server.rs

// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// This file implements a server that can handle multiple connections.

use std::{
    cell::RefCell,
    collections::{HashMap, HashSet, VecDeque},
    fs::OpenOptions,
    mem,
    net::SocketAddr,
    ops::{Deref, DerefMut},
    path::PathBuf,
    rc::{Rc, Weak},
    time::Instant,
};

use neqo_common::{
    self as common, event::Provider, hex, qdebug, qerror, qinfo, qlog::NeqoQlog, qtrace, qwarn,
    Datagram, Decoder, Role,
};
use neqo_crypto::{
    encode_ech_config, AntiReplay, Cipher, PrivateKey, PublicKey, ZeroRttCheckResult,
    ZeroRttChecker,
};
use qlog::streamer::QlogStreamer;

pub use crate::addr_valid::ValidateAddress;
use crate::{
    addr_valid::{AddressValidation, AddressValidationResult},
    cid::{ConnectionId, ConnectionIdDecoder, ConnectionIdGenerator, ConnectionIdRef},
    connection::{Connection, Output, State},
    packet::{PacketBuilder, PacketType, PublicPacket, MIN_INITIAL_PACKET_SIZE},
    ConnectionParameters, Res, Version,
};

pub enum InitialResult {
    Accept,
    Drop,
    Retry(Vec<u8>),
}

type StateRef = Rc<RefCell<ServerConnectionState>>;
type ConnectionTableRef = Rc<RefCell<HashMap<ConnectionId, StateRef>>>;

#[derive(Debug)]
pub struct ServerConnectionState {
    c: Connection,
    active_attempt: Option<AttemptKey>,
    wake_at: Option<Instant>,
}

impl ServerConnectionState {
    fn set_wake_at(&mut self, at: Instant) {
        self.wake_at = Some(at);
    }

    fn needs_waking(&self, now: Instant) -> bool {
        self.wake_at.map_or(false, |t| t <= now)
    }

    fn woken(&mut self) {
        self.wake_at = None;
    }
}

impl Deref for ServerConnectionState {
    type Target = Connection;
    fn deref(&self) -> &Self::Target {
        &self.c
    }
}

impl DerefMut for ServerConnectionState {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.c
    }
}

/// A `AttemptKey` is used to disambiguate connection attempts.
/// Multiple connection attempts with the same key won't produce multiple connections.
#[derive(Clone, Debug, Hash, PartialEq, Eq)]
struct AttemptKey {
    // Using the remote address is sufficient for disambiguation,
    // until we support multiple local socket addresses.
    remote_address: SocketAddr,
    odcid: ConnectionId,
}

/// A `ServerZeroRttChecker` is a simple wrapper around a single checker.
/// It uses `RefCell` so that the wrapped checker can be shared between
/// multiple connections created by the server.
#[derive(Clone, Debug)]
struct ServerZeroRttChecker {
    checker: Rc<RefCell<Box<dyn ZeroRttChecker>>>,
}

impl ServerZeroRttChecker {
    pub fn new(checker: Box<dyn ZeroRttChecker>) -> Self {
        Self {
            checker: Rc::new(RefCell::new(checker)),
        }
    }
}

impl ZeroRttChecker for ServerZeroRttChecker {
    fn check(&self, token: &[u8]) -> ZeroRttCheckResult {
        self.checker.borrow().check(token)
    }
}

/// `InitialDetails` holds important information for processing `Initial` packets.
struct InitialDetails {
    src_cid: ConnectionId,
    dst_cid: ConnectionId,
    token: Vec<u8>,
    version: Version,
}

impl InitialDetails {
    fn new(packet: &PublicPacket) -> Self {
        Self {
            src_cid: ConnectionId::from(packet.scid()),
            dst_cid: ConnectionId::from(packet.dcid()),
            token: packet.token().to_vec(),
            version: packet.version().unwrap(),
        }
    }
}

struct EchConfig {
    config: u8,
    public_name: String,
    sk: PrivateKey,
    pk: PublicKey,
    encoded: Vec<u8>,
}

impl EchConfig {
    fn new(config: u8, public_name: &str, sk: &PrivateKey, pk: &PublicKey) -> Res<Self> {
        let encoded = encode_ech_config(config, public_name, pk)?;
        Ok(Self {
            config,
            public_name: String::from(public_name),
            sk: sk.clone(),
            pk: pk.clone(),
            encoded,
        })
    }
}

