cfdp/

dest.rs

//! # CFDP Destination Entity Module
//!
//! The [DestinationHandler] is the primary component of this module which converts the PDUs sent
//! from a remote source entity back to a file. A file copy operation on the receiver side
//! is started with the reception of a Metadata PDU, for example one generated by the
//! [spacepackets::cfdp::pdu::metadata::MetadataPduCreator]. After that, file packet PDUs, for
//! example generated with the [spacepackets::cfdp::pdu::file_data] module, can be inserted into
//! the destination handler and will be assembled into a file.
//!
//! A destination entity might still generate packets which need to be sent back to the source
//! entity of the file transfer. However, this handler allows freedom of communication like the
//! source entity by using a user-provided [PduSender] instance to send all generated PDUs.
//!
//! The transaction will be finished when following conditions are met:
//!
//!  1. A valid EOF PDU, for example one generated by the [spacepackets::cfdp::pdu::eof::EofPdu]
//!     helper, has been inserted into the class.
//!  2. The file checksum verification has been successful. If this is not the case for the
//!     unacknowledged mode, the handler will re-attempt the checksum calculation up to a certain
//!     threshold called the check limit. If the threshold is reached, the transaction will
//!     finish with a failure.
//!
//! ### Unacknowledged mode with closure
//!
//!  3. Finished PDU has been sent back to the remote side.
//!
//! ### Acknowledged mode
//!
//!  3. An EOF ACK PDU has been sent back to the remote side.
//!  4. A Finished PDU has been sent back to the remote side.
//!  5. A Finished PDU ACK was received.
use crate::{
    DummyPduProvider, FaultInfo, GenericSendError, IndicationConfig, PduProvider,
    PositiveAckParams,
    lost_segments::{LostSegmentError, LostSegmentStore},
    user::TransactionFinishedParams,
};
use core::{
    cell::{Cell, RefCell},
    str::{Utf8Error, from_utf8, from_utf8_unchecked},
};

use super::{
    Countdown, EntityType, LocalEntityConfig, PacketTarget, PduSender, RemoteConfigStore,
    RemoteEntityConfig, State, TimerContext, TimerCreator, TransactionId, UserFaultHook,
    filestore::{FilestoreError, VirtualFilestore},
    user::{CfdpUser, FileSegmentRecvdParams, MetadataReceivedParams},
};
use smallvec::SmallVec;
use spacepackets::{
    cfdp::{
        ChecksumType, ConditionCode, FaultHandlerCode, LargeFileFlag, PduType, TransactionStatus,
        TransmissionMode,
        pdu::{
            CfdpPdu, CommonPduConfig, FileDirectiveType, PduError, PduHeader,
            ack::AckPdu,
            eof::EofPdu,
            file_data::FileDataPdu,
            finished::{DeliveryCode, FileStatus, FinishedPduCreator},
            metadata::{MetadataGenericParams, MetadataPduReader},
            nak::{NakPduCreator, NakPduCreatorWithReservedSeqReqsBuf},
        },
        tlv::{EntityIdTlv, GenericTlv, ReadableTlv, TlvType, msg_to_user::MsgToUserTlv},
    },
    util::{UnsignedByteField, UnsignedEnum},
};

#[derive(Debug)]
struct FileNames {
    src_file_name: [u8; u8::MAX as usize],
    src_file_name_len: usize,
    dest_file_name: [u8; u8::MAX as usize],
    dest_file_name_len: usize,
    dest_path_buf: [u8; u8::MAX as usize * 2],
    dest_file_path_len: usize,
}

#[derive(Debug, Default, Clone, Copy)]
pub struct AnomalyTracker {
    invalid_ack_directive_code: u8,
    lost_segment_errors: u8,
}

impl AnomalyTracker {
    #[inline]
    pub fn invalid_ack_directive_code(&mut self) -> u8 {
        self.invalid_ack_directive_code
    }

    #[inline]
    pub fn lost_segment_errors(&mut self) -> u8 {
        self.lost_segment_errors
    }

    #[inline]
    pub fn increment_lost_segment_errors(&mut self) {
        self.lost_segment_errors = self.lost_segment_errors.wrapping_add(1);
    }

    #[inline]
    pub fn increment_invalid_ack_directive_code(&mut self) {
        self.invalid_ack_directive_code = self.invalid_ack_directive_code.wrapping_add(1);
    }

    #[inline]
    pub fn reset(&mut self) {
        *self = Default::default();
    }
}

#[derive(Debug, PartialEq, Eq, Copy, Clone)]
enum CompletionDisposition {
    Completed = 0,
    Cancelled = 1,
}

/// This enumeration models the different transaction steps of the destination entity handler.
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
pub enum TransactionStep {
    Idle,
    TransactionStart,
    /// Special state which is only used for acknowledged mode. The CFDP entity is still waiting
    /// for a missing metadata PDU to be re-sent. Until then, all arriving file data PDUs will only
    /// update the internal lost segment tracker. When the EOF PDU arrives, the state will be left.
    /// Please note that deferred lost segment handling might also be active when this state is set.
    WaitingForMetadata,
    ReceivingFileDataPdus,
    /// This is the check timer step as specified in chapter 4.6.3.3 b) of the standard.
    /// The destination handler will still check for file data PDUs which might lead to a full
    /// file transfer completion.
    ReceivingFileDataPdusWithCheckLimitHandling,
    WaitingForMissingData,
    //SendingAckPdu,
    TransferCompletion,
    WaitingForFinishedAck,
}

#[derive(Debug)]
struct FinishedParams {
    condition_code: Cell<ConditionCode>,
    delivery_code: Cell<DeliveryCode>,
    fault_location_finished: Cell<Option<EntityIdTlv>>,
    file_status: FileStatus,
}

impl Default for FinishedParams {
    fn default() -> Self {
        Self {
            condition_code: Cell::new(ConditionCode::NoError),
            delivery_code: Cell::new(DeliveryCode::Incomplete),
            fault_location_finished: Cell::new(None),
            file_status: FileStatus::Unreported,
        }
    }
}

impl FinishedParams {
    pub fn reset(&mut self) {
        self.condition_code.set(ConditionCode::NoError);
        self.delivery_code.set(DeliveryCode::Incomplete);
        self.file_status = FileStatus::Unreported;
    }
}

