debug_assert_ne!(port_info.last_instruction, PortInstruction::None); // set by caller

    debug_assert!(port_info.state.is_open()); // checked by caller

    // Check if there are any more messages in the backup buffer

    for message_index in 0..inbox_backup.len() {

        let message = &inbox_backup[message_index];

        if message.data_header.target_port == targeted_port {

            // One more message, place it in the slot

            let message = inbox_backup.remove(message_index);

            debug_assert!(comp_ctx.get_port(port_handle).state.is_blocked()); // since we're removing another message from the backup

            inbox_main[port_index] = Some(message);

            return Ok(());

        }

    }

    // Did not have any more messages, so if we were blocked, then we need to

    // unblock the port now (and inform the peer of this unblocking)

    if port_info.state.is_set(PortStateFlag::BlockedDueToFullBuffers) {

        let port_info = comp_ctx.get_port_mut(port_handle);

        port_info.state.clear(PortStateFlag::BlockedDueToFullBuffers);

        let (peer_handle, message) = control.cancel_port_blocking(comp_ctx, port_handle);

        let peer_info = comp_ctx.get_peer(peer_handle);

        peer_info.handle.send_message_logged(sched_ctx, Message::Control(message), true);

    }

    return Ok(());

}

/// Handles control messages in the default way. Note that this function may

/// take a lot of actions in the name of the caller: pending messages may be

/// sent, ports may become blocked/unblocked, etc. So the execution

/// (`CompExecState`), control (`ControlLayer`) and consensus (`Consensus`)

/// state may all change.

pub(crate) fn default_handle_control_message(

    exec_state: &mut CompExecState, control: &mut ControlLayer, consensus: &mut Consensus,

    message: ControlMessage, sched_ctx: &SchedulerCtx, comp_ctx: &mut CompCtx,

    inbox_main: &mut InboxMain, inbox_backup: &mut InboxBackup

) -> Result<(), (PortInstruction, String)> {

    match message.content {

        ControlMessageContent::Ack => {

            default_handle_ack(exec_state, control, message.id, sched_ctx, comp_ctx, consensus, inbox_main, inbox_backup);

        },

        ControlMessageContent::BlockPort => {

            // One of our messages was accepted, but the port should be

            // blocked.

            let port_to_block = message.target_port_id.unwrap();

            let port_handle = comp_ctx.get_port_handle(port_to_block);

            let port_info = comp_ctx.get_port_mut(port_handle);

            debug_assert_eq!(port_info.kind, PortKind::Putter);

            port_info.state.set(PortStateFlag::BlockedDueToFullBuffers);

        },

        ControlMessageContent::ClosePort(content) => {

            // Request to close the port. We immediately comply and remove

            // the component handle as well

            let port_to_close = message.target_port_id.unwrap();

            let port_handle = comp_ctx.get_port_handle(port_to_close);

            // We're closing the port, so we will always update the peer of the

            // port (in case of error messages)

            let port_info = comp_ctx.get_port_mut(port_handle);

            port_info.peer_comp_id = message.sender_comp_id;

            port_info.close_at_sync_end = true; // might be redundant (we might set it closed now)

            let peer_comp_id = port_info.peer_comp_id;

            let peer_handle = comp_ctx.get_peer_handle(peer_comp_id);

            // One exception to sending an `Ack` is if we just closed the

            // port ourselves, meaning that the `ClosePort` messages got

            // sent to one another.

            if let Some(control_id) = control.has_close_port_entry(port_handle, comp_ctx) {

                // The two components (sender and this component) are closing

                // the channel at the same time. So we don't care about the

                // content of the `ClosePort` message.

                default_handle_ack(exec_state, control, control_id, sched_ctx, comp_ctx, consensus, inbox_main, inbox_backup);

            } else {

                // Respond to the message

                let port_info = comp_ctx.get_port(port_handle);

                let last_instruction = port_info.last_instruction;

                let port_has_had_message = port_info.received_message_for_sync;

                default_send_ack(message.id, peer_handle, sched_ctx, comp_ctx);

                comp_ctx.change_port_peer(sched_ctx, port_handle, None);

                // Handle any possible error conditions (which boil down to: the

                // port has been used, but the peer has died). If not in sync

                // mode then we close the port immediately.

