let port_info = comp_ctx.get_port_mut(port_handle);

    port_info.last_instruction = port_instruction;

    let port_info = comp_ctx.get_port(port_handle);

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

    if port_info.state == PortState::Closed {

        // Note: normally peer is eventually consistent, but if it has shut down

        // then we can be sure it is consistent (I think?)

        return Err((

            port_info.last_instruction,

            format!("Cannot send on this port, as the peer (id:{}) has shut down", port_info.peer_comp_id.0)

        ))

    } else if port_info.state.is_blocked() {

        // Port is blocked, so we cannot send

        exec_state.set_as_blocked_put(transmitting_port_id, value);

        return Ok(CompScheduling::Sleep);

    } else {

        // Port is not blocked, so send to the peer

        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(&sched_ctx.runtime, Message::Data(annotated_message), true);

        return Ok(CompScheduling::Immediate);

    }

}

pub(crate) enum IncomingData {

    PlacedInSlot,

    SlotFull(DataMessage),

}

/// Default handling of receiving a data message. In case there is no room for

/// the message it is returned from this function. Note that this function is

/// different from PDL code performing a `get` on a port; this is the case where

/// the message first arrives at the component.

// NOTE: This is supposed to be a somewhat temporary implementation. It would be

//  nicest if the sending component can figure out it cannot send any more data.

#[must_use]

pub(crate) fn default_handle_incoming_data_message(

    exec_state: &mut CompExecState, inbox_main: &mut InboxMain,

    comp_ctx: &mut CompCtx, incoming_message: DataMessage,

    sched_ctx: &SchedulerCtx, control: &mut ControlLayer

) -> IncomingData {

    let port_handle = comp_ctx.get_port_handle(incoming_message.data_header.target_port);

    let port_index = comp_ctx.get_port_index(port_handle);

    comp_ctx.get_port_mut(port_handle).received_message_for_sync = true;

    let port_value_slot = &mut inbox_main[port_index];

    let target_port_id = incoming_message.data_header.target_port;

    if port_value_slot.is_none() {

        // We can put the value in the slot

        *port_value_slot = Some(incoming_message);

        // Check if we're blocked on receiving this message.

        dbg_code!({

            // Our port cannot have been blocked itself, because we're able to

            // directly insert the message into its slot.

            assert!(!comp_ctx.get_port(port_handle).state.is_blocked());

        });

        if exec_state.is_blocked_on_get(target_port_id) {

            // Return to normal operation

            exec_state.mode = CompMode::Sync;

            exec_state.mode_port = PortId::new_invalid();

            debug_assert!(exec_state.mode_value.values.is_empty());

        }

        return IncomingData::PlacedInSlot

    } else {

        // Slot is already full, so if the port was previously opened, it will

        // now become closed

        let port_info = comp_ctx.get_port_mut(port_handle);

        debug_assert!(port_info.state == PortState::Open || port_info.state.is_blocked()); // i.e. not closed, but will go off if more states are added in the future

        if port_info.state == PortState::Open {

            comp_ctx.set_port_state(port_handle, PortState::BlockedDueToFullBuffers);

            let (peer_handle, message) =

                control.initiate_port_blocking(comp_ctx, port_handle);

            let peer = comp_ctx.get_peer(peer_handle);

            peer.handle.send_message(&sched_ctx.runtime, Message::Control(message), true);

        }

        return IncomingData::SlotFull(incoming_message)

    }

}

pub(crate) enum GetResult {

    Received(DataMessage),

    NoMessage,

    Error((PortInstruction, String)),

}

/// Default attempt at trying to receive from a port (i.e. through a `get`, or

/// the equivalent operation for a builtin component). `target_port` is the port

/// we're trying to receive from, and the `target_port_instruction` is the

/// instruction we're attempting on this port.

pub(crate) fn default_attempt_get(

    exec_state: &mut CompExecState, target_port: PortId, target_port_instruction: PortInstruction,

    inbox_main: &mut InboxMainRef, inbox_backup: &mut InboxBackup, sched_ctx: &SchedulerCtx,

    comp_ctx: &mut CompCtx, control: &mut ControlLayer, consensus: &mut Consensus

) -> GetResult {

    let port_handle = comp_ctx.get_port_handle(target_port);

    let port_index = comp_ctx.get_port_index(port_handle);

    if let Some(message) = &inbox_main[port_index] {

        if consensus.try_receive_data_message(sched_ctx, comp_ctx, message) {

            // We're allowed to receive this message

            let message = inbox_main[port_index].take().unwrap();

            debug_assert_eq!(target_port, message.data_header.target_port);

            // Note: we can still run into an unrecoverable error when actually

            // receiving this message

            match default_handle_received_data_message(

                target_port, target_port_instruction, inbox_main, inbox_backup,

                comp_ctx, sched_ctx, control,

            ) {

                Ok(()) => return GetResult::Received(message),

                Err(location_and_message) => return GetResult::Error(location_and_message)

