citadel_sdk/prefabs/client/

broadcast.rs

//! Group Broadcasting and Management
//!
//! This module provides functionality for creating and managing group-based communication
//! channels in the Citadel Protocol. It implements an owner-based trust model where one
//! peer acts as the group administrator and trust anchor.
//!
//! # Features
//! - Group creation and management
//! - Owner-based trust model
//! - Public and private group support
//! - Automatic member registration
//! - Group invitation system
//! - Dynamic peer discovery
//! - Concurrent group participation
//!
//! # Example
//! ```rust
//! use citadel_sdk::prelude::*;
//! use citadel_sdk::prefabs::client::broadcast::{BroadcastKernel, GroupInitRequestType};
//! use uuid::Uuid;
//!
//! # fn main() -> Result<(), NetworkError> {
//! async fn create_group(local_user: UserIdentifier) -> Result<(), NetworkError> {
//!     let request = GroupInitRequestType::Create {
//!         local_user,
//!         invite_list: vec![],
//!         group_id: Uuid::new_v4(),
//!         accept_registrations: true,
//!     };
//!
//!     let settings = DefaultServerConnectionSettingsBuilder::transient("127.0.0.1:25021")
//!         .build()?;
//!
//!     let kernel = BroadcastKernel::new(
//!         settings,
//!         request,
//!         |group, _remote| async move {
//!             println!("Group created with ID: {}", group.cid());
//!             Ok(())
//!         },
//!     );
//!
//!     Ok(())
//! }
//! # Ok(())
//! # }
//! ```
//!
//! # Important Notes
//! - Each group must have exactly one owner
//! - Members must be registered with the owner
//! - Trust flows transitively through the owner
//! - Group IDs must be unique per owner
//! - Public groups allow automatic registration
//!
//! # Related Components
//! - [`GroupChannel`]: Group communication channel
//! - [`UserIdentifier`]: User identification
//! - [`GroupInitRequestType`]: Group initialization
//!
//! [`GroupChannel`]: crate::prelude::GroupChannel
//! [`UserIdentifier`]: crate::prelude::UserIdentifier
//! [`GroupInitRequestType`]: crate::prefabs::client::broadcast::GroupInitRequestType

use crate::prelude::*;
use crate::test_common::wait_for_peers;
use citadel_io::tokio::sync::Mutex;
use citadel_user::prelude::UserIdentifierExt;
use futures::{Future, StreamExt};
use std::marker::PhantomData;
use std::pin::Pin;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;
use uuid::Uuid;

/// A kernel that streamlines creating, connecting, and interacting with groups
/// Each group has a single owner, and, each connecting peer must at least be registered
/// to the owner alone. The owner thus serves as an "axis of consent", where each member
/// trusts the owner, and through this trust, transitivity of trust flows to all other
/// future members that connect to the group.
pub struct BroadcastKernel<'a, F, Fut, R: Ratchet> {
    inner_kernel: Box<dyn NetKernel<R> + 'a>,
    shared: Arc<BroadcastShared>,
    _pd: PhantomData<fn() -> (F, Fut)>,
}

pub struct BroadcastShared {
    route_registers: AtomicBool,
    register_rx:
        citadel_io::Mutex<Option<citadel_io::tokio::sync::mpsc::UnboundedReceiver<PeerSignal>>>,
    register_tx: citadel_io::tokio::sync::mpsc::UnboundedSender<PeerSignal>,
}

