zebrad/components/sync/downloads.rs
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//! A download stream for Zebra's block syncer.
use std::{
collections::HashMap,
convert,
pin::Pin,
sync::Arc,
task::{Context, Poll},
};
use futures::{
future::{FutureExt, TryFutureExt},
ready,
stream::{FuturesUnordered, Stream},
};
use pin_project::pin_project;
use thiserror::Error;
use tokio::{
sync::{oneshot, watch},
task::JoinHandle,
time::timeout,
};
use tower::{hedge, Service, ServiceExt};
use tracing_futures::Instrument;
use zebra_chain::{
block::{self, Height, HeightDiff},
chain_tip::ChainTip,
};
use zebra_network as zn;
use zebra_state as zs;
use crate::components::sync::{
FINAL_CHECKPOINT_BLOCK_VERIFY_TIMEOUT, FINAL_CHECKPOINT_BLOCK_VERIFY_TIMEOUT_LIMIT,
};
type BoxError = Box<dyn std::error::Error + Send + Sync + 'static>;
/// A multiplier used to calculate the extra number of blocks we allow in the
/// verifier, state, and block commit pipelines, on top of the lookahead limit.
///
/// The extra number of blocks is calculated using
/// `lookahead_limit * VERIFICATION_PIPELINE_SCALING_MULTIPLIER`.
///
/// This allows the verifier and state queues, and the block commit channel,
/// to hold a few extra tips responses worth of blocks,
/// even if the syncer queue is full. Any unused capacity is shared between both queues.
///
/// If this capacity is exceeded, the downloader will tell the syncer to pause new downloads.
///
/// Since the syncer queue is limited to the `lookahead_limit`,
/// the rest of the capacity is reserved for the other queues.
/// There is no reserved capacity for the syncer queue:
/// if the other queues stay full, the syncer will eventually time out and reset.
pub const VERIFICATION_PIPELINE_SCALING_MULTIPLIER: usize = 2;
/// The maximum height difference between Zebra's state tip and a downloaded block.
/// Blocks higher than this will get dropped and return an error.
pub const VERIFICATION_PIPELINE_DROP_LIMIT: HeightDiff = 50_000;
#[derive(Copy, Clone, Debug)]
pub(super) struct AlwaysHedge;
impl<Request: Clone> hedge::Policy<Request> for AlwaysHedge {
fn can_retry(&self, _req: &Request) -> bool {
true
}
fn clone_request(&self, req: &Request) -> Option<Request> {
Some(req.clone())
}
}
/// Errors that can occur while downloading and verifying a block.
#[derive(Error, Debug)]
#[allow(dead_code)]
pub enum BlockDownloadVerifyError {
#[error("permanent readiness error from the network service: {error:?}")]
NetworkServiceError {
#[source]
error: BoxError,
},
#[error("permanent readiness error from the verifier service: {error:?}")]
VerifierServiceError {
#[source]
error: BoxError,
},
#[error("duplicate block hash queued for download: {hash:?}")]
DuplicateBlockQueuedForDownload { hash: block::Hash },
#[error("error downloading block: {error:?} {hash:?}")]
DownloadFailed {
#[source]
error: BoxError,
hash: block::Hash,
},
/// A downloaded block was a long way ahead of the state chain tip.
/// This error should be very rare during normal operation.
///
/// We need to reset the syncer on this error, to allow the verifier and state to catch up,
/// or prevent it following a bad chain.
///
/// If we don't reset the syncer on this error, it will continue downloading blocks from a bad
/// chain, or blocks far ahead of the current state tip.