                // Note that `port_was_used` does not mean that any messages

                // were actually received. It might also mean that e.g. the

                // component attempted a `get`, but there were no messages, so

                // now it is in the `BlockedGet` state.

                let port_was_used = last_instruction != PortInstruction::None;

                if exec_state.mode.is_in_sync_block() {

                    let closed_during_sync_round = content.closed_in_sync_round && port_was_used;

                    if closed_during_sync_round || closed_before_sync_round {

                        return Err((

                            last_instruction,

                            format!("Peer component (id:{}) shut down, so communication cannot (have) succeed(ed)", peer_comp_id.0)

                        ));

                    }

                } else {

                    let port_info = comp_ctx.get_port_mut(port_handle);

                    port_info.state.set(PortStateFlag::Closed);

                }

            }

        },

        ControlMessageContent::UnblockPort => {

            // We were previously blocked (or already closed)

            let port_to_unblock = message.target_port_id.unwrap();

            let port_handle = comp_ctx.get_port_handle(port_to_unblock);

            let port_info = comp_ctx.get_port_mut(port_handle);

            debug_assert_eq!(port_info.kind, PortKind::Putter);

            debug_assert!(port_info.state.is_set(PortStateFlag::BlockedDueToFullBuffers));

            port_info.state.clear(PortStateFlag::BlockedDueToFullBuffers);

            default_handle_recently_unblocked_port(

                exec_state, consensus, port_handle, sched_ctx, comp_ctx,

                inbox_main, inbox_backup

            );

        },

        ControlMessageContent::PortPeerChangedBlock => {

            // The peer of our port has just changed. So we are asked to

            // temporarily block the port (while our original recipient is

            // potentially rerouting some of the in-flight messages) and

            // Ack. Then we wait for the `unblock` call.

            let port_to_change = message.target_port_id.unwrap();

            let port_handle = comp_ctx.get_port_handle(port_to_change);

            let port_info = comp_ctx.get_port_mut(port_handle);

            let peer_comp_id = port_info.peer_comp_id;

            port_info.state.set(PortStateFlag::BlockedDueToPeerChange);

            let peer_handle = comp_ctx.get_peer_handle(peer_comp_id);

            default_send_ack(message.id, peer_handle, sched_ctx, comp_ctx);

        },

        ControlMessageContent::PortPeerChangedUnblock(new_port_id, new_comp_id) => {

            let port_to_change = message.target_port_id.unwrap();

            let port_handle = comp_ctx.get_port_handle(port_to_change);

            let port_info = comp_ctx.get_port(port_handle);

            debug_assert!(port_info.state.is_set(PortStateFlag::BlockedDueToPeerChange));

            let port_info = comp_ctx.get_port_mut(port_handle);

            port_info.peer_port_id = new_port_id;

            port_info.state.clear(PortStateFlag::BlockedDueToPeerChange);

            comp_ctx.change_port_peer(sched_ctx, port_handle, Some(new_comp_id));

            default_handle_recently_unblocked_port(

                exec_state, consensus, port_handle, sched_ctx, comp_ctx,

                inbox_main, inbox_backup

            );

        }

    }

    return Ok(());

}

/// Handles a component entering the synchronous block. Will ensure that the

/// `Consensus` and the `ComponentCtx` are initialized properly.

pub(crate) fn default_handle_sync_start(

    exec_state: &mut CompExecState, inbox_main: &mut InboxMainRef,

    sched_ctx: &SchedulerCtx, comp_ctx: &mut CompCtx, consensus: &mut Consensus

) {

    sched_ctx.info("Component starting sync mode");

    // If any messages are present for this sync round, set the appropriate flag

    // and notify the consensus handler of the present messages

    consensus.notify_sync_start(comp_ctx);

    for (port_index, message) in inbox_main.iter().enumerate() {

        if let Some(message) = message {

            consensus.handle_incoming_data_message(comp_ctx, message);

            let port_info = comp_ctx.get_port_by_index_mut(port_index);

            port_info.received_message_for_sync = true;