#[derive(Debug, Copy, Clone)]
pub struct AcknowledgedModeParams {
    last_start_offset: u64,
    last_end_offset: u64,
    metadata_missing: bool,
    nak_activity_counter: u32,
    deferred_procedure_active: bool,
}

// This contains parameters for destination transaction.
#[derive(Debug)]
struct TransactionParams<CountdownInstance: Countdown> {
    pdu_conf: CommonPduConfig,
    file_names: FileNames,
    msgs_to_user_size: usize,
    // TODO: Should we make this configurable?
    msgs_to_user_buf: [u8; 1024],
    remote_cfg: Option<RemoteEntityConfig>,
    transaction_id: Option<TransactionId>,
    metadata_params: MetadataGenericParams,
    file_size: u64,
    progress: u64,
    acked_params: Option<AcknowledgedModeParams>,
    deferred_procedure_timer: Option<CountdownInstance>,
    finished_params: FinishedParams,
    positive_ack_params: Option<PositiveAckParams>,
    ack_timer: Option<CountdownInstance>,
    metadata_only: bool,
    completion_disposition: Cell<CompletionDisposition>,
    checksum: u32,
    anomaly_tracker: AnomalyTracker,
    current_check_count: u32,
    current_check_timer: Option<CountdownInstance>,
}

impl<CheckTimer: Countdown> TransactionParams<CheckTimer> {
    fn transmission_mode(&self) -> TransmissionMode {
        self.pdu_conf.trans_mode
    }
}

impl Default for FileNames {
    fn default() -> Self {
        Self {
            src_file_name: [0; u8::MAX as usize],
            src_file_name_len: Default::default(),
            dest_file_name: [0; u8::MAX as usize],
            dest_file_name_len: Default::default(),
            dest_path_buf: [0; u8::MAX as usize * 2],
            dest_file_path_len: Default::default(),
        }
    }
}

impl<CheckTimer: Countdown> TransactionParams<CheckTimer> {
    fn file_size(&self) -> u64 {
        self.metadata_params.file_size
    }

    fn metadata_params(&self) -> &MetadataGenericParams {
        &self.metadata_params
    }
}

impl<CheckTimer: Countdown> Default for TransactionParams<CheckTimer> {
    fn default() -> Self {
        Self {
            pdu_conf: Default::default(),
            msgs_to_user_size: 0,
            file_size: 0,
            msgs_to_user_buf: [0; 1024],
            file_names: Default::default(),
            remote_cfg: None,
            transaction_id: None,
            metadata_params: Default::default(),
            progress: Default::default(),
            deferred_procedure_timer: None,
            acked_params: None,
            metadata_only: false,
            positive_ack_params: None,
            ack_timer: None,
            finished_params: FinishedParams::default(),
            completion_disposition: Cell::new(CompletionDisposition::Completed),
            checksum: 0,
            current_check_count: 0,
            anomaly_tracker: AnomalyTracker::default(),
            current_check_timer: None,
        }
    }
}

impl<CheckTimer: Countdown> TransactionParams<CheckTimer> {
    fn reset(&mut self) {
        self.finished_params.reset();
        self.anomaly_tracker.reset();
    }
}

#[derive(Debug, thiserror::Error)]
#[non_exhaustive]
pub enum DestError {
    /// File directive expected, but none specified
    #[error("expected file directive")]
    DirectiveFieldEmpty,
    #[error("can not process packet type {pdu_type:?} with directive type {directive_type:?}")]
    CantProcessPacketType {
        pdu_type: PduType,
        directive_type: Option<FileDirectiveType>,
    },
    #[error("first packet must be metadata PDU for unacknowledged transfers")]
    FirstPacketNotMetadata,
    #[error(
        "can not process PDU with type {pdu_type:?} and directive field {file_directive_type:?} in \
        current transaction step {step:?}"
    )]
    WrongStepForPdu {
        step: TransactionStep,
        pdu_type: PduType,
        file_directive_type: Option<FileDirectiveType>,
    },
    // Received new metadata PDU while being already being busy with a file transfer.
    #[error("busy with transfer")]
    RecvdMetadataButIsBusy,
    #[error("empty source file field")]
    EmptySrcFileField,
    #[error("empty dest file field")]
    EmptyDestFileField,
    #[error("pdu error {0}")]
    Pdu(#[from] PduError),
    #[error("io error {0}")]
    #[cfg(feature = "std")]
    Io(#[from] std::io::Error),
    #[error("file store error: {0}")]
    Filestore(#[from] FilestoreError),
    #[error("unexpected large file size which is not supported for current transfer")]
    UnexpectedLargeFileSize,
    #[error("lost segment error: {0}")]
    LostSegmentError(#[from] LostSegmentError),
    #[error("path conversion error {0}")]
    PathConversion(#[from] Utf8Error),
    #[error("error building dest path from source file name and dest folder")]
    PathConcat,
    #[error("no remote entity configuration found for {0:?}")]
    NoRemoteConfigFound(UnsignedByteField),
    #[error("issue sending PDU: {0}")]
    SendError(#[from] GenericSendError),
    #[error("invalid remote entity configuration: {0:?}")]
    InvalidRemoteConfig(RemoteEntityConfig),
    #[error("cfdp feature not implemented")]
    NotImplemented,
}