#[error("downloaded block was too far ahead of the chain tip: {height:?} {hash:?}")]
AboveLookaheadHeightLimit {
height: block::Height,
hash: block::Hash,
},
#[error("downloaded block was too far behind the chain tip: {height:?} {hash:?}")]
BehindTipHeightLimit {
height: block::Height,
hash: block::Hash,
},
#[error("downloaded block had an invalid height: {hash:?}")]
InvalidHeight { hash: block::Hash },
#[error("block failed consensus validation: {error:?} {height:?} {hash:?}")]
Invalid {
#[source]
error: zebra_consensus::router::RouterError,
height: block::Height,
hash: block::Hash,
},
#[error("block validation request failed: {error:?} {height:?} {hash:?}")]
ValidationRequestError {
#[source]
error: BoxError,
height: block::Height,
hash: block::Hash,
},
#[error("block download & verification was cancelled during download: {hash:?}")]
CancelledDuringDownload { hash: block::Hash },
#[error(
"block download & verification was cancelled while waiting for the verifier service: \
to become ready: {height:?} {hash:?}"
)]
CancelledAwaitingVerifierReadiness {
height: block::Height,
hash: block::Hash,
},
#[error(
"block download & verification was cancelled during verification: {height:?} {hash:?}"
)]
CancelledDuringVerification {
height: block::Height,
hash: block::Hash,
},
#[error(
"timeout during service readiness, download, verification, or internal downloader operation"
)]
Timeout,
}
impl From<tokio::time::error::Elapsed> for BlockDownloadVerifyError {
fn from(_value: tokio::time::error::Elapsed) -> Self {
BlockDownloadVerifyError::Timeout
}
}
/// Represents a [`Stream`] of download and verification tasks during chain sync.
#[pin_project]
#[derive(Debug)]
pub struct Downloads<ZN, ZV, ZSTip>
where
ZN: Service<zn::Request, Response = zn::Response, Error = BoxError> + Send + Sync + 'static,
ZN::Future: Send,
ZV: Service<zebra_consensus::Request, Response = block::Hash, Error = BoxError>
+ Send
+ Sync
+ Clone
+ 'static,
ZV::Future: Send,
ZSTip: ChainTip + Clone + Send + 'static,
{
// Services
//
/// A service that forwards requests to connected peers, and returns their
/// responses.
network: ZN,
/// A service that verifies downloaded blocks.
verifier: ZV,
/// Allows efficient access to the best tip of the blockchain.
latest_chain_tip: ZSTip,
// Configuration
//
/// The configured lookahead limit, after applying the minimum limit.
lookahead_limit: usize,
/// The largest block height for the checkpoint verifier, based on the current config.
max_checkpoint_height: Height,
// Shared syncer state
//
/// Sender that is set to `true` when the downloader is past the lookahead limit.
/// This is based on the downloaded block height and the state tip height.
past_lookahead_limit_sender: Arc<std::sync::Mutex<watch::Sender<bool>>>,
/// Receiver for `past_lookahead_limit_sender`, which is used to avoid accessing the mutex.
past_lookahead_limit_receiver: zs::WatchReceiver<bool>,
// Internal downloads state
//
/// A list of pending block download and verify tasks.
#[pin]
pending: FuturesUnordered<
JoinHandle<Result<(Height, block::Hash), (BlockDownloadVerifyError, block::Hash)>>,
>,
/// A list of channels that can be used to cancel pending block download and
/// verify tasks.
cancel_handles: HashMap<block::Hash, oneshot::Sender<()>>,
}
impl<ZN, ZV, ZSTip> Stream for Downloads<ZN, ZV, ZSTip>
where
ZN: Service<zn::Request, Response = zn::Response, Error = BoxError> + Send + Sync + 'static,
ZN::Future: Send,
ZV: Service<zebra_consensus::Request, Response = block::Hash, Error = BoxError>
+ Send
+ Sync
+ Clone
+ 'static,
ZV::Future: Send,
ZSTip: ChainTip + Clone + Send + 'static,
{
type Item = Result<(Height, block::Hash), BlockDownloadVerifyError>;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>> {
let this = self.project();
// CORRECTNESS
//
// The current task must be scheduled for wakeup every time we return
// `Poll::Pending`.
//
// If no download and verify tasks have exited since the last poll, this
// task is scheduled for wakeup when the next task becomes ready.