        }

    }

    // Modify execution state

    debug_assert_eq!(exec_state.mode, CompMode::NonSync);

    exec_state.mode = CompMode::Sync;

}

/// Handles a component that has reached the end of the sync block. This does

/// not necessarily mean that the component will go into the `NonSync` mode, as

/// it might have to wait for the leader to finish the round for everyone (see

/// `default_handle_sync_decision`)

pub(crate) fn default_handle_sync_end(

    exec_state: &mut CompExecState, sched_ctx: &SchedulerCtx, comp_ctx: &mut CompCtx,

    consensus: &mut Consensus

) {

    sched_ctx.info("Component ending sync mode (but possibly waiting for a solution)");

    debug_assert_eq!(exec_state.mode, CompMode::Sync);

    let decision = consensus.notify_sync_end_success(sched_ctx, comp_ctx);

    exec_state.mode = CompMode::SyncEnd;

    default_handle_sync_decision(sched_ctx, exec_state, comp_ctx, decision, consensus);

}

/// Handles a component initiating the exiting procedure, and closing all of its

/// ports. Should only be called once per component (which is ensured by

/// checking and modifying the mode in the execution state).

#[must_use]

pub(crate) fn default_handle_start_exit(

    exec_state: &mut CompExecState, control: &mut ControlLayer,

    sched_ctx: &SchedulerCtx, comp_ctx: &mut CompCtx, consensus: &mut Consensus

) -> CompScheduling {

    debug_assert_eq!(exec_state.mode, CompMode::StartExit);

    sched_ctx.info(&format!("Component starting exit (reason: {:?})", exec_state.exit_reason));

    exec_state.mode = CompMode::BusyExit;

    let exit_inside_sync = exec_state.exit_reason.is_in_sync();

    // If exiting while inside sync mode, report to the leader of the current

    // round that we've failed.

    if exit_inside_sync {

        let decision = consensus.notify_sync_end_failure(sched_ctx, comp_ctx);

        default_handle_sync_decision(sched_ctx, exec_state, comp_ctx, decision, consensus);

    }

    // Iterating over ports by index to work around borrowing rules

    for port_index in 0..comp_ctx.num_ports() {

        let port = comp_ctx.get_port_by_index_mut(port_index);

            // Already closed, or in the process of being closed

            continue;

        }

        // Mark as closed

        let port_id = port.self_id;

        port.state.set(PortStateFlag::Closed);

        // Notify peer of closing

        let port_handle = comp_ctx.get_port_handle(port_id);

        let (peer, message) = control.initiate_port_closing(port_handle, exit_inside_sync, comp_ctx);

        let peer_info = comp_ctx.get_peer(peer);

        peer_info.handle.send_message_logged(sched_ctx, Message::Control(message), true);

    }

    return CompScheduling::Immediate; // to check if we can shut down immediately

}

/// Handles a component waiting until all peers are notified that it is quitting

/// (i.e. after calling `default_handle_start_exit`).

#[must_use]

pub(crate) fn default_handle_busy_exit(

    exec_state: &mut CompExecState, control: &ControlLayer,

    sched_ctx: &SchedulerCtx

) -> CompScheduling {

    debug_assert_eq!(exec_state.mode, CompMode::BusyExit);

    if control.has_acks_remaining() {

        sched_ctx.info("Component busy exiting, still has `Ack`s remaining");

        return CompScheduling::Sleep;

    } else {

        sched_ctx.info("Component busy exiting, now shutting down");

        exec_state.mode = CompMode::Exit;

        return CompScheduling::Exit;

    }

}

/// Handles a potential synchronous round decision. If there was a decision then

/// the `Some(success)` value indicates whether the round succeeded or not.

/// Might also end up changing the `ExecState`.

///

/// Might be called in two cases:

/// 1. The component is in regular execution mode, at the end of a sync round,

///     and is waiting for a solution to the round.