/// Before running the [`BroadcastKernel`], each peer must send this request
/// to the protocol. One peer can create a group, allowing others to join the group.
///
/// Each peer may create multiple groups.
///
/// Note: When creating a group, a [`GroupType::Public`] will be created. This means any
/// mutually-registered user to the owner may join the group
pub enum GroupInitRequestType {
    /// Create a new group, under owner, with a list of users that are desired to be invited
    ///
    /// if accept_registrations is true, then, any inbound registrations will automatically
    /// be accepted, simulating a publicly open group to all users on the server.
    Create {
        local_user: UserIdentifier,
        invite_list: Vec<UserIdentifier>,
        group_id: Uuid,
        accept_registrations: bool,
    },
    /// Join a pre-existing group as local_user that is administered by owner, and a group_id
    /// that corresponds to a unique group administered by the particular owner
    ///
    /// Note: ordinarily, local_user must be mutually-register to the owner. However, if do_peer_register
    /// is specified, this will ensure both users are registered before continuing (this is not recommended
    /// for production environments, since the timing of registration between users should be approximately
    /// equal, otherwise, the registration request may expire)
    Join {
        local_user: UserIdentifier,
        owner: UserIdentifier,
        group_id: Uuid,
        do_peer_register: bool,
    },
}

#[async_trait]
impl<'a, F, Fut, R: Ratchet> PrefabFunctions<'a, GroupInitRequestType, R>
    for BroadcastKernel<'a, F, Fut, R>
where
    F: FnOnce(GroupChannel, CitadelClientServerConnection<R>) -> Fut + Send + 'a,
    Fut: Future<Output = Result<(), NetworkError>> + Send + 'a,
{
    type UserLevelInputFunction = F;
    type SharedBundle = Arc<BroadcastShared>;

    fn get_shared_bundle(&self) -> Self::SharedBundle {
        self.shared.clone()
    }

    #[allow(unreachable_code, clippy::blocks_in_conditions)]
    #[cfg_attr(
        feature = "localhost-testing",
        tracing::instrument(level = "trace", target = "citadel", skip_all, ret, err(Debug))
    )]
    async fn on_c2s_channel_received(
        connect_success: CitadelClientServerConnection<R>,
        arg: GroupInitRequestType,
        fx: Self::UserLevelInputFunction,
        shared: Arc<BroadcastShared>,
    ) -> Result<(), NetworkError> {
        let session_cid = connect_success.cid;
        wait_for_peers().await;
        let mut creator_only_accept_inbound_registers = false;

        let mut is_owner = false;
        let request = match arg {
            GroupInitRequestType::Create {
                local_user,
                invite_list,
                group_id,
                accept_registrations,
            } => {
                is_owner = true;
                // ensure local user is registered to each on the invite list
                let mut peers_registered = vec![];

                for peer in &invite_list {
                    let peer = peer
                        .search_peer(session_cid, connect_success.account_manager())
                        .await?
                        .ok_or_else(|| {
                            citadel_io::error!(
                                citadel_io::ErrorCode::BroadcastCreateUserNotRegistered,
                                format!("{peer:?}"),
                                format!("{local_user:?}")
                            )
                        })?;

                    peers_registered.push(peer.cid)
                }

                creator_only_accept_inbound_registers = accept_registrations;

                GroupBroadcast::Create {
                    initial_invitees: peers_registered,
                    options: MessageGroupOptions {
                        group_type: GroupType::Public,
                        id: group_id.as_u128(),
                        ..Default::default()
                    },
                }
            }

            GroupInitRequestType::Join {
                local_user,
                owner,
                group_id,
                do_peer_register,
            } => {
                // ensure local is registered to owner
                let owner_orig = owner;
                let owner_find = owner_orig
                    .search_peer(session_cid, connect_success.account_manager())
                    .await?;

                let owner = if let Some(owner) = owner_find {
                    Some(owner)
                } else if do_peer_register {
                    let handle = connect_success
                        .propose_target(local_user.clone(), owner_orig.clone())
                        .await?;
                    let _ = handle.register_to_peer().await?;
                    // wait_for_peers().await;
                    owner_orig
                        .search_peer(session_cid, connect_success.account_manager())
                        .await?
                } else {
                    None
                };

                let owner = owner.ok_or_else(|| {
                    citadel_io::error!(
                        citadel_io::ErrorCode::BroadcastJoinUserNotRegistered,
                        format!("{owner_orig:?}"),
                        format!("{local_user:?}")
                    )
                })?;

                let expected_message_group_key = MessageGroupKey {
                    cid: owner.cid,
                    mgid: group_id.as_u128(),
                };