/// This is the primary CFDP destination handler. It models the CFDP destination entity, which is
/// primarily responsible for receiving files sent from another CFDP entity. It performs the
/// reception side of File Copy Operations.
///
/// The [DestinationHandler::state_machine] function is the primary function to drive the
/// destination handler. It can be used to insert packets into the destination
/// handler and driving the state machine, which might generate new packets to be sent to the
/// remote entity. Please note that the destination handler can also only process Metadata, EOF and
/// Prompt PDUs in addition to ACK PDUs where the acknowledged PDU is the Finished PDU.
/// All generated packets are sent using the user provided [PduSender].
///
///
/// The handler has an internal buffer for PDU generation and checksum generation. The size of this
/// buffer is select via Cargo features and defaults to 2048 bytes. It does not allocate
/// memory during run-time and thus is suitable for embedded systems where  allocation is
/// not possible.  Furthermore, it uses the [VirtualFilestore] abstraction to allow
/// usage on systems without a [std] filesystem.
///
/// This handler is able to deal with file copy operations to directories, similarly to how the
/// UNIX tool `cp` works. If the destination path is a directory instead of a regular  full path,
/// the source path base file name will be appended to the destination path to form the resulting
/// new full path.
///
// This handler also does not support concurrency out of the box but is flexible enough to be used
/// in different concurrent contexts. For example, you can dynamically create new handlers and
/// run them inside a thread pool, or move the newly created handler to a new thread."""
pub struct DestinationHandler<
    PduSenderInstance: PduSender,
    UserFaultHookInstance: UserFaultHook,
    VirtualFileStoreInstance: VirtualFilestore,
    RemoteConfigStoreInstance: RemoteConfigStore,
    TimerCreatorInstance: TimerCreator<Countdown = CountdownInstance>,
    CountdownInstance: Countdown,
    LostSegmentTracker: LostSegmentStore,
> {
    local_cfg: LocalEntityConfig<UserFaultHookInstance>,
    step: core::cell::Cell<TransactionStep>,
    state: State,
    transaction_params: TransactionParams<CountdownInstance>,
    pdu_and_cksum_buffer: RefCell<[u8; crate::buf_len::PACKET_BUF_LEN]>,
    pub pdu_sender: PduSenderInstance,
    pub vfs: VirtualFileStoreInstance,
    pub remote_cfg_table: RemoteConfigStoreInstance,
    pub check_timer_creator: TimerCreatorInstance,
    lost_segment_tracker: LostSegmentTracker,
}

#[cfg(feature = "std")]
pub type StdDestinationHandler<PduSender, UserFaultHook> = DestinationHandler<
    PduSender,
    UserFaultHook,
    crate::filestore::NativeFilestore,
    crate::RemoteConfigStoreStd,
    crate::StdTimerCreator,
    crate::StdCountdown,
    crate::lost_segments::LostSegmentsList,
>;

#[cfg(feature = "std")]
impl<PduSenderInstance: PduSender, UserFaultHookInstance: UserFaultHook>
    StdDestinationHandler<PduSenderInstance, UserFaultHookInstance>
{
    #[cfg(feature = "std")]
    pub fn new_std(
        local_cfg: LocalEntityConfig<UserFaultHookInstance>,
        pdu_sender: PduSenderInstance,
    ) -> Self {
        Self::new(
            local_cfg,
            pdu_sender,
            crate::filestore::NativeFilestore::default(),
            crate::RemoteConfigStoreStd::default(),
            crate::StdTimerCreator::default(),
            crate::lost_segments::LostSegmentsList::default(),
        )
    }
}

impl<
    PduSenderInstance: PduSender,
    UserFaultHookInstance: UserFaultHook,
    VirtualFilestoreInstance: VirtualFilestore,
    RemoteConfigStoreInstance: RemoteConfigStore,
    TimerCreatorInstance: TimerCreator<Countdown = CountdownInstance>,
    CountdownInstance: Countdown,
    LostSegmentTracker: LostSegmentStore,
>
    DestinationHandler<
        PduSenderInstance,
        UserFaultHookInstance,
        VirtualFilestoreInstance,
        RemoteConfigStoreInstance,
        TimerCreatorInstance,
        CountdownInstance,
        LostSegmentTracker,
    >
{
    /// Constructs a new destination handler.
    ///
    /// # Arguments
    ///
    /// * `local_cfg` - The local CFDP entity configuration.
    ///
    /// * `pdu_and_cksum_buf_size` - The handler requires a buffer to generate PDUs and perform
    ///   checksum calculations. This parameter can either be a known upper bound for the packet
    ///   size, for example 2048 or 4096 bytes.
    ///   It can also specifically be determined by the largest packet size parameter of all
    ///   remote entity configurations in the passed `remote_cfg_table`.
    /// * `pdu_sender` - [PduSender] used to send generated PDU packets.
    /// * `vfs` - [VirtualFilestore] implementation used by the handler, which decouples the CFDP
    ///   implementation from the underlying filestore/filesystem. This allows to use this handler
    ///   for embedded systems where a standard runtime might not be available.
    /// * `remote_cfg_table` - The [RemoteConfigStore] used to look up remote
    ///   entities and target specific configuration for file copy operations.
    /// * `check_timer_creator` - [TimerCreator] used by the CFDP handler to generate
    ///   timers required by various tasks. This allows to use this handler for embedded systems
    ///   where the standard time APIs might not be available.
    pub fn new(
        local_cfg: LocalEntityConfig<UserFaultHookInstance>,
        pdu_sender: PduSenderInstance,
        vfs: VirtualFilestoreInstance,
        remote_cfg_table: RemoteConfigStoreInstance,
        timer_creator: TimerCreatorInstance,
        lost_segment_tracker: LostSegmentTracker,
    ) -> Self {
        Self {
            local_cfg,
            step: Cell::new(TransactionStep::Idle),
            state: State::Idle,
            transaction_params: Default::default(),
            pdu_and_cksum_buffer: core::cell::RefCell::new([0; crate::buf_len::PACKET_BUF_LEN]),
            pdu_sender,
            vfs,
            remote_cfg_table,
            check_timer_creator: timer_creator,
            lost_segment_tracker,
        }
    }

    pub fn state_machine_no_packet(
        &mut self,
        cfdp_user: &mut impl CfdpUser,
    ) -> Result<u32, DestError> {
        self.state_machine(cfdp_user, None::<&DummyPduProvider>)
    }

