ozzy-replication 0.1.0

//! Quorum-supported timeout suspicion, independent of durable election evidence.

use super::{Action, DriverError, ReplicaDriver, Role};
use crate::wire::Control;
use crate::{Scope, Status, ViewChangeError};

impl ReplicaDriver {
    pub(super) fn receive_exit(&mut self, voter: usize, scope: Scope) -> Result<(), DriverError> {
        match self.role.as_ref().expect("owned role") {
            Role::Normal(normal) if normal.snapshot().status == Status::Faulted => {
                return Err(ViewChangeError::Faulted.into());
            }
            Role::ViewChanging(changing)
                if changing.normal_snapshot().status == Status::Faulted =>
            {
                return Err(ViewChangeError::Faulted.into());
            }
            Role::Installing(installing) => installing.require_pending()?,
            _ => {}
        }
        // Neither a distant wish nor a duplicate from another view is evidence
        // that anyone has entered that view. START_VIEW_CHANGE supplies that fence.
        if scope.view < self.scope().view {
            // This voter left that view, possibly on the sender's own request
            // plus a local timeout, before its suspicion went out. Without
            // support the sender cannot leave until this voter's durable
            // promise lets START_VIEW_CHANGE reach it. After that promise,
            // election and normal traffic carry the newer view themselves.
            //
            // A peer that also left the view cannot tell this answer from a
            // request. Answer only the sender, and each sender at most once
            // per retransmission interval, so two such voters fall silent.
            if self.promise_pending() && self.now >= self.exit_echo_at[voter] {
                self.exit_echo_at[voter] = self.now + self.timing.retransmit;
                let asked = match self.exit_echo {
                    Some((old, asked)) if old == scope => asked,
                    _ => 0,
                };
                self.exit_echo = Some((scope, asked | 1 << voter));
            }
            return Ok(());
        }
        if scope != self.scope() {
            return Ok(());
        }
        if let Some(normal) = self.normal() {
            let snapshot = normal.snapshot();
            if !self.normal_timed_out(&snapshot) {
                self.withdraw_exit();
            }
        }
        self.exit_votes |= 1 << voter;
        self.advance_if_exit_quorum()
    }

    pub(super) fn request_exit(&mut self) -> Result<(), DriverError> {
        self.exit_retry_at.get_or_insert(self.now);
        self.exit_votes |= 1 << self.configuration.voter_index(self.local)?;
        self.advance_if_exit_quorum()
    }

    fn advance_if_exit_quorum(&mut self) -> Result<(), DriverError> {
        if self.exit_votes.count_ones() >= 2 {
            if self
                .normal()
                .is_none_or(|normal| !normal.snapshot().ready_for_appends)
            {
                self.election_delay = self
                    .election_delay
                    .saturating_mul(2)
                    .min(self.timing.max_election_timeout);
            }
            self.next_view()?;
        }
        Ok(())
    }

    pub(super) fn withdraw_exit(&mut self) {
        let local = self
            .configuration
            .voter_index(self.local)
            .expect("configured local voter");
        self.exit_votes &= !(1 << local);
        self.exit_retry_at = None;
    }

    fn promise_pending(&self) -> bool {
        matches!(
            self.role.as_ref().expect("owned role"),
            Role::ViewChanging(changing) if changing.promised_view() != changing.scope().view
        )
    }

    /// Support one sender's exit from an older view per call. The sender
    /// retransmits, so a lost answer is repeated after that interval.
    pub(super) fn exit_echo_action(&mut self) -> Option<Action> {
        let (scope, asked) = self.exit_echo.take()?;
        let voter = asked.trailing_zeros() as usize;
        let remaining = asked & !(1 << voter);
        if remaining != 0 {
            self.exit_echo = Some((scope, remaining));
        }
        Some(Action::Send {
            to: self.configuration.voters()[voter],
            message: Control::ExitView(scope),
        })
    }

    pub(super) fn exit_action(&mut self) -> Option<Action> {
        if self.now < self.exit_retry_at? {
            return None;
        }
        self.exit_retry_at = Some(self.now + self.timing.retransmit);
        Some(Action::Broadcast(Control::ExitView(self.scope())))
    }
}