pub struct Server {
    /// The names of certificates.
    certs: Vec<String>,
    /// The ALPN values that the server supports.
    protocols: Vec<String>,
    /// The cipher suites that the server supports.
    ciphers: Vec<Cipher>,
    /// Anti-replay configuration for 0-RTT.
    anti_replay: AntiReplay,
    /// A function for determining if 0-RTT can be accepted.
    zero_rtt_checker: ServerZeroRttChecker,
    /// A connection ID generator.
    cid_generator: Rc<RefCell<dyn ConnectionIdGenerator>>,
    /// Connection parameters.
    conn_params: ConnectionParameters,
    /// Active connection attempts, keyed by `AttemptKey`.  Initial packets with
    /// the same key are routed to the connection that was first accepted.
    /// This is cleared out when the connection is closed or established.
    active_attempts: HashMap<AttemptKey, StateRef>,
    /// All connections, keyed by `ConnectionId`.
    connections: ConnectionTableRef,
    /// The connections that have new events.
    active: HashSet<ActiveConnectionRef>,
    /// The set of connections that need immediate processing.
    waiting: VecDeque<StateRef>,
    /// The latest [`Output::Callback`] returned from [`Server::process`].
    wake_at: Option<Instant>,
    /// Address validation logic, which determines whether we send a Retry.
    address_validation: Rc<RefCell<AddressValidation>>,
    /// Directory to create qlog traces in
    qlog_dir: Option<PathBuf>,
    /// Encrypted client hello (ECH) configuration.
    ech_config: Option<EchConfig>,
}

impl Server {
    /// Construct a new server.
    /// * `now` is the time that the server is instantiated.
    /// * `certs` is a list of the certificates that should be configured.
    /// * `protocols` is the preference list of ALPN values.
    /// * `anti_replay` is an anti-replay context.
    /// * `zero_rtt_checker` determines whether 0-RTT should be accepted. This will be passed the
    ///   value of the `extra` argument that was passed to `Connection::send_ticket` to see if it is
    ///   OK.
    /// * `cid_generator` is responsible for generating connection IDs and parsing them; connection
    ///   IDs produced by the manager cannot be zero-length.
    /// # Errors
    /// When address validation state cannot be created.
    pub fn new(
        now: Instant,
        certs: &[impl AsRef<str>],
        protocols: &[impl AsRef<str>],
        anti_replay: AntiReplay,
        zero_rtt_checker: Box<dyn ZeroRttChecker>,
        cid_generator: Rc<RefCell<dyn ConnectionIdGenerator>>,
        conn_params: ConnectionParameters,
    ) -> Res<Self> {
        let validation = AddressValidation::new(now, ValidateAddress::Never)?;
        Ok(Self {
            certs: certs.iter().map(|x| String::from(x.as_ref())).collect(),
            protocols: protocols.iter().map(|x| String::from(x.as_ref())).collect(),
            ciphers: Vec::new(),
            anti_replay,
            zero_rtt_checker: ServerZeroRttChecker::new(zero_rtt_checker),
            cid_generator,
            conn_params,
            active_attempts: HashMap::default(),
            connections: Rc::default(),
            active: HashSet::default(),
            waiting: VecDeque::default(),
            address_validation: Rc::new(RefCell::new(validation)),
            qlog_dir: None,
            ech_config: None,
            wake_at: None,
        })
    }

    /// Set or clear directory to create logs of connection events in QLOG format.
    pub fn set_qlog_dir(&mut self, dir: Option<PathBuf>) {
        self.qlog_dir = dir;
    }

    /// Set the policy for address validation.
    pub fn set_validation(&mut self, v: ValidateAddress) {
        self.address_validation.borrow_mut().set_validation(v);
    }

    /// Set the cipher suites that should be used.  Set an empty value to use
    /// default values.
    pub fn set_ciphers(&mut self, ciphers: impl AsRef<[Cipher]>) {
        self.ciphers = Vec::from(ciphers.as_ref());
    }

    /// # Errors
    /// When the configuration is invalid.
    pub fn enable_ech(
        &mut self,
        config: u8,
        public_name: &str,
        sk: &PrivateKey,
        pk: &PublicKey,
    ) -> Res<()> {
        self.ech_config = Some(EchConfig::new(config, public_name, sk, pk)?);
        Ok(())
    }

