self.request_blocks(download_set).await?;
|
|
|
|
Ok(())
|
|
}
|
|
|
|
/// Download and verify the genesis block, if it isn't currently known to
|
|
/// our node.
|
|
async fn request_genesis(&mut self) -> Result<(), Report> {
|
|
// Due to Bitcoin protocol limitations, we can't request the genesis
|
|
// block using our standard tip-following algorithm:
|
|
// - getblocks requires at least one hash
|
|
// - responses start with the block *after* the requested block, and
|
|
// - the genesis hash is used as a placeholder for "no matches".
|
|
//
|
|
// So we just download and verify the genesis block here.
|
|
while !self.state_contains(self.genesis_hash).await? {
|
|
self.downloads
|
|
.download_and_verify(self.genesis_hash)
|
|
.await
|
|
.map_err(|e| eyre!(e))?;
|
|
match self.downloads.next().await.expect("downloads is nonempty") {
|
|
Ok(hash) => tracing::trace!(?hash, "verified and committed block to state"),
|
|
Err(e) => {
|
|
tracing::warn!(?e, "could not download or verify genesis block, retrying")
|
|
}
|
|
}
|
|
}
|
|
|
|
Ok(())
|
|
}
|
|
|
|
/// Queue download and verify tasks for each block that isn't currently known to our node
|
|
async fn request_blocks(&mut self, hashes: HashSet<block::Hash>) -> Result<(), Report> {
|
|
tracing::debug!(hashes.len = hashes.len(), "requesting blocks");
|
|
for hash in hashes.into_iter() {
|
|
self.downloads
|
|
.download_and_verify(hash)
|
|
.await
|
|
.map_err(|e| eyre!(e))?;
|
|
}
|
|
|
|
Ok(())
|
|
}
|
|
|
|
/// Returns `true` if the hash is present in the state, and `false`
|
|
/// if the hash is not present in the state.
|
|
///
|
|
/// TODO: handle multiple tips in the state.
|
|
async fn state_contains(&mut self, hash: block::Hash) -> Result<bool, Report> {
|
|
match self
|
|
.state
|
|
.ready_and()
|
|
.await
|
|
.map_err(|e| eyre!(e))?
|
|
.call(zebra_state::Request::Depth(hash))
|
|
.await
|
|
.map_err(|e| eyre!(e))?
|
|
{
|
|
zs::Response::Depth(Some(_)) => Ok(true),
|
|
zs::Response::Depth(None) => Ok(false),
|
|
_ => unreachable!("wrong response to depth request"),
|
|
}
|
|
}
|
|
|
|
fn update_metrics(&self) {
|
|
metrics::gauge!(
|
|
"sync.prospective_tips.len",
|
|
self.prospective_tips.len() as i64
|
|
);
|
|
metrics::gauge!(
|
|
"sync.downloads.in_flight",
|
|
self.downloads.in_flight() as i64
|
|
);
|
|
}
|
|
}
|
|
|
|
#[cfg(test)]
|
|
mod test {
|
|
use super::*;
|
|
|
|
/// Make sure the timeout values are consistent with each other.
|
|
#[test]
|
|
fn ensure_timeouts_consistent() {
|
|
let max_download_retry_time =
|
|
BLOCK_DOWNLOAD_TIMEOUT.as_secs() * (BLOCK_DOWNLOAD_RETRY_LIMIT as u64);
|
|
assert!(
|
|
max_download_retry_time < BLOCK_VERIFY_TIMEOUT.as_secs(),
|
|
"Verify timeout should allow for previous block download retries"
|
|
);
|
|
assert!(
|
|
BLOCK_DOWNLOAD_TIMEOUT.as_secs() * 2 < SYNC_RESTART_TIMEOUT.as_secs(),
|
|
"Sync restart should allow for pending and buffered requests to complete"
|
|
);
|
|
|
|
assert!(
|
|
SYNC_RESTART_TIMEOUT < BLOCK_VERIFY_TIMEOUT,
|
|
"Verify timeout should allow for a sync restart"
|
|
);
|
|
}
|
|
}
|