                // Exponential backoff, waiting for owner to create group
                let mut retries = 0;
                let group_owner_handle = connect_success
                    .propose_target(local_user.clone(), owner.cid)
                    .await?;
                loop {
                    let owned_groups = group_owner_handle.list_owned_groups().await?;
                    if owned_groups.contains(&expected_message_group_key) {
                        break;
                    } else {
                        citadel_io::time::sleep(std::time::Duration::from_secs(2u64.pow(retries)))
                            .await;

                        retries += 1;
                        if retries > 4 {
                            return Err(citadel_io::error!(
                                citadel_io::ErrorCode::BroadcastOwnerGroupMissing,
                                citadel_io::Dbg(owner),
                                citadel_io::Dbg(group_id)
                            ));
                        }
                    }
                }

                GroupBroadcast::RequestJoin {
                    sender: local_user.get_cid(),
                    key: expected_message_group_key,
                }
            }
        };

        let request = NodeRequest::GroupBroadcastCommand(GroupBroadcastCommand {
            session_cid,
            command: request,
        });

        let subscription = &Mutex::new(Some(
            connect_success.send_callback_subscription(request).await?,
        ));

        log::trace!(target: "citadel", "Peer {session_cid} is attempting to join group");
        let acceptor_task = if creator_only_accept_inbound_registers {
            shared.route_registers.store(true, Ordering::Relaxed);
            let mut reg_rx = shared.register_rx.lock().take().unwrap();
            let remote = connect_success.remote_ref().clone();
            Box::pin(async move {
                let mut subscription = subscription.lock().await.take().unwrap();
                // Merge the reg_rx stream and the subscription stream
                let mut count_registered = 0;
                loop {
                    let post_register = citadel_io::tokio::select! {
                        reg_request = reg_rx.recv() => {
                            reg_request.ok_or_else(|| citadel_io::error!(citadel_io::ErrorCode::BroadcastStreamEndedUnexpectedly, "reg_rx"))?
                        },

                        reg_request2 = subscription.next() => {
                            let signal = reg_request2.ok_or_else(|| citadel_io::error!(citadel_io::ErrorCode::BroadcastStreamEndedUnexpectedly, "subscription"))?;
                            if let NodeResult::PeerEvent(PeerEvent { event: sig @ PeerSignal::PostRegister { .. }, .. }) = &signal {
                                sig.clone()
                            } else {
                                continue;
                            }
                        }
                    };

                    log::trace!(target: "citadel", "ACCEPTOR {session_cid} RECV reg_request: {post_register:?}");
                    if let PeerSignal::PostRegister {
                        peer_conn_type: peer_conn,
                        inviter_username: _,
                        invitee_username: _,
                        ticket_opt: _,
                        invitee_response: None,
                    } = &post_register
                    {
                        let cid = peer_conn.get_original_target_cid();
                        if cid != session_cid {
                            log::warn!(target: "citadel", "Received the wrong CID. Will not accept request");
                            continue;
                        }

                        let _ = responses::peer_register(post_register, true, &remote).await?;
                        if cfg!(feature = "localhost-testing") {
                            count_registered += 1;
                            if count_registered == crate::test_common::num_local_test_peers() - 1 {
                                // wait_for_peers().await;
                                break;
                            }
                        }
                    }
                }