    #[must_use]
    pub fn ech_config(&self) -> &[u8] {
        self.ech_config.as_ref().map_or(&[], |cfg| &cfg.encoded)
    }

    fn process_connection(
        &mut self,
        c: &StateRef,
        dgram: Option<&Datagram>,
        now: Instant,
    ) -> Option<Datagram> {
        qtrace!([self], "Process connection {:?}", c);
        let out = c.borrow_mut().process(dgram, now);
        match out {
            Output::Datagram(_) => {
                qtrace!([self], "Sending packet, added to waiting connections");
                self.waiting.push_back(Rc::clone(c));
            }
            Output::Callback(delay) => {
                let next = now + delay;
                c.borrow_mut().set_wake_at(next);
                if self.wake_at.map_or(true, |c| c > next) {
                    self.wake_at = Some(next);
                }
            }
            Output::None => {}
        }
        if c.borrow().has_events() {
            qtrace!([self], "Connection active: {:?}", c);
            self.active.insert(ActiveConnectionRef { c: Rc::clone(c) });
        }

        if *c.borrow().state() > State::Handshaking {
            // Remove any active connection attempt now that this is no longer handshaking.
            if let Some(k) = c.borrow_mut().active_attempt.take() {
                self.active_attempts.remove(&k);
            }
        }

        if matches!(c.borrow().state(), State::Closed(_)) {
            c.borrow_mut().set_qlog(NeqoQlog::disabled());
            self.connections
                .borrow_mut()
                .retain(|_, v| !Rc::ptr_eq(v, c));
        }
        out.dgram()
    }

    fn connection(&self, cid: ConnectionIdRef) -> Option<StateRef> {
        self.connections.borrow().get(&cid[..]).cloned()
    }

    fn handle_initial(
        &mut self,
        initial: InitialDetails,
        dgram: &Datagram,
        now: Instant,
    ) -> Option<Datagram> {
        qdebug!([self], "Handle initial");
        let res = self
            .address_validation
            .borrow()
            .validate(&initial.token, dgram.source(), now);
        match res {
            AddressValidationResult::Invalid => None,
            AddressValidationResult::Pass => self.connection_attempt(initial, dgram, None, now),
            AddressValidationResult::ValidRetry(orig_dcid) => {
                self.connection_attempt(initial, dgram, Some(orig_dcid), now)
            }
            AddressValidationResult::Validate => {
                qinfo!([self], "Send retry for {:?}", initial.dst_cid);

                let res = self.address_validation.borrow().generate_retry_token(
                    &initial.dst_cid,
                    dgram.source(),
                    now,
                );
                let Ok(token) = res else {
                    qerror!([self], "unable to generate token, dropping packet");
                    return None;
                };
                if let Some(new_dcid) = self.cid_generator.borrow_mut().generate_cid() {
                    let packet = PacketBuilder::retry(
                        initial.version,
                        &initial.src_cid,
                        &new_dcid,
                        &token,
                        &initial.dst_cid,
                    );
                    if let Ok(p) = packet {
                        let retry =
                            Datagram::new(dgram.destination(), dgram.source(), dgram.tos(), p);
                        Some(retry)
                    } else {
                        qerror!([self], "unable to encode retry, dropping packet");
                        None
                    }
                } else {
                    qerror!([self], "no connection ID for retry, dropping packet");
                    None
                }
            }
        }
    }

    fn connection_attempt(
        &mut self,
        initial: InitialDetails,
        dgram: &Datagram,
        orig_dcid: Option<ConnectionId>,
        now: Instant,
    ) -> Option<Datagram> {
        let attempt_key = AttemptKey {
            remote_address: dgram.source(),
            odcid: orig_dcid.as_ref().unwrap_or(&initial.dst_cid).clone(),
        };
        if let Some(c) = self.active_attempts.get(&attempt_key) {
            qdebug!(
                [self],
                "Handle Initial for existing connection attempt {:?}",
                attempt_key
            );
            let c = Rc::clone(c);
            self.process_connection(&c, Some(dgram), now)
        } else {
            self.accept_connection(attempt_key, initial, dgram, orig_dcid, now)
        }
    }

    fn create_qlog_trace(&self, odcid: ConnectionIdRef<'_>) -> NeqoQlog {
        if let Some(qlog_dir) = &self.qlog_dir {
            let mut qlog_path = qlog_dir.clone();

            qlog_path.push(format!("{odcid}.qlog"));

