reth_snap_sync/

session.rs

//! Drives one snap synchronization attempt: what it targets, and when it stops.

use crate::{SnapAttemptStore, SnapGeneration, SnapPivotPolicy, SnapSyncError, SnapWrite};
use reth_storage_api::{BlockHashReader, HeaderProvider, MetadataProvider, MetadataWriter};

/// One snap synchronization attempt, from the pivot it targets to the work it owns.
///
/// Targets come from the local chain, never from a peer: peers only supply state, which is
/// authenticated against the target's state root.
#[derive(Debug)]
pub struct SnapSyncSession {
    // Decides which blocks are eligible targets.
    policy: SnapPivotPolicy,
    // How far the attempt has got.
    state: SnapSyncSessionState,
}

impl SnapSyncSession {
    /// Creates a session waiting for its first eligible target.
    pub const fn new(policy: SnapPivotPolicy) -> Self {
        Self { policy, state: SnapSyncSessionState::Waiting }
    }

    /// What the session is doing.
    pub const fn state(&self) -> &SnapSyncSessionState {
        &self.state
    }

    /// Pivot this session is anchored to, if it has one.
    pub const fn target(&self) -> Option<&SnapGeneration> {
        self.state.target()
    }

    /// Selects a target under `head`, or waits while no block is eligible.
    ///
    /// A target no work has taken yet is replaced by a newer eligible one and dropped when none is
    /// eligible, since nothing authenticates against its root so far. Moving a target that work
    /// has taken is instead pivot advancement, which has to carry the downloaded state forward.
    pub fn select(
        &mut self,
        provider: &impl HeaderProvider,
        head: u64,
        finalized: Option<u64>,
    ) -> Result<&SnapSyncSessionState, SnapSyncError> {
        if matches!(self.state, SnapSyncSessionState::Waiting | SnapSyncSessionState::Selected(_)) {
            self.state = match self.policy.select(provider, head, finalized)? {
                Some(generation) => SnapSyncSessionState::Selected(generation),
                None => SnapSyncSessionState::Waiting,
            };
        }
        Ok(&self.state)
    }

    /// Hands the selected target to the work downloading against it.
    ///
    /// Taking the target is what starts it, so only the first caller gets one: a target already
    /// being downloaded has an owner, and a waiting session has nothing to hand out.
    pub const fn start(&mut self) -> Option<SnapGeneration> {
        let SnapSyncSessionState::Selected(generation) = self.state else { return None };
        self.state = SnapSyncSessionState::Downloading(generation);
        Some(generation)
    }

    /// Takes over the target of an attempt a previous run recorded, as [`Self::start`] does for a
    /// selected one.
    ///
    /// Only a session that has not handed out a target can take one over, so this returns whether
    /// it did.
    pub const fn resume(&mut self, generation: SnapGeneration) -> bool {
        if !matches!(self.state, SnapSyncSessionState::Waiting | SnapSyncSessionState::Selected(_))
        {
            return false
        }
        self.state = SnapSyncSessionState::Downloading(generation);
        true
    }

    /// Re-anchors the downloaded target once it lags too far behind `head`, returning the new
    /// write.
    ///
    /// The target is re-read from the attempt record first, so an advance whose transaction was
    /// dropped is retried rather than assumed.
    pub fn advance<P>(
        &mut self,
        provider: &P,
        write: SnapWrite,
        head: u64,
        finalized: Option<u64>,
    ) -> Result<Option<SnapWrite>, SnapSyncError>
    where
        P: HeaderProvider + BlockHashReader + MetadataProvider + MetadataWriter,
    {
        let SnapSyncSessionState::Downloading(mut current) = self.state else { return Ok(None) };
        let attempt = provider.authorize_snap_write(write)?;
        if attempt.pivot() != current.target() {
            current = SnapGeneration::new(attempt.pivot(), attempt.state_root());
            self.state = SnapSyncSessionState::Downloading(current);
        }
        if !self.policy.needs_advance(current, head) {
            return Ok(None)
        }
        let Some(next) = self.policy.select(provider, head, finalized)? else { return Ok(None) };
        // A policy anchoring further back than it re-anchors can select a block behind the pivot.
        if next.target().number <= current.target().number {
            return Ok(None)
        }
        let write = provider.advance_snap_pivot(write, next)?;
        self.state = SnapSyncSessionState::Downloading(next);
        Ok(Some(write))
    }
}

/// What a [`SnapSyncSession`] is doing.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum SnapSyncSessionState {
    /// No block is eligible yet, so the session holds no target.
    Waiting,
    /// A target is selected, but no work has taken it yet.
    Selected(SnapGeneration),
    /// Work is outstanding against the target.
    Downloading(SnapGeneration),
}

impl SnapSyncSessionState {
    /// Target of this state, if it has one.
    pub const fn target(&self) -> Option<&SnapGeneration> {
        match self {
            Self::Selected(generation) | Self::Downloading(generation) => Some(generation),
            Self::Waiting => None,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::test_utils::{chain, hashed_factory, policy, provider_with};
    use reth_primitives_traits::SealedHeader;
    use reth_provider::{
        test_utils::{insert_headers, MockNodeTypesWithDB},
        DBProvider, DatabaseProviderFactory, ProviderFactory,
    };

    fn session() -> SnapSyncSession {
        SnapSyncSession::new(policy())
    }

    // A session downloading block 1 under head 2, re-anchoring once its pivot lags by two blocks,
    // with the attempt recorded in a database holding blocks 0 through 3.
    fn downloading() -> (ProviderFactory<MockNodeTypesWithDB>, SnapSyncSession, SnapWrite) {
        let factory = hashed_factory();
        let headers: Vec<_> = chain(Some(0)).into_iter().map(SealedHeader::seal_slow).collect();
        insert_headers(&factory, &headers);
        let provider = factory.database_provider_rw().unwrap();
        let mut session = SnapSyncSession::new(policy().with_advance_after(1));
        session.select(&provider, 2, None).unwrap();
        let generation = session.start().unwrap();
        let write = provider.start_snap_attempt(generation).unwrap();
        provider.commit().unwrap();
        (factory, session, write)
    }