                Ok::<_, NetworkError>(())
            })
                as Pin<
                    Box<
                        dyn futures::Future<
                                Output = Result<(), citadel_proto::prelude::NetworkError>,
                            > + Send,
                    >,
                >
        } else {
            Box::pin(async move { Ok::<_, NetworkError>(()) })
                as Pin<
                    Box<
                        dyn futures::Future<
                                Output = Result<(), citadel_proto::prelude::NetworkError>,
                            > + Send,
                    >,
                >
        };

        let mut lock = subscription.lock().await;
        let subscription = lock.as_mut().unwrap();
        while let Some(event) = subscription.next().await {
            match event.into_result()? {
                NodeResult::PeerEvent(PeerEvent {
                    event: ref ps @ PeerSignal::PostRegister { .. },
                    ..
                }) => {
                    shared
                        .register_tx
                        .send(ps.clone())
                        .map_err(|err| NetworkError::generic(err.to_string()))?;
                }
                NodeResult::GroupChannelCreated(GroupChannelCreated {
                    ticket: _,
                    channel,
                    session_cid: _,
                }) => {
                    // in either case, whether owner or not, we get a channel
                    // Drop the lock to allow the acceptor task to gain access to the subscription
                    drop(lock);
                    return if is_owner {
                        citadel_io::tokio::try_join!(fx(channel, connect_success), acceptor_task)
                            .map(|_| ())
                    } else {
                        fx(channel, connect_success).await.map(|_| ())
                    };
                }

                NodeResult::GroupEvent(GroupEvent {
                    session_cid: _,
                    ticket: _,
                    event: GroupBroadcast::CreateResponse { key: None },
                }) => {
                    return Err(citadel_io::error!(
                        citadel_io::ErrorCode::BroadcastCreateGroupFailed
                    ))
                }

                _ => {}
            }
        }

        Ok(())
    }

    fn construct(kernel: Box<dyn NetKernel<R> + 'a>) -> Self {
        let (tx, rx) = citadel_io::tokio::sync::mpsc::unbounded_channel();
        Self {
            shared: Arc::new(BroadcastShared {
                route_registers: AtomicBool::new(false),
                register_rx: citadel_io::Mutex::new(Some(rx)),
                register_tx: tx,
            }),
            inner_kernel: kernel,
            _pd: Default::default(),
        }
    }
}

#[async_trait]
impl<F, Fut, R: Ratchet> NetKernel<R> for BroadcastKernel<'_, F, Fut, R> {
    fn load_remote(&mut self, node_remote: NodeRemote<R>) -> Result<(), NetworkError> {
        self.inner_kernel.load_remote(node_remote)
    }

    async fn on_start(&self) -> Result<(), NetworkError> {
        self.inner_kernel.on_start().await
    }

    async fn on_node_event_received(&self, message: NodeResult<R>) -> Result<(), NetworkError> {
        if let NodeResult::PeerEvent(PeerEvent {
            event: ps @ PeerSignal::PostRegister { .. },
            ..
        }) = &message
        {
            if self.shared.route_registers.load(Ordering::Relaxed) {
                return self
                    .shared
                    .register_tx
                    .send(ps.clone())
                    .map_err(|err| NetworkError::generic(err.to_string()));
            }
        }

        self.inner_kernel.on_node_event_received(message).await
    }

    async fn on_stop(&mut self) -> Result<(), NetworkError> {
        self.inner_kernel.on_stop().await
    }
}

#[cfg(all(test, feature = "localhost-testing"))]
mod tests {
    use crate::prefabs::client::broadcast::{BroadcastKernel, GroupInitRequestType};
    use crate::prefabs::client::peer_connection::PeerConnectionKernel;
    use crate::prefabs::client::DefaultServerConnectionSettingsBuilder;
    use crate::prelude::*;
    use crate::test_common::{server_info, wait_for_peers, TestBarrier};
    use citadel_io::tokio;
    use futures::prelude::stream::FuturesUnordered;
    use futures::TryStreamExt;
    use rstest::rstest;
    use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
    use uuid::Uuid;

