nordisk-pairing 0.5.27

//! **The pairing loop a box runs itself.** A pure state machine: the box
//! asks it "what now?", does that one thing through its own doors (the
//! tunnel driver, the join client, gunnar's control plane), reports what
//! happened, and asks again. Every step is idempotent and retried by the box,
//! for as long as it takes — the laptop is not in the loop.
//!
//! ```text
//! Booted ──join (utility net)──▶ Joined ──tunnel up (assigned address)──▶ RoleKnown ──┬─ primary: admin present ──▶ Paired
//!                                                                                  └─ twin: BecomeTwin → certify → follows ──▶ Paired
//! ```
//!
//! The ORDER is the medium-wide seed's (owner 2026-09-24): a box knows its
//! tunnel address only once the door has told it, so it joins over the
//! utility network first, raises the tunnel second, and pairs inside it.
//! A join answered without a role yet (the election waits for the other box)
//! is a transient: the box stays `Booted` and asks again.
//!
//! A tunnel that goes down sends the machine back to `Joined` (the door
//! already knows it; only the tunnel is raised again); a join that is
//! refused BY NAME as spent or forged is terminal (`Refused`), because no
//! retry changes a spent token — the operator has to re-seed.

use crate::Role;
use serde::{Deserialize, Serialize};
use std::net::Ipv4Addr;

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum State {
    Booted,
    /// The door admitted this box and named its role; the tunnel is not up yet.
    Joined { role: Role, primary: Option<Ipv4Addr>, all_joined: bool },
    RoleKnown { role: Role, primary: Option<Ipv4Addr> },
    /// Twin only: the role is conferred on the primary's side.
    TwinConferred { primary: Ipv4Addr },
    /// Twin only: it holds a certificate signed by the primary's issuer.
    TwinCertified { primary: Ipv4Addr },
    Paired { role: Role },
    /// A refusal no retry can change (a spent or forged join).
    Refused { why: String },
}

/// What the box should do now.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Step {
    BringTunnelUp,
    Join,
    /// The front's own pairing is the join: once it has admitted every peer it is paired.
    FrontAwaitPeers,
    /// The primary has the admin pre-enrolled from the seed; it only checks
    /// that this is so, then waits for its twin.
    PrimaryCheckAdmin,
    TwinBecome { primary: Ipv4Addr },
    TwinCertify { primary: Ipv4Addr },
    TwinFollow { primary: Ipv4Addr },
    Nothing,
}

impl Step {
    /// The step in plain words, for the signal.
    pub fn name(&self) -> &'static str {
        match self {
            Step::BringTunnelUp => "tunnel-up",
            Step::Join => "join",
            Step::FrontAwaitPeers => "await-peers",
            Step::PrimaryCheckAdmin => "check-admin",
            Step::TwinBecome { .. } => "become-twin",
            Step::TwinCertify { .. } => "certify",
            Step::TwinFollow { .. } => "follow",
            Step::Nothing => "nothing",
        }
    }
}

/// What happened when the box did the step.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Event {
    TunnelUp,
    TunnelDown,
    Joined { role: Role, primary: Option<Ipv4Addr>, peers_admitted: u8, peers_expected: u8 },
    JoinRefusedForGood(String),
    AdminPresent,
    TwinConferred,
    TwinCertified,
    /// The twin has applied the primary's head (the primary's heartbeat says so).
    Following { applied: u64, head: u64 },
    /// A transient failure of the step; the machine stays where it is and the
    /// box tries again after its backoff.
    Transient(String),
}

/// The first retry of a step that waits on a peer.
pub const PEER_WAIT_FIRST_S: u64 = 2;
/// Every later retry: the cap while a peer is expected.
pub const PEER_WAIT_CAP_S: u64 = 3;