    /// This is the core function to drive the destination handler. It is also used to insert
    /// packets into the destination handler.
    ///
    /// The state machine should either be called if a packet with the appropriate destination ID
    /// is received and periodically to perform all CFDP related tasks, for example
    /// checking for timeouts or missed file segments.
    ///
    /// The function returns the number of sent PDU packets on success.
    pub fn state_machine(
        &mut self,
        cfdp_user: &mut impl CfdpUser,
        packet_to_insert: Option<&impl PduProvider>,
    ) -> Result<u32, DestError> {
        let mut sent_packets = 0;
        if let Some(packet) = packet_to_insert {
            sent_packets += self.insert_packet(cfdp_user, packet)?;
        }
        match self.state {
            State::Idle => {
                // TODO: In acknowledged mode, add timer handling.
            }
            State::Busy => {
                sent_packets += self.fsm_busy(cfdp_user)?;
            }
            State::Suspended => {
                // There is now way to suspend the handler currently anyway.
            }
        }
        Ok(sent_packets)
    }

    /// This function models the Cancel.request CFDP primitive and is the recommended way
    /// to cancel a transaction. It will cause a Notice Of Cancellation at this entity.
    /// Please note that the state machine might still be active because a cancelled transfer
    /// might still require some packets to be sent to the remote sender entity.
    ///
    /// If no unexpected errors occur, this function returns whether the current transfer
    /// was cancelled. It returns [false] if the state machine is currently idle or if there
    /// is a transaction ID missmatch.
    pub fn cancel_request(&mut self, transaction_id: &TransactionId) -> bool {
        if self.state() == super::State::Idle {
            return false;
        }
        if let Some(active_id) = self.transaction_id() {
            if active_id == *transaction_id {
                self.trigger_notice_of_completion_cancelled(
                    ConditionCode::CancelRequestReceived,
                    EntityIdTlv::new(self.local_cfg.id),
                );

                self.set_step(TransactionStep::TransferCompletion);
                return true;
            }
        }
        false
    }

    /// Returns [None] if the state machine is IDLE, and the transmission mode of the current
    /// request otherwise.
    #[inline]
    pub fn transmission_mode(&self) -> Option<TransmissionMode> {
        if self.state == State::Idle {
            return None;
        }
        Some(self.transaction_params.transmission_mode())
    }

    #[inline]
    pub fn transaction_id(&self) -> Option<TransactionId> {
        self.transaction_params.transaction_id
    }

    /// Get the step, which denotes the exact step of a pending CFDP transaction when applicable.
    #[inline]
    pub fn step(&self) -> TransactionStep {
        self.step.get()
    }

    /// Get the step, which denotes whether the CFDP handler is active, and which CFDP class
    /// is used if it is active.
    #[inline]
    pub fn state(&self) -> State {
        self.state
    }

    #[inline]
    pub fn local_cfg(&self) -> &LocalEntityConfig<UserFaultHookInstance> {
        &self.local_cfg
    }

    #[inline]
    pub fn local_cfg_mut(&mut self) -> &mut LocalEntityConfig<UserFaultHookInstance> {
        &mut self.local_cfg
    }

    /// This function is public to allow completely resetting the handler, but it is explicitely
    /// discouraged to do this. CFDP has mechanism to detect issues and errors on itself.
    /// Resetting the handler might interfere with these mechanisms and lead to unexpected
    /// behaviour.
    pub fn reset(&mut self) {
        self.set_step(TransactionStep::Idle);
        self.state = State::Idle;
        self.transaction_params.reset();
    }

    fn insert_packet(
        &mut self,
        cfdp_user: &mut impl CfdpUser,
        packet_to_insert: &impl PduProvider,
    ) -> Result<u32, DestError> {
        let mut sent_packets = 0;
        if packet_to_insert.packet_target()? != PacketTarget::DestEntity {
            // Unwrap is okay here, a PacketInfo for a file data PDU should always have the
            // destination as the target.
            return Err(DestError::CantProcessPacketType {
                pdu_type: packet_to_insert.pdu_type(),
                directive_type: packet_to_insert.file_directive_type(),
            });
        }
        match packet_to_insert.pdu_type() {
            PduType::FileDirective => {
                if packet_to_insert.file_directive_type().is_none() {
                    return Err(DestError::DirectiveFieldEmpty);
                }
                sent_packets += self.handle_file_directive(
                    cfdp_user,
                    packet_to_insert.file_directive_type().unwrap(),
                    packet_to_insert.raw_pdu(),
                )?;
            }
            PduType::FileData => {
                let fd_pdu = FileDataPdu::from_bytes(packet_to_insert.raw_pdu())?;
                sent_packets += self.handle_file_data(cfdp_user, fd_pdu)?;
            }
        }
        Ok(sent_packets)
    }

    fn handle_file_directive(
        &mut self,
        cfdp_user: &mut impl CfdpUser,
        pdu_directive: FileDirectiveType,
        raw_packet: &[u8],
    ) -> Result<u32, DestError> {
        let mut sent_packets = 0;
        match pdu_directive {
            FileDirectiveType::EofPdu => {
                let eof_pdu = EofPdu::from_bytes(raw_packet)?;
                sent_packets += self.handle_eof_pdu(cfdp_user, eof_pdu)?
            }
            FileDirectiveType::FinishedPdu
            | FileDirectiveType::NakPdu
            | FileDirectiveType::KeepAlivePdu => {
                return Err(DestError::CantProcessPacketType {
                    pdu_type: PduType::FileDirective,
                    directive_type: Some(pdu_directive),
                });
            }
            FileDirectiveType::AckPdu => {
                let ack_pdu = AckPdu::from_bytes(raw_packet)?;
                self.handle_ack_pdu(ack_pdu)?;
            }
            FileDirectiveType::MetadataPdu => {
                let metadata_pdu = MetadataPduReader::from_bytes(raw_packet)?;
                self.handle_metadata_pdu(metadata_pdu)?
            }
            FileDirectiveType::PromptPdu => self.handle_prompt_pdu(raw_packet)?,
        };
        Ok(sent_packets)
    }