    #[test]
    fn a_target_no_work_took_is_replaced_as_the_head_advances() {
        let provider = provider_with(chain(Some(0)));
        let mut session = session();

        session.select(&provider, 2, None).unwrap();
        assert_eq!(session.target().unwrap().target().number, 1);

        session.select(&provider, 3, None).unwrap();
        assert_eq!(session.target().unwrap().target().number, 2);
    }

    #[test]
    fn a_target_no_work_took_is_dropped_once_it_is_no_longer_eligible() {
        let provider = provider_with(chain(Some(0)));
        let mut session = session();
        session.select(&provider, 3, None).unwrap();

        // The head has run past the downloaded headers, so no candidate confirms the old target.
        session.select(&provider, 9, None).unwrap();

        assert_eq!(session.state(), &SnapSyncSessionState::Waiting);
        assert!(session.start().is_none());
    }

    #[test]
    fn a_target_work_took_is_kept() {
        let provider = provider_with(chain(Some(0)));
        let mut session = session();
        session.select(&provider, 2, None).unwrap();
        let started = session.start().unwrap();

        session.select(&provider, 3, None).unwrap();

        assert_eq!(session.state(), &SnapSyncSessionState::Downloading(started));
    }

    #[test]
    fn a_recorded_target_is_resumed_once() {
        let (factory, _, write) = downloading();
        let provider = factory.database_provider_rw().unwrap();
        let attempt = provider.authorize_snap_write(write).unwrap();
        let recorded = SnapGeneration::new(attempt.pivot(), attempt.state_root());
        let mut session = SnapSyncSession::new(policy().with_advance_after(1));

        assert!(session.resume(recorded));
        assert!(!session.resume(recorded));
        assert!(session.start().is_none());
        assert_eq!(session.state(), &SnapSyncSessionState::Downloading(recorded));
        // A resumed target advances like a started one.
        assert!(session.advance(&provider, write, 3, None).unwrap().is_some());
        assert_eq!(session.target().unwrap().target().number, 2);
    }

    #[test]
    fn a_lagging_target_advances_with_the_attempt() {
        let (factory, mut session, write) = downloading();
        let provider = factory.database_provider_rw().unwrap();

        let advanced = session.advance(&provider, write, 3, None).unwrap().unwrap();

        let target = *session.target().unwrap();
        assert_eq!(target.target().number, 2);
        assert!(matches!(session.state(), SnapSyncSessionState::Downloading(_)));
        let attempt = provider.authorize_snap_write(advanced).unwrap();
        assert_eq!((attempt.pivot(), attempt.state_root()), (target.target(), target.state_root()));
        assert!(matches!(
            provider.authorize_snap_write(write),
            Err(SnapSyncError::StaleWrite { .. })
        ));
    }

    #[test]
    fn a_target_within_the_window_is_kept() {
        let (factory, mut session, write) = downloading();
        let provider = factory.database_provider_rw().unwrap();
        let before = *session.target().unwrap();

        assert_eq!(session.advance(&provider, write, 2, None).unwrap(), None);

        assert_eq!(session.target(), Some(&before));
        provider.authorize_snap_write(write).unwrap();
    }

    #[test]
    fn a_refused_advance_keeps_the_target() {
        let (factory, mut session, write) = downloading();
        let provider = factory.database_provider_rw().unwrap();
        let before = *session.target().unwrap();
        provider.abandon_snap_attempt().unwrap();

        let refused = session.advance(&provider, write, 3, None);

        assert!(matches!(refused, Err(SnapSyncError::StaleWrite { .. })));
        assert_eq!(session.state(), &SnapSyncSessionState::Downloading(before));
    }

    #[test]
    fn a_rolled_back_advance_is_retried() {
        let (factory, mut session, write) = downloading();
        let provider = factory.database_provider_rw().unwrap();
        session.advance(&provider, write, 3, None).unwrap().unwrap();
        drop(provider);

        let provider = factory.database_provider_rw().unwrap();
        let advanced = session.advance(&provider, write, 3, None).unwrap().unwrap();

        assert_eq!(session.target().unwrap().target().number, 2);
        assert_eq!(provider.authorize_snap_write(advanced).unwrap().pivot().number, 2);
    }

    #[test]
    fn a_target_no_work_took_is_not_advanced() {
        let (factory, _, write) = downloading();
        let provider = factory.database_provider_rw().unwrap();
        let mut session = SnapSyncSession::new(policy().with_advance_after(1));
        session.select(&provider, 2, None).unwrap();

        assert_eq!(session.advance(&provider, write, 3, None).unwrap(), None);
        assert_eq!(session.target().unwrap().target().number, 1);
    }
}