            // The original DCID is chosen by the client. Using create_new()
            // prevents attackers from overwriting existing logs.
            match OpenOptions::new()
                .write(true)
                .create_new(true)
                .open(&qlog_path)
            {
                Ok(f) => {
                    qinfo!("Qlog output to {}", qlog_path.display());

                    let streamer = QlogStreamer::new(
                        qlog::QLOG_VERSION.to_string(),
                        Some("Neqo server qlog".to_string()),
                        Some("Neqo server qlog".to_string()),
                        None,
                        std::time::Instant::now(),
                        common::qlog::new_trace(Role::Server),
                        qlog::events::EventImportance::Base,
                        Box::new(f),
                    );
                    let n_qlog = NeqoQlog::enabled(streamer, qlog_path);
                    match n_qlog {
                        Ok(nql) => nql,
                        Err(e) => {
                            // Keep going but w/o qlogging
                            qerror!("NeqoQlog error: {}", e);
                            NeqoQlog::disabled()
                        }
                    }
                }
                Err(e) => {
                    qerror!(
                        "Could not open file {} for qlog output: {}",
                        qlog_path.display(),
                        e
                    );
                    NeqoQlog::disabled()
                }
            }
        } else {
            NeqoQlog::disabled()
        }
    }

    fn setup_connection(
        &mut self,
        c: &mut Connection,
        attempt_key: &AttemptKey,
        initial: InitialDetails,
        orig_dcid: Option<ConnectionId>,
    ) {
        let zcheck = self.zero_rtt_checker.clone();
        if c.server_enable_0rtt(&self.anti_replay, zcheck).is_err() {
            qwarn!([self], "Unable to enable 0-RTT");
        }
        if let Some(odcid) = orig_dcid {
            // There was a retry, so set the connection IDs for.
            c.set_retry_cids(&odcid, initial.src_cid, &initial.dst_cid);
        }
        c.set_validation(&self.address_validation);
        c.set_qlog(self.create_qlog_trace(attempt_key.odcid.as_cid_ref()));
        if let Some(cfg) = &self.ech_config {
            if c.server_enable_ech(cfg.config, &cfg.public_name, &cfg.sk, &cfg.pk)
                .is_err()
            {
                qwarn!([self], "Unable to enable ECH");
            }
        }
    }

    fn accept_connection(
        &mut self,
        attempt_key: AttemptKey,
        initial: InitialDetails,
        dgram: &Datagram,
        orig_dcid: Option<ConnectionId>,
        now: Instant,
    ) -> Option<Datagram> {
        qinfo!([self], "Accept connection {:?}", attempt_key);
        // The internal connection ID manager that we use is not used directly.
        // Instead, wrap it so that we can save connection IDs.

        let cid_mgr = Rc::new(RefCell::new(ServerConnectionIdGenerator {
            c: Weak::new(),
            cid_generator: Rc::clone(&self.cid_generator),
            connections: Rc::clone(&self.connections),
            saved_cids: Vec::new(),
        }));

        let mut params = self.conn_params.clone();
        params.get_versions_mut().set_initial(initial.version);
        let sconn = Connection::new_server(
            &self.certs,
            &self.protocols,
            Rc::clone(&cid_mgr) as _,
            params,
        );

        match sconn {
            Ok(mut c) => {
                self.setup_connection(&mut c, &attempt_key, initial, orig_dcid);
                let c = Rc::new(RefCell::new(ServerConnectionState {
                    c,
                    wake_at: None,
                    active_attempt: Some(attempt_key.clone()),
                }));
                cid_mgr.borrow_mut().set_connection(&c);
                let previous_attempt = self.active_attempts.insert(attempt_key, Rc::clone(&c));
                debug_assert!(previous_attempt.is_none());
                self.process_connection(&c, Some(dgram), now)
            }
            Err(e) => {
                qwarn!([self], "Unable to create connection");
                if e == crate::Error::VersionNegotiation {
                    crate::qlog::server_version_information_failed(
                        &mut self.create_qlog_trace(attempt_key.odcid.as_cid_ref()),
                        self.conn_params.get_versions().all(),
                        initial.version.wire_version(),
                    );
                }
                None
            }
        }
    }