/// 2. The component has encountered an error during a sync round and is

///     exiting, hence is waiting for a "Failure" message from the leader.

pub(crate) fn default_handle_sync_decision(

    sched_ctx: &SchedulerCtx, exec_state: &mut CompExecState, comp_ctx: &mut CompCtx,

    decision: SyncRoundDecision, consensus: &mut Consensus

) -> Option<bool> {

    let success = match decision {

        SyncRoundDecision::None => return None,

        SyncRoundDecision::Solution => true,

        SyncRoundDecision::Failure => false,

    };

    debug_assert!(

        exec_state.mode == CompMode::SyncEnd || (

            exec_state.mode.is_busy_exiting() && exec_state.exit_reason.is_error()

        ) || (

            exec_state.mode.is_in_sync_block() && decision == SyncRoundDecision::Failure

        )

    );

    sched_ctx.info(&format!("Handling decision {:?} (in mode: {:?})", decision, exec_state.mode));

    consensus.notify_sync_decision(decision);

    if success {

        // We cannot get a success message if the component has encountered an

        // error.

        for port_index in 0..comp_ctx.num_ports() {

            let port_info = comp_ctx.get_port_by_index_mut(port_index);

            if port_info.close_at_sync_end {

                port_info.state.set(PortStateFlag::Closed);

            }

            port_info.state.clear(PortStateFlag::Received);

        }

        debug_assert_eq!(exec_state.mode, CompMode::SyncEnd);

        exec_state.mode = CompMode::NonSync;

        return Some(true);

    } else {

        // We may get failure both in all possible cases. But we should only

        // modify the execution state if we're not already in exit mode

        if !exec_state.mode.is_busy_exiting() {

            sched_ctx.error("failed synchronous round, initiating exit");

            exec_state.set_as_start_exit(ExitReason::ErrorNonSync);

        }

        return Some(false);

    }

}

/// Performs the default action of printing the provided error, and then putting

/// the component in the state where it will shut down. Only to be used for

/// builtin components: their error message construction is simpler (and more

/// common) as they don't have any source code.

pub(crate) fn default_handle_error_for_builtin(

    exec_state: &mut CompExecState, sched_ctx: &SchedulerCtx,

    location_and_message: (PortInstruction, String)

        ExitReason::ErrorNonSync

    };

    exec_state.set_as_start_exit(exit_reason);

}

#[inline]

pub(crate) fn default_handle_exit(_exec_state: &CompExecState) -> CompScheduling {

    debug_assert_eq!(_exec_state.mode, CompMode::Exit);

    return CompScheduling::Exit;

}

// -----------------------------------------------------------------------------

// Internal messaging/state utilities

// -----------------------------------------------------------------------------

/// Sends a message without any transmitted ports. Does not check if sending

/// is actually valid.

fn send_message_without_ports(

    sending_port_handle: LocalPortHandle, value: ValueGroup,

    comp_ctx: &CompCtx, sched_ctx: &SchedulerCtx, consensus: &mut Consensus,

) {

    let port_info = comp_ctx.get_port(sending_port_handle);

    debug_assert!(port_info.state.can_send());

    let peer_handle = comp_ctx.get_peer_handle(port_info.peer_comp_id);

    let peer_info = comp_ctx.get_peer(peer_handle);

    let annotated_message = consensus.annotate_data_message(comp_ctx, port_info, value);

    peer_info.handle.send_message_logged(sched_ctx, Message::Data(annotated_message), true);

}

/// Prepares sending a message that contains ports. Only once a particular

/// protocol has completed (where we notify all the peers that the ports will

/// be transferred) will we actually send the message to the recipient.

fn prepare_send_message_with_ports(

    sending_port_id: PortId, sending_port_instruction: PortInstruction, value: ValueGroup,

    exec_state: &mut CompExecState, comp_ctx: &mut CompCtx, sched_ctx: &SchedulerCtx,

    control: &mut ControlLayer

) -> Result<(), (PortInstruction, String)> {

    debug_assert_eq!(exec_state.mode, CompMode::Sync); // busy in sync, trying to send

    let sending_port_handle = comp_ctx.get_port_handle(sending_port_id);

    let sending_port_info = comp_ctx.get_port_mut(sending_port_handle);

    sending_port_info.last_instruction = sending_port_instruction;

    let mut transmit_ports = Vec::new();

    find_ports_in_value_group(&value, &mut transmit_ports);

    debug_assert!(!transmit_ports.is_empty()); // requisite for calling this function