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Machine {
    pub state: State,
    /// Transient failures of the CURRENT step, for the backoff and the signal.
    pub tries: u32,
    pub last_transient: Option<String>,
}

impl Default for Machine {
    fn default() -> Self {
        Machine { state: State::Booted, tries: 0, last_transient: None }
    }
}

impl Machine {
    pub fn next(&self) -> Step {
        match &self.state {
            State::Booted => Step::Join,
            State::Joined { .. } => Step::BringTunnelUp,
            State::RoleKnown { role: Role::Front, .. } => Step::FrontAwaitPeers,
            State::RoleKnown { role: Role::Primary, .. } => Step::PrimaryCheckAdmin,
            State::RoleKnown { role: Role::Twin, primary: Some(p) } => Step::TwinBecome { primary: *p },
            State::RoleKnown { role: Role::Twin, primary: None } => Step::Join,
            State::TwinConferred { primary } => Step::TwinCertify { primary: *primary },
            State::TwinCertified { primary } => Step::TwinFollow { primary: *primary },
            State::Paired { .. } | State::Refused { .. } => Step::Nothing,
        }
    }

    /// Backoff for the current step's next try: 2 s, then 3 s for as long as it
    /// takes. Every step before PAIRED waits on a PEER (the election, the
    /// primary's listener, the front's certificate, the first applied entry),
    /// and a peer that is late is late by the seconds its own boot takes — not
    /// by a minute. MEASURED 2026-09-26 on mock-upcloud-kvm: with 5, 10, 20,
    /// 40, 60 s the twin reached PAIRED 35-60 s after its gunnar started while
    /// its last wait was only for the primary's next heartbeat. Each try is one
    /// local call over the overlay; a try every 3 s costs nothing.
    pub fn backoff_s(&self) -> u64 {
        if self.tries <= 1 {
            PEER_WAIT_FIRST_S
        } else {
            PEER_WAIT_CAP_S
        }
    }

    pub fn apply(&mut self, e: Event) {
        let next = match (&self.state, e) {
            (_, Event::JoinRefusedForGood(why)) => State::Refused { why },
            // The door already knows this box: only the tunnel is raised again.
            (State::RoleKnown { role, primary }, Event::TunnelDown) => State::Joined { role: *role, primary: *primary, all_joined: false },
            (State::TwinConferred { primary } | State::TwinCertified { primary }, Event::TunnelDown) => {
                State::Joined { role: Role::Twin, primary: Some(*primary), all_joined: false }
            }
            (State::Booted, Event::Joined { role, primary, peers_admitted, peers_expected }) => {
                State::Joined { role, primary, all_joined: peers_admitted >= peers_expected }
            }
            (State::Joined { role, primary, all_joined }, Event::TunnelUp) => {
                if *role == Role::Front && *all_joined {
                    State::Paired { role: Role::Front }
                } else {
                    State::RoleKnown { role: *role, primary: *primary }
                }
            }
            (State::RoleKnown { role: Role::Front, .. }, Event::Joined { role: Role::Front, peers_admitted, peers_expected, .. }) if peers_admitted >= peers_expected => {
                State::Paired { role: Role::Front }
            }
            (State::RoleKnown { role: Role::Twin, primary: None }, Event::Joined { role: Role::Twin, primary, .. }) => State::RoleKnown { role: Role::Twin, primary },
            (State::RoleKnown { role: Role::Primary, .. }, Event::AdminPresent) => State::Paired { role: Role::Primary },
            (State::RoleKnown { role: Role::Twin, primary: Some(p) }, Event::TwinConferred) => State::TwinConferred { primary: *p },
            (State::TwinConferred { primary }, Event::TwinCertified) => State::TwinCertified { primary: *primary },
            (State::TwinCertified { .. }, Event::Following { applied, head }) if head > 0 && applied == head => State::Paired { role: Role::Twin },
            (_, Event::Transient(why)) => {
                self.tries += 1;
                self.last_transient = Some(why);
                return;
            }
            // An event that does not belong to the current step is ignored —
            // a late answer to a step already done changes nothing.
            (s, _) => s.clone(),
        };
        if next != self.state {
            self.tries = 0;
            self.last_transient = None;
        }
        self.state = next;
    }

    pub fn paired(&self) -> bool {
        matches!(self.state, State::Paired { .. })
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::OVERLAY_BOX_A;

    fn joined(role: Role, primary: Option<std::net::Ipv4Addr>, admitted: u8) -> Event {
        Event::Joined { role, primary, peers_admitted: admitted, peers_expected: 3 }
    }