            }

        } else {

            // We're not allowed to receive this message. This means that the

            // receiver is attempting to receive something out of order with

            // respect to the sender.

            return GetResult::Error((target_port_instruction, String::from(

                "Cannot get from this port, as this causes a deadlock. This happens if you `get` in a different order as another component `put`s"

            )));

        }

    } else {

        exec_state.set_as_blocked_get(target_port);

        return GetResult::NoMessage;

    }

}

/// Default handling that has been received through a `get`. Will check if any

/// more messages are waiting, and if the corresponding port was blocked because

/// of full buffers (hence, will use the control layer to make sure the peer

/// will become unblocked).

pub(crate) fn default_handle_received_data_message(

    targeted_port: PortId, port_instruction: PortInstruction,

    inbox_main: &mut InboxMainRef, inbox_backup: &mut InboxBackup,

    comp_ctx: &mut CompCtx, sched_ctx: &SchedulerCtx, control: &mut ControlLayer

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

    let port_handle = comp_ctx.get_port_handle(targeted_port);

    let port_index = comp_ctx.get_port_index(port_handle);

    let slot = &mut inbox_main[port_index];

    debug_assert!(slot.is_none()); // because we've just received from it

    // Modify last-known location where port instruction was retrieved

    let port_info = comp_ctx.get_port_mut(port_handle);

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

    let port_info = comp_ctx.get_port(port_handle);

    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!(port_info.state.is_blocked()); // since we're removing another message from the backup

            *slot = 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 == PortState::BlockedDueToFullBuffers {

        comp_ctx.set_port_state(port_handle, PortState::Open);

        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(&sched_ctx.runtime, 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

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

    match message.content {

        ControlMessageContent::Ack => {

            default_handle_ack(control, message.id, sched_ctx, comp_ctx);

        },

        ControlMessageContent::BlockPort(port_id) => {

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

            // blocked.

            let port_handle = comp_ctx.get_port_handle(port_id);

            let port_info = comp_ctx.get_port(port_handle);

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

            if port_info.state == PortState::Open {

                // only when open: we don't do this when closed, and we we don't do this if we're blocked due to peer changes

                comp_ctx.set_port_state(port_handle, PortState::BlockedDueToFullBuffers);

            }

        },

        ControlMessageContent::ClosePort(content) => {

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

            // the component handle as well

            let port_handle = comp_ctx.get_port_handle(content.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 port_info = comp_ctx.get_port(port_handle);

            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(control, control_id, sched_ctx, comp_ctx);

            } else {

                // Respond to the message

                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.remove_peer(sched_ctx, port_handle, peer_comp_id, false); // do not remove if closed

                // 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;

                    let closed_before_sync_round = !content.closed_in_sync_round && !port_has_had_message;

                    if closed_during_sync_round || closed_before_sync_round {

                        return Err((

                            last_instruction,

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

                        ));

                    }

                } else {

                    comp_ctx.set_port_state(port_handle, PortState::Closed);

                }

            }

        },

        ControlMessageContent::UnblockPort(port_id) => {

            // We were previously blocked (or already closed)

            let port_handle = comp_ctx.get_port_handle(port_id);

            let port_info = comp_ctx.get_port(port_handle);

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

            if port_info.state == PortState::BlockedDueToFullBuffers {

                default_handle_unblock_put(exec_state, consensus, port_handle, sched_ctx, comp_ctx);

            }

        },

        ControlMessageContent::PortPeerChangedBlock(port_id) => {

            // 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.

            debug_assert_eq!(message.target_port_id, Some(port_id));

            let port_handle = comp_ctx.get_port_handle(port_id);

            comp_ctx.set_port_state(port_handle, PortState::BlockedDueToPeerChange);

            let port_info = comp_ctx.get_port(port_handle);

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

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

        },

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

            let port_handle = comp_ctx.get_port_handle(message.target_port_id.unwrap());

            let port_info = comp_ctx.get_port(port_handle);

            debug_assert!(port_info.state == PortState::BlockedDueToPeerChange);

            let old_peer_id = port_info.peer_comp_id;

            comp_ctx.remove_peer(sched_ctx, port_handle, old_peer_id, false);

            let port_info = comp_ctx.get_port_mut(port_handle);

            port_info.peer_comp_id = new_comp_id;

/// 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.log("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.log("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);

    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);

        if port.state == PortState::Closed || port.close_at_sync_end {

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

            continue;

        }

        // Mark as closed

        let port_id = port.self_id;

        port.state = PortState::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(&sched_ctx.runtime, 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.log("Component busy exiting, still has `Ack`s remaining");

        return CompScheduling::Sleep;

    } else {

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

        exec_state.mode = CompMode::Exit;

        return CompScheduling::Exit;

    }

                ));

                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

        let (location, message) = location_and_message;

        let error = match location {

            PortInstruction::None => CompError::Component(message),

            PortInstruction::NoSource => unreachable!(), // for debugging: all in-sync errors are associated with a source location

            PortInstruction::SourceLocation(expression_id) => {