    // Set up the final Ack that triggers us to send our final message

    let unblock_put_control_id = control.add_unblock_put_with_ports_entry();

    for (_, port_id) in &transmit_ports {

        let transmit_port_handle = comp_ctx.get_port_handle(*port_id);

        let transmit_port_info = comp_ctx.get_port_mut(transmit_port_handle);

        let peer_comp_id = transmit_port_info.peer_comp_id;

        let peer_port_id = transmit_port_info.peer_port_id;

        // Note: we checked earlier that we are currently in sync mode. Now we

        // will check if we've already used the port we're about to transmit.

        if !transmit_port_info.last_instruction.is_none() {

            return Err((

                sending_port_instruction,

                String::from("Cannot transmit one of the ports in this message, as it is used in this sync round")

            ));

        }

        if transmit_port_info.state.is_set(PortStateFlag::Transmitted) {

            return Err((

                sending_port_instruction,

                String::from("Cannot transmit one of the ports in this message, as that port is already transmitted")

            ));

        }

        // Set the flag for transmission

        transmit_port_info.state.set(PortStateFlag::Transmitted);

        // Block the peer of the port

        let message = control.create_port_transfer_message(unblock_put_control_id, comp_ctx.id, peer_port_id);

        println!("DEBUG: Port transfer message\nControl ID: {:?}\nMessage: {:?}", unblock_put_control_id, message);

        let peer_handle = comp_ctx.get_peer_handle(peer_comp_id);

        let peer_info = comp_ctx.get_peer(peer_handle);

        peer_info.handle.send_message_logged(sched_ctx, message, true);

    }

    // We've set up the protocol, once all the PPC's are blocked we are supposed

    // to transfer the message to the recipient. So store it temporarily

    exec_state.set_as_blocked_put_with_ports(sending_port_id, value);

    return Ok(());

}

/// Performs the transmission of a data message that contains ports. These were

/// all stored in the component's execution state by the

/// `prepare_send_message_with_ports` function. Port must be ready to send!

fn perform_send_message_with_ports(

    inbox_main: &mut InboxMain, inbox_backup: &mut InboxBackup

) {

    debug_assert_eq!(exec_state.mode, CompMode::BlockedPutPortsReady);

    // Find all ports again

    let mut transmit_ports = Vec::new();

    find_ports_in_value_group(&exec_state.mode_value, &mut transmit_ports);

    let port_handle = comp_ctx.get_port_handle(exec_state.mode_port);

    let port_info = comp_ctx.get_port(port_handle);

    debug_assert!(port_info.state.can_send() && !port_info.state.is_blocked());

    let peer_handle = comp_ctx.get_peer_handle(port_info.peer_comp_id);

    // Annotate the data message

    let message_value = exec_state.mode_value.take();

    let mut annotated_message = consensus.annotate_data_message(comp_ctx, port_info, message_value);

    // And further enhance the message by adding data about the ports that are

    // being transferred

    for (port_locations, transmit_port_id) in transmit_ports {

        let transmit_port_handle = comp_ctx.get_port_handle(transmit_port_id);

        let transmit_port_info = comp_ctx.get_port(transmit_port_handle);

        let transmit_messages = take_port_messages(comp_ctx, transmit_port_id, inbox_main, inbox_backup);

        let mut transmit_port_state = transmit_port_info.state;

        debug_assert!(transmit_port_state.is_set(PortStateFlag::Transmitted));

        transmit_port_state.clear(PortStateFlag::Transmitted);

        annotated_message.ports.push(TransmittedPort{

            locations: port_locations,

            messages: transmit_messages,

            peer_comp: transmit_port_info.peer_comp_id,

            peer_port: transmit_port_info.peer_port_id,

            kind: transmit_port_info.kind,

            state: transmit_port_state

    }

    // And finally, send the message to the peer

    let peer_info = comp_ctx.get_peer(peer_handle);

    peer_info.handle.send_message_logged(sched_ctx, Message::Data(annotated_message), true);