    #[test]
    fn a_twin_walks_join_tunnel_become_certify_follow_and_a_late_event_changes_nothing() {
        let mut m = Machine::default();
        assert_eq!(m.next(), Step::Join, "the door first: the tunnel address comes from it");
        m.apply(Event::Transient("admitted; the election has not spoken yet".into()));
        assert_eq!((m.next(), m.tries), (Step::Join, 1));
        m.apply(joined(Role::Twin, Some(OVERLAY_BOX_A), 2));
        assert_eq!(m.next(), Step::BringTunnelUp, "then the tunnel, with the assigned address");
        m.apply(Event::TunnelUp);
        assert_eq!(m.next(), Step::TwinBecome { primary: OVERLAY_BOX_A });
        m.apply(Event::Transient("primary not listening".into()));
        assert_eq!((m.tries, m.backoff_s()), (1, PEER_WAIT_FIRST_S));
        m.apply(Event::TwinConferred);
        assert_eq!((m.tries, m.next()), (0, Step::TwinCertify { primary: OVERLAY_BOX_A }));
        m.apply(Event::TwinCertified);
        assert_eq!(m.next(), Step::TwinFollow { primary: OVERLAY_BOX_A });
        m.apply(Event::Following { applied: 10, head: 12 });
        assert!(!m.paired(), "behind is not paired");
        m.apply(Event::Following { applied: 12, head: 12 });
        assert!(m.paired());
        m.apply(Event::TwinConferred);
        assert!(m.paired(), "a late event is ignored");
        assert_eq!(m.next(), Step::Nothing);
    }

    #[test]
    fn a_primary_is_paired_once_its_admin_is_there_and_the_front_once_every_peer_joined() {
        let mut p = Machine::default();
        p.apply(joined(Role::Primary, None, 1));
        p.apply(Event::TunnelUp);
        assert_eq!(p.next(), Step::PrimaryCheckAdmin);
        p.apply(Event::AdminPresent);
        assert!(p.paired());

        let mut f = Machine::default();
        f.apply(joined(Role::Front, None, 1));
        f.apply(Event::TunnelUp);
        assert_eq!(f.next(), Step::FrontAwaitPeers);
        f.apply(joined(Role::Front, None, 3));
        assert!(f.paired());

        let mut late = Machine::default();
        late.apply(joined(Role::Front, None, 3));
        late.apply(Event::TunnelUp);
        assert!(late.paired(), "a front that joins last is paired as its tunnel comes up");
    }

    #[test]
    fn a_tunnel_that_drops_is_raised_again_without_a_new_join_and_a_spent_token_is_terminal() {
        let mut m = Machine::default();
        m.apply(joined(Role::Twin, Some(OVERLAY_BOX_A), 2));
        m.apply(Event::TunnelUp);
        m.apply(Event::TwinConferred);
        m.apply(Event::TunnelDown);
        assert_eq!(m.next(), Step::BringTunnelUp, "the door already knows this box");
        m.apply(Event::TunnelUp);
        assert_eq!(m.next(), Step::TwinBecome { primary: OVERLAY_BOX_A }, "BecomeTwin is idempotent; it is asked again");
        let mut r = Machine::default();
        r.apply(Event::JoinRefusedForGood("REFUSED [join-spent] …".into()));
        assert!(matches!(r.state, State::Refused { .. }));
        assert_eq!(r.next(), Step::Nothing);
        assert_eq!(Machine { tries: 9, ..Machine::default() }.backoff_s(), PEER_WAIT_CAP_S, "a peer is waited for every 3 s, never a minute");
        assert_eq!(Machine { tries: 2, ..Machine::default() }.backoff_s(), PEER_WAIT_CAP_S);
    }
}