    /// Handle 0-RTT packets that were sent with the client's choice of connection ID.
    /// Most 0-RTT will arrive this way.  A client can usually send 1-RTT after it
    /// receives a connection ID from the server.
    fn handle_0rtt(
        &mut self,
        dgram: &Datagram,
        dcid: ConnectionId,
        now: Instant,
    ) -> Option<Datagram> {
        let attempt_key = AttemptKey {
            remote_address: dgram.source(),
            odcid: dcid,
        };
        if let Some(c) = self.active_attempts.get(&attempt_key) {
            qdebug!(
                [self],
                "Handle 0-RTT for existing connection attempt {:?}",
                attempt_key
            );
            let c = Rc::clone(c);
            self.process_connection(&c, Some(dgram), now)
        } else {
            qdebug!([self], "Dropping 0-RTT for unknown connection");
            None
        }
    }

    fn process_input(&mut self, dgram: &Datagram, now: Instant) -> Option<Datagram> {
        qtrace!("Process datagram: {}", hex(&dgram[..]));

        // This is only looking at the first packet header in the datagram.
        // All packets in the datagram are routed to the same connection.
        let res = PublicPacket::decode(&dgram[..], self.cid_generator.borrow().as_decoder());
        let Ok((packet, _remainder)) = res else {
            qtrace!([self], "Discarding {:?}", dgram);
            return None;
        };

        // Finding an existing connection. Should be the most common case.
        if let Some(c) = self.connection(packet.dcid()) {
            return self.process_connection(&c, Some(dgram), now);
        }

        if packet.packet_type() == PacketType::Short {
            // TODO send a stateless reset here.
            qtrace!([self], "Short header packet for an unknown connection");
            return None;
        }

        if packet.packet_type() == PacketType::OtherVersion
            || (packet.packet_type() == PacketType::Initial
                && !self
                    .conn_params
                    .get_versions()
                    .all()
                    .contains(&packet.version().unwrap()))
        {
            if dgram.len() < MIN_INITIAL_PACKET_SIZE {
                qdebug!([self], "Unsupported version: too short");
                return None;
            }

            qdebug!([self], "Unsupported version: {:x}", packet.wire_version());
            let vn = PacketBuilder::version_negotiation(
                &packet.scid()[..],
                &packet.dcid()[..],
                packet.wire_version(),
                self.conn_params.get_versions().all(),
            );

            crate::qlog::server_version_information_failed(
                &mut self.create_qlog_trace(packet.dcid()),
                self.conn_params.get_versions().all(),
                packet.wire_version(),
            );

            return Some(Datagram::new(
                dgram.destination(),
                dgram.source(),
                dgram.tos(),
                vn,
            ));
        }

        match packet.packet_type() {
            PacketType::Initial => {
                if dgram.len() < MIN_INITIAL_PACKET_SIZE {
                    qdebug!([self], "Drop initial: too short");
                    return None;
                }
                // Copy values from `packet` because they are currently still borrowing from
                // `dgram`.
                let initial = InitialDetails::new(&packet);
                self.handle_initial(initial, dgram, now)
            }
            PacketType::ZeroRtt => {
                let dcid = ConnectionId::from(packet.dcid());
                self.handle_0rtt(dgram, dcid, now)
            }
            PacketType::OtherVersion => unreachable!(),
            _ => {
                qtrace!([self], "Not an initial packet");
                None
            }
        }
    }

    /// Iterate through the pending connections looking for any that might want
    /// to send a datagram.  Stop at the first one that does.
    fn process_next_output(&mut self, now: Instant) -> Option<Datagram> {
        qtrace!([self], "No packet to send, look at waiting connections");
        while let Some(c) = self.waiting.pop_front() {
            if let Some(d) = self.process_connection(&c, None, now) {
                return Some(d);
            }
        }

        qtrace!([self], "No packet to send still, check wake up times");
        loop {
            let connection = self
                .connections
                .borrow()
                .values()
                .find(|c| c.borrow().needs_waking(now))
                .cloned()?;
            let datagram = self.process_connection(&connection, None, now);
            connection.borrow_mut().woken();
            if datagram.is_some() {
                return datagram;
            }
        }
    }