//
// TODO: this would be cleaner with poll_map (#2693)
if let Some(join_result) = ready!(this.pending.poll_next(cx)) {
match join_result.expect("block download and verify tasks must not panic") {
Ok((height, hash)) => {
this.cancel_handles.remove(&hash);
Poll::Ready(Some(Ok((height, hash))))
}
Err((e, hash)) => {
this.cancel_handles.remove(&hash);
Poll::Ready(Some(Err(e)))
}
}
} else {
Poll::Ready(None)
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
self.pending.size_hint()
}
}
impl<ZN, ZV, ZSTip> Downloads<ZN, ZV, ZSTip>
where
ZN: Service<zn::Request, Response = zn::Response, Error = BoxError> + Send + Sync + 'static,
ZN::Future: Send,
ZV: Service<zebra_consensus::Request, Response = block::Hash, Error = BoxError>
+ Send
+ Sync
+ Clone
+ 'static,
ZV::Future: Send,
ZSTip: ChainTip + Clone + Send + 'static,
{
/// Initialize a new download stream with the provided `network` and
/// `verifier` services.
///
/// Uses the `latest_chain_tip` and `lookahead_limit` to drop blocks
/// that are too far ahead of the current state tip.
/// Uses `max_checkpoint_height` to work around a known block timeout (#5125).
///
/// The [`Downloads`] stream is agnostic to the network policy, so retry and
/// timeout limits should be applied to the `network` service passed into
/// this constructor.
pub fn new(
network: ZN,
verifier: ZV,
latest_chain_tip: ZSTip,
past_lookahead_limit_sender: watch::Sender<bool>,
lookahead_limit: usize,
max_checkpoint_height: Height,
) -> Self {
let past_lookahead_limit_receiver =
zs::WatchReceiver::new(past_lookahead_limit_sender.subscribe());
Self {
network,
verifier,
latest_chain_tip,
lookahead_limit,
max_checkpoint_height,
past_lookahead_limit_sender: Arc::new(std::sync::Mutex::new(
past_lookahead_limit_sender,
)),
past_lookahead_limit_receiver,
pending: FuturesUnordered::new(),
cancel_handles: HashMap::new(),
}
}
/// Queue a block for download and verification.
///
/// This method waits for the network to become ready, and returns an error
/// only if the network service fails. It returns immediately after queuing
/// the request.
#[instrument(level = "debug", skip(self), fields(%hash))]
pub async fn download_and_verify(
&mut self,
hash: block::Hash,
) -> Result<(), BlockDownloadVerifyError> {
if self.cancel_handles.contains_key(&hash) {
metrics::counter!("sync.already.queued.dropped.block.hash.count").increment(1);
return Err(BlockDownloadVerifyError::DuplicateBlockQueuedForDownload { hash });
}
// We construct the block requests sequentially, waiting for the peer
// set to be ready to process each request. This ensures that we start
// block downloads in the order we want them (though they may resolve
// out of order), and it means that we respect backpressure. Otherwise,
// if we waited for readiness and did the service call in the spawned
// tasks, all of the spawned tasks would race each other waiting for the
// network to become ready.
let block_req = self
.network
.ready()
.await
.map_err(|error| BlockDownloadVerifyError::NetworkServiceError { error })?
.call(zn::Request::BlocksByHash(std::iter::once(hash).collect()));
// This oneshot is used to signal cancellation to the download task.
let (cancel_tx, mut cancel_rx) = oneshot::channel::<()>();
let mut verifier = self.verifier.clone();
let latest_chain_tip = self.latest_chain_tip.clone();
let lookahead_limit = self.lookahead_limit;
let max_checkpoint_height = self.max_checkpoint_height;
let past_lookahead_limit_sender = self.past_lookahead_limit_sender.clone();
let past_lookahead_limit_receiver = self.past_lookahead_limit_receiver.clone();
let task = tokio::spawn(
async move {
// Download the block.
// Prefer the cancel handle if both are ready.
let rsp = tokio::select! {
biased;
_ = &mut cancel_rx => {
trace!("task cancelled prior to download completion");
metrics::counter!("sync.cancelled.download.count").increment(1);
return Err(BlockDownloadVerifyError::CancelledDuringDownload { hash })
}
rsp = block_req => rsp.map_err(|error| BlockDownloadVerifyError::DownloadFailed { error, hash})?,
};
let block = if let zn::Response::Blocks(blocks) = rsp {
assert_eq!(
blocks.len(),
1,
"wrong number of blocks in response to a single hash"
);
blocks
.first()
.expect("just checked length")
.available()
.expect("unexpected missing block status: single block failures should be errors")
} else {
unreachable!("wrong response to block request");
};
metrics::counter!("sync.downloaded.block.count").increment(1);
// Security & Performance: reject blocks that are too far ahead of our tip.