    fn handle_ack_pdu(&mut self, ack_pdu: AckPdu) -> Result<(), DestError> {
        if ack_pdu.directive_code_of_acked_pdu() != FileDirectiveType::FinishedPdu {
            self.transaction_params
                .anomaly_tracker
                .increment_invalid_ack_directive_code();
        }
        // We are done.
        self.reset();
        Ok(())
    }

    fn first_packet_handling(&mut self, pdu_config: &CommonPduConfig) -> Result<(), DestError> {
        self.transaction_params.reset();
        let remote_cfg = self.remote_cfg_table.get(pdu_config.source_id().value());
        if remote_cfg.is_none() {
            return Err(DestError::NoRemoteConfigFound(pdu_config.source_id()));
        }
        self.transaction_params.remote_cfg = Some(*remote_cfg.unwrap());
        self.transaction_params.transaction_id = Some(TransactionId::new(
            pdu_config.source_id(),
            pdu_config.transaction_seq_num,
        ));
        self.transaction_params.pdu_conf = *pdu_config;
        self.transaction_params.pdu_conf.direction = spacepackets::cfdp::Direction::TowardsSender;
        self.state = State::Busy;
        Ok(())
    }

    fn handle_metadata_pdu(&mut self, metadata_pdu: MetadataPduReader) -> Result<(), DestError> {
        let first_packet = self.step() == TransactionStep::Idle;
        if first_packet {
            self.first_packet_handling(metadata_pdu.pdu_header().common_pdu_conf())?;
        }
        match self.transmission_mode() {
            Some(transmission_mode) => {
                if !first_packet && transmission_mode == TransmissionMode::Unacknowledged {
                    return Err(DestError::RecvdMetadataButIsBusy);
                }
                match self.transaction_params.acked_params.as_mut() {
                    Some(acked_params) => {
                        if acked_params.metadata_missing {
                            if let Some(timer) =
                                self.transaction_params.deferred_procedure_timer.as_mut()
                            {
                                acked_params.nak_activity_counter = 0;
                                timer.reset();
                            }
                            acked_params.metadata_missing = false;
                        } else {
                            // Quietly ignore.
                            return Ok(());
                        }
                    }
                    None => {
                        self.transaction_params.acked_params = Some(AcknowledgedModeParams {
                            last_start_offset: 0,
                            last_end_offset: 0,
                            metadata_missing: false,
                            nak_activity_counter: 0,
                            deferred_procedure_active: false,
                        });
                    }
                }
            }
            None => {
                return Err(DestError::RecvdMetadataButIsBusy);
            }
        }

        self.transaction_params.metadata_params = *metadata_pdu.metadata_params();
        let remote_cfg = self.remote_cfg_table.get(metadata_pdu.source_id().value());
        if remote_cfg.is_none() {
            return Err(DestError::NoRemoteConfigFound(metadata_pdu.dest_id()));
        }

        let src_name = metadata_pdu.src_file_name();
        let dest_name = metadata_pdu.dest_file_name();
        if src_name.is_empty() && dest_name.is_empty() {
            self.transaction_params.metadata_only = true;
        }
        if !self.transaction_params.metadata_only && src_name.is_empty() {
            return Err(DestError::EmptySrcFileField);
        }
        if !self.transaction_params.metadata_only && dest_name.is_empty() {
            return Err(DestError::EmptyDestFileField);
        }
        if !self.transaction_params.metadata_only {
            self.transaction_params.file_names.src_file_name[..src_name.len_value()]
                .copy_from_slice(src_name.value());
            self.transaction_params.file_names.src_file_name_len = src_name.len_value();
            if dest_name.is_empty() {
                return Err(DestError::EmptyDestFileField);
            }
            self.transaction_params.file_names.dest_file_name[..dest_name.len_value()]
                .copy_from_slice(dest_name.value());
            self.transaction_params.file_names.dest_file_name_len = dest_name.len_value();
            self.transaction_params.msgs_to_user_size = 0;
        }
        if !metadata_pdu.options().is_empty() {
            for option_tlv in metadata_pdu.options_iter().unwrap() {
                if option_tlv.is_standard_tlv()
                    && option_tlv.tlv_type().unwrap() == TlvType::MsgToUser
                {
                    self.transaction_params
                        .msgs_to_user_buf
                        .copy_from_slice(option_tlv.raw_data().unwrap());
                    self.transaction_params.msgs_to_user_size += option_tlv.len_full();
                }
            }
        }
        self.set_step(TransactionStep::TransactionStart);
        Ok(())
    }

    fn reset_nak_activity_parameters_if_active(&mut self) {
        if let Some(acked_params) = self.transaction_params.acked_params.as_mut() {
            if acked_params.metadata_missing {
                if let Some(timer) = self.transaction_params.deferred_procedure_timer.as_mut() {
                    acked_params.nak_activity_counter = 0;
                    timer.reset();
                }
            }
        }
    }