}

/// Handles an `Ack` for the control layer.

fn default_handle_ack(

    exec_state: &mut CompExecState, control: &mut ControlLayer, control_id: ControlId,

    sched_ctx: &SchedulerCtx, comp_ctx: &mut CompCtx, consensus: &mut Consensus,

    inbox_main: &mut InboxMain, inbox_backup: &mut InboxBackup

) {

    // Since an `Ack` may cause another one, handle them in a loop

    let mut to_ack = control_id;

    loop {

        let (action, new_to_ack) = control.handle_ack(to_ack, sched_ctx, comp_ctx);

        match action {

            AckAction::SendMessage(target_comp, message) => {

                // FIX @NoDirectHandle

                let mut handle = sched_ctx.runtime.get_component_public(target_comp);

                handle.send_message_logged(sched_ctx, Message::Control(message), true);

                let _should_remove = handle.decrement_users();

                debug_assert!(_should_remove.is_none());

            },

            AckAction::ScheduleComponent(to_schedule) => {

                // FIX @NoDirectHandle

                let mut handle = sched_ctx.runtime.get_component_public(to_schedule);

                // Note that the component is intentionally not

                // sleeping, so we just wake it up

                debug_assert!(!handle.sleeping.load(std::sync::atomic::Ordering::Acquire));

                let key = unsafe { to_schedule.upgrade() };

                sched_ctx.runtime.enqueue_work(key);

                let _should_remove = handle.decrement_users();

                debug_assert!(_should_remove.is_none());

            },

            AckAction::UnblockPutWithPorts => {

                // Send the message (containing ports) stored in the component

                // execution state to the recipient

                println!("DEBUG: Unblocking put with ports");

                exec_state.mode = CompMode::BlockedPutPortsReady;

                let port_handle = comp_ctx.get_port_handle(exec_state.mode_port);

                default_handle_recently_unblocked_port(

                    exec_state, consensus, port_handle, sched_ctx, comp_ctx,

                    inbox_main, inbox_backup

                );

            },

            AckAction::None => {}

        }

        match new_to_ack {

            Some(new_to_ack) => to_ack = new_to_ack,

            None => break,

        }

    }

}

/// Little helper for sending the most common kind of `Ack`

fn default_send_ack(

    causer_of_ack_id: ControlId, peer_handle: LocalPeerHandle,

    sched_ctx: &SchedulerCtx, comp_ctx: &CompCtx

) {

    let peer_info = comp_ctx.get_peer(peer_handle);

    peer_info.handle.send_message_logged(sched_ctx, Message::Control(ControlMessage{

        id: causer_of_ack_id,

        sender_comp_id: comp_ctx.id,

        target_port_id: None,

        content: ControlMessageContent::Ack

    }), true);

}

/// Handles the unblocking of a putter port. In case there is a pending message

/// on that port then it will be sent. There are two reasons for calling this

/// function: either a port was blocked (i.e. the Blocked state flag was

/// cleared), or the component is ready to send a message containing ports

/// (stored in the execution state). In this latter case we might still have

/// a blocked port.

fn default_handle_recently_unblocked_port(

    exec_state: &mut CompExecState, consensus: &mut Consensus,

    port_handle: LocalPortHandle, sched_ctx: &SchedulerCtx, comp_ctx: &mut CompCtx,

    inbox_main: &mut InboxMain, inbox_backup: &mut InboxBackup

) {

    let port_info = comp_ctx.get_port_mut(port_handle);

    let port_id = port_info.self_id;

    if port_info.state.is_blocked() {

        // Port is still blocked. We wait until the next control message where

        // we unblock the port.

        return;

    }

    if exec_state.is_blocked_on_put_without_ports(port_id) {

        // Return to the regular execution mode

                }

            },

            EC::Put(expr_id, port_id, value) => {

                debug_assert_eq!(self.exec_state.mode, CompMode::Sync);

                sched_ctx.info(&format!("Putting value {:?}", value));