    #[citadel_io::tokio::test(flavor = "multi_thread")]
    async fn group_connect_list_members() -> Result<(), Box<dyn std::error::Error>> {
        let peer_count = 3;
        assert!(peer_count > 1);
        citadel_logging::setup_log();
        TestBarrier::setup(peer_count);

        let client_success = &AtomicUsize::new(0);
        let (server, server_addr) = server_info::<StackedRatchet>();

        let client_kernels = FuturesUnordered::new();
        let total_peers = (0..peer_count)
            .map(|_| Uuid::new_v4())
            .collect::<Vec<Uuid>>();
        let group_id = Uuid::new_v4();

        for idx in 0..peer_count {
            let uuid = total_peers.get(idx).cloned().unwrap();

            let request = if idx == 0 {
                // invite list is empty since we will expect the users to post_register to us before attempting to join
                GroupInitRequestType::Create {
                    local_user: UserIdentifier::from(uuid),
                    invite_list: vec![],
                    group_id,
                    accept_registrations: true,
                }
            } else {
                GroupInitRequestType::Join {
                    local_user: UserIdentifier::from(uuid),
                    owner: total_peers.first().cloned().unwrap().into(),
                    group_id,
                    do_peer_register: true,
                }
            };

            let server_connection_settings =
                DefaultServerConnectionSettingsBuilder::transient_with_id(server_addr, uuid)
                    .build()
                    .unwrap();

            let client_kernel = BroadcastKernel::new(
                server_connection_settings,
                request,
                move |channel, connection| async move {
                    wait_for_peers().await;
                    log::trace!(target: "citadel", "***GROUP PEER {}={}={} CONNECT SUCCESS***", idx, uuid, connection.conn_type.get_session_cid());

                    let owned_groups = connection.list_owned_groups().await.unwrap();

                    if idx == 0 {
                        assert_eq!(owned_groups.len(), 1);
                    } else {
                        assert_eq!(owned_groups.len(), 0);
                    }

                    log::trace!(target: "citadel", "Peer {idx}={} is COMPLETE!", connection.conn_type.get_session_cid());

                    let _ = client_success.fetch_add(1, Ordering::Relaxed);
                    wait_for_peers().await;
                    drop(channel);
                    connection.shutdown_kernel().await
                },
            );

            let client = DefaultNodeBuilder::default().build(client_kernel).unwrap();

            client_kernels.push(async move { client.await.map(|_| ()) });
        }

        let clients = Box::pin(async move { client_kernels.try_collect::<()>().await.map(|_| ()) });

        let res = futures::future::try_select(server, clients).await;
        if let Err(err) = res {
            return match err {
                futures::future::Either::Left(left) => Err(left.0.into_string().into()),
                futures::future::Either::Right(right) => Err(right.0.into_string().into()),
            };
        }

        assert_eq!(client_success.load(Ordering::Relaxed), peer_count);
        Ok(())
    }

    #[rstest]
    #[case(2)]
    #[timeout(std::time::Duration::from_secs(90))]
    #[citadel_io::tokio::test(flavor = "multi_thread")]
    async fn test_manual_group_connect(
        #[case] peer_count: usize,
    ) -> Result<(), Box<dyn std::error::Error>> {
        /*
           Test a group connection between two registered peers
           who engage in a manual mode
        */
        assert!(peer_count > 1);
        citadel_logging::setup_log();
        TestBarrier::setup(peer_count);

        let client_success = &AtomicBool::new(false);
        let receiver_success = &AtomicBool::new(false);

        let (server, server_addr) = server_info::<StackedRatchet>();

        let client_kernels = FuturesUnordered::new();
        let total_peers = (0..peer_count)
            .map(|_| Uuid::new_v4())
            .collect::<Vec<Uuid>>();

        for idx in 0..peer_count {
            let uuid = total_peers.get(idx).cloned().unwrap();
            let peers = total_peers
                .clone()
                .into_iter()
                .filter(|r| r != &uuid)
                .map(UserIdentifier::from)
                .collect::<Vec<UserIdentifier>>();

            let server_connection_settings =
                DefaultServerConnectionSettingsBuilder::transient_with_id(server_addr, uuid)
                    .build()
                    .unwrap();

            let client_kernel = PeerConnectionKernel::new(
                server_connection_settings,
                peers,
                move |mut results, remote| async move {
                    let _sender = remote.conn_type.get_session_cid();
                    let mut signals = remote.get_unprocessed_signals_receiver().unwrap();

                    wait_for_peers().await;
                    let conn = results.recv().await.unwrap()?;
                    log::trace!(target: "citadel", "User {uuid} received {conn:?}");