    fn handle_file_data_without_previous_metadata(
        &mut self,
        fd_pdu: &FileDataPdu,
    ) -> Result<u32, DestError> {
        let mut packets_sent = 0;
        if fd_pdu.transmission_mode() == TransmissionMode::Unacknowledged {
            // In acknowledged mode, we need to wait for the metadata PDU first.
            return Err(DestError::FirstPacketNotMetadata);
        }
        self.first_packet_handling(fd_pdu.pdu_header().common_pdu_conf())?;
        self.transaction_params.progress = fd_pdu.offset() + fd_pdu.file_data().len() as u64;
        self.transaction_params.acked_params = Some(AcknowledgedModeParams {
            last_start_offset: 0,
            last_end_offset: 0,
            nak_activity_counter: 0,
            metadata_missing: true,
            deferred_procedure_active: false,
        });
        if fd_pdu.file_data().len() as u64 > 0 {
            self.lost_segment_tracker
                .add_lost_segment((0, self.transaction_params.progress))?;
            let ack_params = self.transaction_params.acked_params.as_mut().unwrap();
            ack_params.last_start_offset = self.transaction_params.progress;
            ack_params.last_end_offset = self.transaction_params.progress;
        }
        if self
            .transaction_params
            .remote_cfg
            .as_ref()
            .unwrap()
            .immediate_nak_mode
        {
            let mut num_seg_reqs = 1;
            if fd_pdu.file_data().len() as u64 > 0 {
                num_seg_reqs += 1;
            }
            let pdu_header = PduHeader::new_for_file_directive(self.transaction_params.pdu_conf, 0);
            let mut nak_pdu = NakPduCreatorWithReservedSeqReqsBuf::new(
                self.pdu_and_cksum_buffer.get_mut(),
                pdu_header,
                num_seg_reqs,
            )
            .map_err(PduError::from)?;
            let buf_mut = nak_pdu.segment_request_buffer_mut();
            let mut current_offset = 0;
            let increment = if pdu_header.common_pdu_conf().file_flag == LargeFileFlag::Large {
                8
            } else {
                4
            };
            buf_mut[0..current_offset].fill(0);
            current_offset += increment;
            buf_mut[current_offset..current_offset + increment].fill(0);
            current_offset += increment;
            if fd_pdu.file_data().len() as u64 > 0 {
                buf_mut[0..current_offset].fill(0);
                current_offset += increment;
                if pdu_header.common_pdu_conf().file_flag == LargeFileFlag::Large {
                    buf_mut[current_offset..current_offset + increment]
                        .copy_from_slice(&self.transaction_params.progress.to_be_bytes());
                } else {
                    buf_mut[current_offset..current_offset + increment]
                        .copy_from_slice(&(self.transaction_params.progress as u32).to_be_bytes());
                }
            }
            let written_size = nak_pdu
                .finish(0, self.transaction_params.progress)
                .map_err(PduError::from)?;
            self.pdu_sender.send_file_directive_pdu(
                FileDirectiveType::NakPdu,
                &self.pdu_and_cksum_buffer.borrow()[0..written_size],
            )?;
            packets_sent += 1;
        }
        Ok(packets_sent)
    }

    fn handle_file_data(
        &mut self,
        user: &mut impl CfdpUser,
        fd_pdu: FileDataPdu,
    ) -> Result<u32, DestError> {
        let mut sent_packets = 0;
        let mut handle_indication = |id: TransactionId, indication_config: &IndicationConfig| {
            if indication_config.file_segment_recv {
                user.file_segment_recvd_indication(&FileSegmentRecvdParams {
                    id,
                    offset: fd_pdu.offset(),
                    length: fd_pdu.file_data().len(),
                    segment_metadata: fd_pdu.segment_metadata(),
                });
            }
        };
        let step = self.step.get();
        if self.state == State::Idle {
            if step == TransactionStep::Idle {
                sent_packets += self.handle_file_data_without_previous_metadata(&fd_pdu)?;
                self.set_step(TransactionStep::WaitingForMetadata);
                handle_indication(
                    self.transaction_id().unwrap(),
                    &self.local_cfg.indication_cfg,
                );
                return Ok(sent_packets);
            }
            if step != TransactionStep::ReceivingFileDataPdus
                && step != TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling
            {
                return Err(DestError::WrongStepForPdu {
                    pdu_type: PduType::FileData,
                    file_directive_type: None,
                    step,
                });
            }
        }
        handle_indication(
            self.transaction_id().unwrap(),
            &self.local_cfg.indication_cfg,
        );
        if let Err(e) = self.vfs.write_data(
            // Safety: It was already verified that the path is valid during the transaction start.
            unsafe {
                from_utf8_unchecked(
                    //from_utf8(
                    &self.transaction_params.file_names.dest_path_buf
                        [0..self.transaction_params.file_names.dest_file_path_len],
                )
            },
            fd_pdu.offset(),
            fd_pdu.file_data(),
        ) {
            if self.declare_fault(ConditionCode::FilestoreRejection)
                == FaultHandlerCode::AbandonTransaction
            {
                self.abandon_transaction();
            }
            return Err(e.into());
        }
        self.transaction_params.progress = core::cmp::max(
            self.transaction_params.progress,
            fd_pdu.offset() + fd_pdu.file_data().len() as u64,
        );
        if self.transmission_mode().unwrap() == TransmissionMode::Acknowledged {
            sent_packets += self.lost_segment_handling(&fd_pdu)?;
        }
        Ok(sent_packets)
    }

    fn lost_segment_handling(&mut self, fd_pdu: &FileDataPdu) -> Result<u32, DestError> {
        // Lost segment detection: 4.6.4.3.1 a) and b) are covered by this code. c) is covered
        //by dedicated code which is run when the EOF PDU is handled.
        if self.transaction_params.acked_params.is_none() {
            return Ok(0);
        }