    pub fn process(&mut self, dgram: Option<&Datagram>, now: Instant) -> Output {
        if self.wake_at.map_or(false, |c| c <= now) {
            self.wake_at = None;
        }

        dgram
            .and_then(|d| self.process_input(d, now))
            .or_else(|| self.process_next_output(now))
            .map(|d| {
                qtrace!([self], "Send packet: {:?}", d);
                Output::Datagram(d)
            })
            .or_else(|| self.wake_at.take().map(|c| Output::Callback(c - now)))
            .unwrap_or_else(|| {
                qtrace!([self], "Go dormant");
                Output::None
            })
    }

    /// This lists the connections that have received new events
    /// as a result of calling `process()`.
    pub fn active_connections(&mut self) -> Vec<ActiveConnectionRef> {
        mem::take(&mut self.active).into_iter().collect()
    }

    /// Whether any connections have received new events as a result of calling
    /// `process()`.
    #[must_use]
    pub fn has_active_connections(&self) -> bool {
        !self.active.is_empty()
    }

    pub fn add_to_waiting(&mut self, c: &ActiveConnectionRef) {
        self.waiting.push_back(c.connection());
    }
}

#[derive(Clone, Debug)]
pub struct ActiveConnectionRef {
    c: StateRef,
}

impl ActiveConnectionRef {
    #[must_use]
    pub fn borrow(&self) -> impl Deref<Target = Connection> + '_ {
        std::cell::Ref::map(self.c.borrow(), |c| &c.c)
    }

    pub fn borrow_mut(&mut self) -> impl DerefMut<Target = Connection> + '_ {
        std::cell::RefMut::map(self.c.borrow_mut(), |c| &mut c.c)
    }

    #[must_use]
    pub fn connection(&self) -> StateRef {
        Rc::clone(&self.c)
    }
}

impl std::hash::Hash for ActiveConnectionRef {
    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
        let ptr: *const _ = self.c.as_ref();
        ptr.hash(state);
    }
}

impl PartialEq for ActiveConnectionRef {
    fn eq(&self, other: &Self) -> bool {
        Rc::ptr_eq(&self.c, &other.c)
    }
}

impl Eq for ActiveConnectionRef {}

struct ServerConnectionIdGenerator {
    c: Weak<RefCell<ServerConnectionState>>,
    connections: ConnectionTableRef,
    cid_generator: Rc<RefCell<dyn ConnectionIdGenerator>>,
    saved_cids: Vec<ConnectionId>,
}

impl ServerConnectionIdGenerator {
    pub fn set_connection(&mut self, c: &StateRef) {
        let saved = std::mem::replace(&mut self.saved_cids, Vec::with_capacity(0));
        for cid in saved {
            qtrace!("ServerConnectionIdGenerator inserting saved cid {}", cid);
            self.insert_cid(cid, Rc::clone(c));
        }
        self.c = Rc::downgrade(c);
    }

    fn insert_cid(&mut self, cid: ConnectionId, rc: StateRef) {
        debug_assert!(!cid.is_empty());
        self.connections.borrow_mut().insert(cid, rc);
    }
}

impl ConnectionIdDecoder for ServerConnectionIdGenerator {
    fn decode_cid<'a>(&self, dec: &mut Decoder<'a>) -> Option<ConnectionIdRef<'a>> {
        self.cid_generator.borrow_mut().decode_cid(dec)
    }
}

impl ConnectionIdGenerator for ServerConnectionIdGenerator {
    fn generate_cid(&mut self) -> Option<ConnectionId> {
        let maybe_cid = self.cid_generator.borrow_mut().generate_cid();
        if let Some(cid) = maybe_cid {
            if let Some(rc) = self.c.upgrade() {
                self.insert_cid(cid.clone(), rc);
            } else {
                // This function can be called before the connection is set.
                // So save any connection IDs until that hookup happens.
                qtrace!("ServerConnectionIdGenerator saving cid {}", cid);
                self.saved_cids.push(cid.clone());
            }
            Some(cid)
        } else {
            None
        }
    }

    fn as_decoder(&self) -> &dyn ConnectionIdDecoder {
        self
    }
}

impl ::std::fmt::Display for Server {
    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
        write!(f, "Server")
    }
}