                    // one user will create the group, the other will respond
                    if idx == 0 {
                        let _channel = remote
                            .create_group(Some(vec![conn.channel.get_peer_cid().into()]))
                            .await?;
                        log::info!(target: "citadel", "The designated node has finished creating a group");

                        wait_for_peers().await;
                        client_success.store(true, Ordering::Relaxed);
                        return remote.shutdown_kernel().await;
                    } else {
                        // wait until the group host finishes setting up the group
                        while let Some(evt) = signals.recv().await {
                            log::info!(target: "citadel", "Received unprocessed signal: {evt:?}");
                            match evt {
                                NodeResult::GroupEvent(GroupEvent {
                                    session_cid: _,
                                    ticket: _,
                                    event:
                                        GroupBroadcast::Invitation {
                                            sender: _,
                                            key: _key,
                                        },
                                }) => {
                                    let _ =
                                        crate::responses::group_invite(evt, true, &remote.inner)
                                            .await?;
                                }

                                NodeResult::GroupChannelCreated(GroupChannelCreated {
                                    ticket: _,
                                    channel: _chan,
                                    session_cid: _,
                                }) => {
                                    receiver_success.store(true, Ordering::Relaxed);
                                    log::trace!(target: "citadel", "***PEER {uuid} CONNECT***");
                                    wait_for_peers().await;
                                    return remote.shutdown_kernel().await;
                                }

                                val => {
                                    log::warn!(target: "citadel", "Unhandled response: {val:?}")
                                }
                            }
                        }
                    }

                    Err(citadel_io::error!(
                        citadel_io::ErrorCode::BroadcastStreamEndedUnexpectedly,
                        "signals_recv"
                    ))
                },
            );

            let client = DefaultNodeBuilder::default().build(client_kernel).unwrap();
            client_kernels.push(async move { client.await.map(|_| ()) });
        }

        let clients = Box::pin(async move { client_kernels.try_collect::<()>().await.map(|_| ()) });

        if let Err(err) = futures::future::try_select(server, clients).await {
            return match err {
                futures::future::Either::Left(res) => Err(res.0.into_string().into()),
                futures::future::Either::Right(res) => Err(res.0.into_string().into()),
            };
        }

        assert!(client_success.load(Ordering::Relaxed));
        assert!(receiver_success.load(Ordering::Relaxed));
        Ok(())
    }

    /// End-to-end Decentralized Hierarchy Encryption: an owner founds a `SuperiorOnly` command-hierarchy
    /// group and assigns a subordinate to `/alpha`. The subordinate sends a `DheEnvelope` message; the
    /// owner (the hierarchy root, a superior of every node) reads it through the opaque relay — proving
    /// the full network path: KeyPackage → HierarchyAssign → Welcome → DheEnvelope → relay → superior read.
    #[citadel_io::tokio::test(flavor = "multi_thread")]
    async fn group_command_hierarchy_superior_reads_subordinate(
    ) -> Result<(), Box<dyn std::error::Error>> {
        use crate::prelude::GroupBroadcastPayload;
        use citadel_types::crypto::SecBuffer;
        use citadel_types::proto::{
            CommandPath, GroupHierarchyMode, MessageGroupOptions, ReadPolicy,
        };
        use std::collections::HashMap;

        let peer_count = 2;
        citadel_logging::setup_log();
        TestBarrier::setup(peer_count);