        let mut sent_packets = 0;
        let acked_params = self.transaction_params.acked_params.as_mut().unwrap();
        if fd_pdu.offset() > acked_params.last_end_offset {
            let lost_segment = (acked_params.last_end_offset, fd_pdu.offset());
            self.lost_segment_tracker.add_lost_segment(lost_segment)?;
            if self
                .transaction_params
                .remote_cfg
                .as_ref()
                .unwrap()
                .immediate_nak_mode
            {
                let pdu_header =
                    PduHeader::new_for_file_directive(self.transaction_params.pdu_conf, 0);
                if self.transaction_params.pdu_conf.file_flag == LargeFileFlag::Normal
                    && lost_segment.0 > u32::MAX as u64
                    || lost_segment.1 > u32::MAX as u64
                {
                    return Err(DestError::UnexpectedLargeFileSize);
                }
                let seg32 = [(lost_segment.0 as u32, lost_segment.1 as u32)];
                let seg64 = [lost_segment];
                let nak_pdu = if self.transaction_params.pdu_conf.file_flag == LargeFileFlag::Normal
                {
                    NakPduCreator::new_normal_file_size(
                        pdu_header,
                        0,
                        self.transaction_params.progress as u32,
                        &seg32,
                    )
                    .unwrap()
                } else {
                    NakPduCreator::new_large_file_size(
                        pdu_header,
                        0,
                        self.transaction_params.progress,
                        &seg64,
                    )
                    .unwrap()
                };
                let written_len = nak_pdu.write_to_bytes(self.pdu_and_cksum_buffer.get_mut())?;
                self.pdu_sender.send_file_directive_pdu(
                    FileDirectiveType::NakPdu,
                    &self.pdu_and_cksum_buffer.borrow()[0..written_len],
                )?;
                sent_packets += 1;
            }
        }
        if fd_pdu.offset() >= acked_params.last_end_offset {
            acked_params.last_start_offset = fd_pdu.offset();
            acked_params.last_end_offset = fd_pdu.offset() + fd_pdu.file_data().len() as u64;
        }
        if fd_pdu.offset() + fd_pdu.file_data().len() as u64 <= acked_params.last_start_offset {
            // Might be a re-requested FD PDU.
            let removed = self.lost_segment_tracker.remove_lost_segment((
                fd_pdu.offset(),
                fd_pdu.offset() + fd_pdu.file_data().len() as u64,
            ))?;
            // Reception of missing segments resets the NAK activity parameters.
            // See CFDP 4.6.4.7.
            if removed && acked_params.deferred_procedure_active {
                self.reset_nak_activity_parameters_if_active();
            }
        }
        Ok(sent_packets)
    }

    fn handle_eof_pdu(
        &mut self,
        cfdp_user: &mut impl CfdpUser,
        eof_pdu: EofPdu,
    ) -> Result<u32, DestError> {
        let sent_packets = 0;
        let first_packet = self.step() == TransactionStep::Idle;
        if first_packet {
            self.first_packet_handling(eof_pdu.pdu_header().common_pdu_conf())?;
        }
        if self.local_cfg.indication_cfg.eof_recv {
            // Unwrap is okay here, application logic ensures that transaction ID is valid here.
            cfdp_user
                .eof_recvd_indication(self.transaction_params.transaction_id.as_ref().unwrap());
        }
        if first_packet {
            if self.transmission_mode().unwrap() == TransmissionMode::Unacknowledged {
                return Err(DestError::WrongStepForPdu {
                    pdu_type: PduType::FileDirective,
                    file_directive_type: Some(FileDirectiveType::EofPdu),
                    step: self.step(),
                });
            }
            return self.handle_eof_without_previous_metadata_in_acked_mode(&eof_pdu);
        }
        if eof_pdu.condition_code() == ConditionCode::NoError {
            if !self.handle_eof_no_error(&eof_pdu)? {
                return Ok(sent_packets);
            }
        } else {
            self.handle_eof_cancel(&eof_pdu);
        };
        let mut sent_packets = 0;
        match self.transaction_params.transmission_mode() {
            TransmissionMode::Acknowledged => {
                self.acknowledge_eof_pdu(&eof_pdu)?;
                sent_packets += 1;
                match self.transaction_params.acked_params.as_ref() {
                    Some(acked_params) => {
                        if acked_params.metadata_missing || !self.lost_segment_tracker.is_empty() {
                            self.start_deferred_lost_segment_handling();
                        } else {
                            self.set_step(TransactionStep::TransferCompletion);
                        }
                    }
                    // This can happen when the EOF is the first packet to arrive.
                    None => {
                        self.start_deferred_lost_segment_handling();
                    }
                }
            }
            TransmissionMode::Unacknowledged => {
                self.set_step(TransactionStep::TransferCompletion);
            }
        }
        Ok(sent_packets)
    }

    fn handle_eof_without_previous_metadata_in_acked_mode(
        &mut self,
        eof_pdu: &EofPdu,
    ) -> Result<u32, DestError> {
        let mut sent_packets = 0;
        self.transaction_params.file_size = eof_pdu.file_size();
        self.transaction_params.checksum = eof_pdu.file_checksum();
        self.transaction_params.acked_params = Some(AcknowledgedModeParams {
            last_start_offset: 0,
            last_end_offset: 0,
            nak_activity_counter: 0,
            metadata_missing: true,
            deferred_procedure_active: false,
        });
        if self.transaction_params.file_size > 0 {
            self.lost_segment_tracker.reset();
            // Add the whole file to the lost segments map for now.
            self.lost_segment_tracker
                .add_lost_segment((0, eof_pdu.file_size()))?;
        }
        if eof_pdu.condition_code() != ConditionCode::NoError {
            self.handle_eof_cancel(eof_pdu);
        }
        self.acknowledge_eof_pdu(eof_pdu)?;
        sent_packets += 1;
        if self.transaction_params.completion_disposition.get() == CompletionDisposition::Cancelled
        {
            self.set_step(TransactionStep::TransferCompletion);
            return Ok(sent_packets);
        }
        self.set_step(TransactionStep::WaitingForMetadata);
        self.start_deferred_lost_segment_handling();
        Ok(sent_packets)
    }

    fn handle_eof_cancel(&mut self, eof_pdu: &EofPdu) {
        // This is an EOF (Cancel), perform Cancel Response Procedures according to chapter
        // 4.6.6 of the standard. Set remote ID as fault location.
        self.trigger_notice_of_completion_cancelled(
            eof_pdu.condition_code(),
            EntityIdTlv::new(self.transaction_params.remote_cfg.unwrap().entity_id),
        );
        // Store this as progress for the checksum calculation as well.
        self.transaction_params.progress = eof_pdu.file_size();
        if let Some(ack_params) = &self.transaction_params.acked_params {
            if ack_params.metadata_missing {
                return;
            }
        }
        if self.transaction_params.progress == 0 {
            // Empty file, no file data PDU.
            self.transaction_params
                .finished_params
                .delivery_code
                .set(DeliveryCode::Complete);
            return;
        }
        if self.checksum_verify(self.transaction_params.progress, eof_pdu.file_checksum()) {
            self.transaction_params
                .finished_params
                .delivery_code
                .set(DeliveryCode::Complete);
            return;
        }
        self.transaction_params
            .finished_params
            .delivery_code
            .set(DeliveryCode::Incomplete);
    }