                // Send the message

                let target_port_id = port_id_from_eval(port_id);

                let send_result = component::default_send_data_message(

                    &mut self.exec_state, target_port_id,

                    PortInstruction::SourceLocation(expr_id), value,

                    sched_ctx, &mut self.consensus, &mut self.control, comp_ctx

                );

                if let Err(location_and_message) = send_result {

                    self.handle_generic_component_error(sched_ctx, location_and_message);

                    return CompScheduling::Immediate;

                } else {

                    // When `run` is called again (potentially after becoming

                    // unblocked) we need to instruct the executor that we performed

                    // the `put`

                    let scheduling = send_result.unwrap();

                    self.exec_ctx.stmt = ExecStmt::PerformedPut;

                    return scheduling;

                }

            },

            EC::SelectStart(num_cases, _num_ports) => {

                debug_assert_eq!(self.exec_state.mode, CompMode::Sync);

                self.select_state.handle_select_start(num_cases);

                return CompScheduling::Requeue;

            },

            EC::SelectRegisterPort(expr_id, case_index, port_index, port_id) => {

                debug_assert_eq!(self.exec_state.mode, CompMode::Sync);

                let port_id = port_id_from_eval(port_id);

                let port_handle = comp_ctx.get_port_handle(port_id);

                // Note: we register the "last_instruction" here already. This

                // way if we get a `ClosePort` message, the condition to fail

                // the synchronous round is satisfied.

                let port_info = comp_ctx.get_port_mut(port_handle);

                port_info.last_instruction = PortInstruction::SourceLocation(expr_id);

                let port_is_closed = port_info.state.is_closed();

                // Register port as part of select guard

                if let Err(_err) = self.select_state.register_select_case_port(comp_ctx, case_index, port_index, port_id) {

                    // Failure occurs if a port is used twice in the same guard

                    let protocol = &sched_ctx.runtime.protocol;

                    self.handle_component_error(sched_ctx, CompError::Executor(EvalError::new_error_at_expr(

                        &self.prompt, &protocol.modules, &protocol.heap, expr_id,

                        String::from("Cannot have the one port appear in the same guard twice")

                    )));

                } else if port_is_closed {

                    // Port is closed

                    let peer_id = comp_ctx.get_port(port_handle).peer_comp_id;

                    let protocol = &sched_ctx.runtime.protocol;

                    self.handle_component_error(sched_ctx, CompError::Executor(EvalError::new_error_at_expr(

                        &self.prompt, &protocol.modules, &protocol.heap, expr_id,

                        format!("Cannot register port, as the peer component (id:{}) has shut down", peer_id.0)

                    )));

                }

                return CompScheduling::Immediate;

            },

            EC::SelectWait => {

                debug_assert_eq!(self.exec_state.mode, CompMode::Sync);

                let select_decision = self.select_state.handle_select_waiting_point(&self.inbox_main, comp_ctx);

                if let SelectDecision::Case(case_index) = select_decision {

                    // Reached a conclusion, so we can continue immediately

                    self.exec_ctx.stmt = ExecStmt::PerformedSelectWait(case_index);

                    self.exec_state.mode = CompMode::Sync;

                    return CompScheduling::Immediate;

                } else {

                    // No decision yet

                    self.exec_state.mode = CompMode::BlockedSelect;

                    return CompScheduling::Sleep;

                }

            },

            // Results that can be returned outside of sync mode

            EC::ComponentTerminated => {

                self.exec_state.set_as_start_exit(ExitReason::Termination);

                return CompScheduling::Immediate;

            },

            EC::SyncBlockStart => {

                component::default_handle_sync_start(

                    &mut self.exec_state, &mut self.inbox_main, sched_ctx, comp_ctx, &mut self.consensus

                );

                return CompScheduling::Immediate;

            },

            EC::NewComponent(definition_id, type_id, arguments) => {

                debug_assert_eq!(self.exec_state.mode, CompMode::NonSync);

                self.create_component_and_transfer_ports(

                    sched_ctx, comp_ctx,

                    definition_id, type_id, arguments

                );

                return CompScheduling::Requeue;