// Avoids denial of service attacks, and reduces wasted work on high blocks
// that will timeout before being verified.
let tip_height = latest_chain_tip.best_tip_height();
let (lookahead_drop_height, lookahead_pause_height, lookahead_reset_height) = if let Some(tip_height) = tip_height {
// Scale the height limit with the lookahead limit,
// so users with low capacity or under DoS can reduce them both.
let lookahead_pause = HeightDiff::try_from(
lookahead_limit + lookahead_limit * VERIFICATION_PIPELINE_SCALING_MULTIPLIER,
)
.expect("fits in HeightDiff");
((tip_height + VERIFICATION_PIPELINE_DROP_LIMIT).expect("tip is much lower than Height::MAX"),
(tip_height + lookahead_pause).expect("tip is much lower than Height::MAX"),
(tip_height + lookahead_pause/2).expect("tip is much lower than Height::MAX"))
} else {
let genesis_drop = VERIFICATION_PIPELINE_DROP_LIMIT.try_into().expect("fits in u32");
let genesis_lookahead =
u32::try_from(lookahead_limit - 1).expect("fits in u32");
(block::Height(genesis_drop),
block::Height(genesis_lookahead),
block::Height(genesis_lookahead/2))
};
// Get the finalized tip height, assuming we're using the non-finalized state.
//
// It doesn't matter if we're a few blocks off here, because blocks this low
// are part of a fork with much less work. So they would be rejected anyway.
//
// And if we're still checkpointing, the checkpointer will reject blocks behind
// the finalized tip anyway.
//
// TODO: get the actual finalized tip height
let min_accepted_height = tip_height
.map(|tip_height| {
block::Height(tip_height.0.saturating_sub(zs::MAX_BLOCK_REORG_HEIGHT))
})
.unwrap_or(block::Height(0));
let block_height = if let Some(block_height) = block.coinbase_height() {
block_height
} else {
debug!(
?hash,
"synced block with no height: dropped downloaded block"
);
metrics::counter!("sync.no.height.dropped.block.count").increment(1);
return Err(BlockDownloadVerifyError::InvalidHeight { hash });
};
if block_height > lookahead_drop_height {
Err(BlockDownloadVerifyError::AboveLookaheadHeightLimit { height: block_height, hash })?;
} else if block_height > lookahead_pause_height {
// This log can be very verbose, usually hundreds of blocks are dropped.
// So we only log at info level for the first above-height block.
if !past_lookahead_limit_receiver.cloned_watch_data() {
info!(
?hash,
?block_height,
?tip_height,
?lookahead_pause_height,
?lookahead_reset_height,
lookahead_limit = ?lookahead_limit,
"synced block height too far ahead of the tip: \
waiting for downloaded blocks to commit to the state",
);
// Set the watched value to true, since we're over the limit.
//
// It is ok to block here, because we're going to pause new downloads anyway.
// But if Zebra is shutting down, ignore the send error.
let _ = past_lookahead_limit_sender.lock().expect("thread panicked while holding the past_lookahead_limit_sender mutex guard").send(true);
} else {
debug!(
?hash,
?block_height,
?tip_height,
?lookahead_pause_height,
?lookahead_reset_height,
lookahead_limit = ?lookahead_limit,
"synced block height too far ahead of the tip: \
waiting for downloaded blocks to commit to the state",
);
}
metrics::counter!("sync.max.height.limit.paused.count").increment(1);
} else if block_height <= lookahead_reset_height && past_lookahead_limit_receiver.cloned_watch_data() {
// Reset the watched value to false, since we're well under the limit.
// We need to block here, because if we don't the syncer can hang.