        let owner_read = &AtomicBool::new(false);
        let (server, server_addr) = server_info::<StackedRatchet>();
        let client_kernels = FuturesUnordered::new();
        let total_peers = (0..peer_count)
            .map(|_| Uuid::new_v4())
            .collect::<Vec<Uuid>>();

        for idx in 0..peer_count {
            let uuid = total_peers.get(idx).cloned().unwrap();
            let peers = total_peers
                .clone()
                .into_iter()
                .filter(|r| r != &uuid)
                .map(UserIdentifier::from)
                .collect::<Vec<UserIdentifier>>();
            let server_connection_settings =
                DefaultServerConnectionSettingsBuilder::transient_with_id(server_addr, uuid)
                    .build()
                    .unwrap();

            let client_kernel = PeerConnectionKernel::new(
                server_connection_settings,
                peers,
                move |mut results, remote| async move {
                    let mut signals = remote.get_unprocessed_signals_receiver().unwrap();
                    wait_for_peers().await;
                    let conn = results.recv().await.unwrap()?;

                    if idx == 0 {
                        // Owner: create a SuperiorOnly hierarchy group, assigning the subordinate to /alpha.
                        let sub_cid: u64 = conn.channel.get_peer_cid();
                        let mut ranks = HashMap::new();
                        let _ = ranks.insert(sub_cid, CommandPath::parse("/alpha"));
                        let options = MessageGroupOptions {
                            hierarchy: GroupHierarchyMode::CommandHierarchy {
                                read_policy: ReadPolicy::SuperiorOnly,
                                ranks,
                            },
                            ..Default::default()
                        };
                        let mut channel = remote
                            .create_group_with_options(Some(vec![sub_cid.into()]), options)
                            .await?;

                        // The owner is the hierarchy root: it reads the subordinate's DheEnvelope.
                        loop {
                            match channel.recv().await {
                                Some(GroupBroadcastPayload::Message { payload, sender: _ }) => {
                                    assert_eq!(payload.as_ref(), b"sitrep from subordinate");
                                    owner_read.store(true, Ordering::Relaxed);
                                    break;
                                }
                                Some(_) => continue,
                                None => break,
                            }
                        }
                        wait_for_peers().await;
                        return remote.shutdown_kernel().await;
                    }

                    // Subordinate: accept the invitation, then send once its (hierarchy-mode) channel is up.
                    while let Some(evt) = signals.recv().await {
                        match evt {
                            NodeResult::GroupEvent(GroupEvent {
                                event: GroupBroadcast::Invitation { .. },
                                ..
                            }) => {
                                let _ = crate::responses::group_invite(evt, true, &remote.inner)
                                    .await?;
                            }
                            NodeResult::GroupChannelCreated(GroupChannelCreated {
                                channel,
                                ..
                            }) => {
                                channel
                                    .send_message(SecBuffer::from(
                                        b"sitrep from subordinate".to_vec(),
                                    ))
                                    .await?;
                                wait_for_peers().await;
                                return remote.shutdown_kernel().await;
                            }
                            _ => {}
                        }
                    }

                    Err(citadel_io::error!(
                        citadel_io::ErrorCode::BroadcastStreamEndedUnexpectedly,
                        "signals"
                    ))
                },
            );
            let client = DefaultNodeBuilder::default().build(client_kernel).unwrap();
            client_kernels.push(async move { client.await.map(|_| ()) });
        }

        let clients = Box::pin(async move { client_kernels.try_collect::<()>().await.map(|_| ()) });
        if let Err(err) = futures::future::try_select(server, clients).await {
            return match err {
                futures::future::Either::Left(res) => Err(res.0.into_string().into()),
                futures::future::Either::Right(res) => Err(res.0.into_string().into()),
            };
        }

        assert!(
            owner_read.load(Ordering::Relaxed),
            "owner (hierarchy root) must read the subordinate's DHE message"
        );
        Ok(())
    }
}