            },

            EC::NewChannel => {

                debug_assert!(self.exec_ctx.stmt.is_none());

                let channel = comp_ctx.create_channel();

                self.exec_ctx.stmt = ExecStmt::CreatedChannel((

                    Value::Output(port_id_to_eval(channel.putter_id)),

                    Value::Input(port_id_to_eval(channel.getter_id))

                ));

                self.inbox_main.push(None);

                self.inbox_main.push(None);

                return CompScheduling::Immediate;

            }

        }

    }

}

impl CompPDL {

    pub(crate) fn new(initial_state: Prompt, num_ports: usize) -> Self {

        let mut inbox_main = Vec::new();

        inbox_main.reserve(num_ports);

        for _ in 0..num_ports {

            inbox_main.push(None);

        }

        return Self{

            exec_state: CompExecState::new(),

            select_state: SelectState::new(),

            prompt: initial_state,

            control: ControlLayer::default(),

            consensus: Consensus::new(),

            sync_counter: 0,

            exec_ctx: ExecCtx{

                stmt: ExecStmt::None,

            },

            inbox_main,

            inbox_backup: Vec::new(),

        }

    }

    // -------------------------------------------------------------------------

    // Running component and handling changes in global component state

    // -------------------------------------------------------------------------

    fn execute_prompt(&mut self, sched_ctx: &SchedulerCtx) -> EvalResult {

        let mut step_result = EvalContinuation::Stepping;

        while let EvalContinuation::Stepping = step_result {

            step_result = self.prompt.step(

                &sched_ctx.runtime.protocol.types, &sched_ctx.runtime.protocol.heap,

                &sched_ctx.runtime.protocol.modules, &mut self.exec_ctx,

            )?;

        }

        return Ok(step_result)

    }

    // -------------------------------------------------------------------------

    // Handling messages

    // -------------------------------------------------------------------------

    /// Handles a message that came in through the public inbox. This function

    /// will handle putting it in the correct place, and potentially blocking

    /// the port in case too many messages are being received.

    fn handle_incoming_data_message(&mut self, sched_ctx: &SchedulerCtx, comp_ctx: &mut CompCtx, message: DataMessage) {

        use component::IncomingData;

        // Whatever we do, glean information from headers in message

        if self.exec_state.mode.is_in_sync_block() {

            self.consensus.handle_incoming_data_message(comp_ctx, &message);

        }

        match component::default_handle_incoming_data_message(

            &mut self.exec_state, &mut self.inbox_main, comp_ctx, message,

            sched_ctx, &mut self.control

        ) {

            IncomingData::PlacedInSlot => {

                if self.exec_state.mode == CompMode::BlockedSelect {

                    let select_decision = self.select_state.handle_updated_inbox(&self.inbox_main, comp_ctx);

                    if let SelectDecision::Case(case_index) = select_decision {

                        self.exec_ctx.stmt = ExecStmt::PerformedSelectWait(case_index);

                        self.exec_state.mode = CompMode::Sync;

                    }

                }

            },

            IncomingData::SlotFull(message) => {

                self.inbox_backup.push(message);

            }

        }

    }

    fn handle_incoming_sync_message(&mut self, sched_ctx: &SchedulerCtx, comp_ctx: &mut CompCtx, message: SyncMessage) {

        let decision = self.consensus.receive_sync_message(sched_ctx, comp_ctx, message);

        component::default_handle_sync_decision(sched_ctx, &mut self.exec_state, comp_ctx, decision, &mut self.consensus);

    }

    /// Handles an error coming from the generic `component::handle_xxx`

    /// functions. Hence accepts argument as a tuple.

    fn handle_generic_component_error(&mut self, sched_ctx: &SchedulerCtx, location_and_message: (PortInstruction, String)) {

        // Retrieve location and message, display in terminal

use super::*;

#[test]

fn test_transfer_precreated_port_with_owned_peer() {

    compile_and_create_component("

    primitive port_sender(out<in<u32>> tx) {

        channel a -> b;

        sync put(tx, b);

    }

    primitive port_receiver(in<in<u32>> rx) {

        sync auto a = get(rx);

    }

    composite constructor() {

        channel a -> b;

        new port_sender(a);

        new port_receiver(b);

    }

    ", "constructor", no_args());

}