// But if Zebra is shutting down, ignore the send error.
let _ = past_lookahead_limit_sender.lock().expect("thread panicked while holding the past_lookahead_limit_sender mutex guard").send(false);
metrics::counter!("sync.max.height.limit.reset.count").increment(1);
metrics::counter!("sync.max.height.limit.reset.attempt.count").increment(1);
}
if block_height < min_accepted_height {
debug!(
?hash,
?block_height,
?tip_height,
?min_accepted_height,
behind_tip_limit = ?zs::MAX_BLOCK_REORG_HEIGHT,
"synced block height behind the finalized tip: dropped downloaded block"
);
metrics::counter!("gossip.min.height.limit.dropped.block.count").increment(1);
Err(BlockDownloadVerifyError::BehindTipHeightLimit { height: block_height, hash })?;
}
// Wait for the verifier service to be ready.
let readiness = verifier.ready();
// Prefer the cancel handle if both are ready.
let verifier = tokio::select! {
biased;
_ = &mut cancel_rx => {
trace!("task cancelled waiting for verifier service readiness");
metrics::counter!("sync.cancelled.verify.ready.count").increment(1);
return Err(BlockDownloadVerifyError::CancelledAwaitingVerifierReadiness { height: block_height, hash })
}
verifier = readiness => verifier,
};
// Verify the block.
let mut rsp = verifier
.map_err(|error| BlockDownloadVerifyError::VerifierServiceError { error })?
.call(zebra_consensus::Request::Commit(block)).boxed();
// Add a shorter timeout to workaround a known bug (#5125)
let short_timeout_max = (max_checkpoint_height + FINAL_CHECKPOINT_BLOCK_VERIFY_TIMEOUT_LIMIT).expect("checkpoint block height is in valid range");
if block_height >= max_checkpoint_height && block_height <= short_timeout_max {
rsp = timeout(FINAL_CHECKPOINT_BLOCK_VERIFY_TIMEOUT, rsp)
.map_err(|timeout| format!("initial fully verified block timed out: retrying: {timeout:?}").into())
.map(|nested_result| nested_result.and_then(convert::identity)).boxed();
}
let verification = tokio::select! {
biased;
_ = &mut cancel_rx => {
trace!("task cancelled prior to verification");
metrics::counter!("sync.cancelled.verify.count").increment(1);
return Err(BlockDownloadVerifyError::CancelledDuringVerification { height: block_height, hash })
}
verification = rsp => verification,
};
if verification.is_ok() {
metrics::counter!("sync.verified.block.count").increment(1);
}
verification
.map(|hash| (block_height, hash))
.map_err(|err| {
match err.downcast::<zebra_consensus::router::RouterError>() {
Ok(error) => BlockDownloadVerifyError::Invalid { error: *error, height: block_height, hash },
Err(error) => BlockDownloadVerifyError::ValidationRequestError { error, height: block_height, hash },
}
})
}
.in_current_span()
// Tack the hash onto the error so we can remove the cancel handle
// on failure as well as on success.
.map_err(move |e| (e, hash)),
);
// Try to start the spawned task before queueing the next block request
tokio::task::yield_now().await;
self.pending.push(task);
assert!(
self.cancel_handles.insert(hash, cancel_tx).is_none(),
"blocks are only queued once"
);
Ok(())
}
/// Cancel all running tasks and reset the downloader state.
pub fn cancel_all(&mut self) {
// Replace the pending task list with an empty one and drop it.
let _ = std::mem::take(&mut self.pending);
// Signal cancellation to all running tasks.
// Since we already dropped the JoinHandles above, they should
// fail silently.
for (_hash, cancel) in self.cancel_handles.drain() {
let _ = cancel.send(());
}
assert!(self.pending.is_empty());
assert!(self.cancel_handles.is_empty());
// Set the lookahead limit to false, since we're empty (so we're under the limit).
//
// It is ok to block here, because we're doing a reset and sleep anyway.
// But if Zebra is shutting down, ignore the send error.
let _ = self
.past_lookahead_limit_sender
.lock()
.expect("thread panicked while holding the past_lookahead_limit_sender mutex guard")
.send(false);
}
/// Get the number of currently in-flight download and verify tasks.
pub fn in_flight(&mut self) -> usize {
self.pending.len()
}
/// Returns true if there are no in-flight download and verify tasks.
#[allow(dead_code)]
pub fn is_empty(&mut self) -> bool {
self.pending.is_empty()
}
}