    fn acknowledge_eof_pdu(&mut self, eof_pdu: &EofPdu) -> Result<(), DestError> {
        let pdu_header = PduHeader::new_for_file_directive(self.transaction_params.pdu_conf, 0);
        let ack_pdu = AckPdu::new_for_eof_pdu(
            pdu_header,
            eof_pdu.condition_code(),
            TransactionStatus::Active,
        );
        let written_len = ack_pdu.write_to_bytes(self.pdu_and_cksum_buffer.get_mut())?;
        self.pdu_sender.send_file_directive_pdu(
            FileDirectiveType::AckPdu,
            &self.pdu_and_cksum_buffer.borrow()[0..written_len],
        )?;
        Ok(())
    }

    fn start_deferred_lost_segment_handling(&mut self) {
        match &mut self.transaction_params.acked_params {
            Some(params) => {
                params.last_start_offset = self.transaction_params.file_size;
                params.last_end_offset = self.transaction_params.file_size;
                params.deferred_procedure_active = true;
                params.nak_activity_counter = 0;
            }
            None => {
                self.transaction_params.acked_params = Some(AcknowledgedModeParams {
                    last_start_offset: self.transaction_params.file_size,
                    last_end_offset: self.transaction_params.file_size,
                    metadata_missing: false,
                    nak_activity_counter: 0,
                    deferred_procedure_active: true,
                })
            }
        }
    }

    fn check_for_deferred_lost_segment_completion(&mut self, metadata_missing: bool) -> bool {
        if self.lost_segment_tracker.is_empty() && !metadata_missing {
            // We are done and have received everything.
            match self.checksum_verify(
                self.transaction_params.progress,
                self.transaction_params.checksum,
            ) {
                true => {
                    self.transaction_params
                        .finished_params
                        .condition_code
                        .set(ConditionCode::NoError);
                    self.transaction_params
                        .finished_params
                        .delivery_code
                        .set(DeliveryCode::Complete);
                }
                false => {
                    self.transaction_params
                        .finished_params
                        .condition_code
                        .set(ConditionCode::FileChecksumFailure);
                    self.transaction_params
                        .finished_params
                        .delivery_code
                        .set(DeliveryCode::Incomplete);
                }
            }
            self.transaction_params
                .acked_params
                .as_mut()
                .unwrap()
                .deferred_procedure_active = false;
            self.set_step(TransactionStep::TransferCompletion);
            return true;
        }
        false
    }

    fn deferred_lost_segment_handling(&mut self) -> Result<u32, DestError> {
        assert!(
            self.transaction_params.acked_params.is_some(),
            "acknowledged parameters unexpectedly None"
        );
        let acked_params = self.transaction_params.acked_params.unwrap();
        if self.check_for_deferred_lost_segment_completion(acked_params.metadata_missing) {
            return Ok(0);
        }
        let mut sent_packets = 0;
        let first_nak_issuance = self.transaction_params.deferred_procedure_timer.is_none();
        if first_nak_issuance {
            self.transaction_params.deferred_procedure_timer = Some(
                self.check_timer_creator
                    .create_countdown(TimerContext::NakActivity {
                        expiry_time: self
                            .transaction_params
                            .remote_cfg
                            .as_ref()
                            .unwrap()
                            .nak_timer_interval,
                    }),
            );
        } else if !self
            .transaction_params
            .deferred_procedure_timer
            .as_ref()
            .unwrap()
            .has_expired()
        {
            return Ok(sent_packets);
        }
        if !first_nak_issuance
            && acked_params.nak_activity_counter + 1
                == self
                    .transaction_params
                    .remote_cfg
                    .as_ref()
                    .unwrap()
                    .nak_timer_expiration_limit
        {
            self.transaction_params
                .finished_params
                .delivery_code
                .set(DeliveryCode::Incomplete);
            if self.declare_fault(ConditionCode::NakLimitReached)
                == FaultHandlerCode::AbandonTransaction
            {
                self.abandon_transaction();
            }
            return Ok(sent_packets);
        }
        if !first_nak_issuance {
            self.transaction_params
                .acked_params
                .as_mut()
                .unwrap()
                .nak_activity_counter += 1;
            self.transaction_params
                .deferred_procedure_timer
                .as_mut()
                .unwrap()
                .reset();
        }
        let pdu_header = PduHeader::new_for_file_directive(self.transaction_params.pdu_conf, 0);
        let max_segment_reqs = NakPduCreatorWithReservedSeqReqsBuf::calculate_max_segment_requests(
            self.transaction_params
                .remote_cfg
                .as_ref()
                .unwrap()
                .max_packet_len,
            &pdu_header,
        )
        .map_err(|_| DestError::InvalidRemoteConfig(self.transaction_params.remote_cfg.unwrap()))?;
        let mut segments_to_send = self.lost_segment_tracker.number_of_segments();
        if acked_params.metadata_missing {
            segments_to_send += 1;
        }
        if segments_to_send <= max_segment_reqs {
            // Should never fail because we calculcated the allowed maximum number of segment
            // requests for the buffer.
            let mut nak_pdu_creator = NakPduCreatorWithReservedSeqReqsBuf::new(
                self.pdu_and_cksum_buffer.get_mut(),
                pdu_header,
                segments_to_send,
            )
            .unwrap();
            // All error conditions were checked so this should never fail.
            //
            // 1. Number of segments was calculated.
            // 2. Buffer size was calculated based on PDU header and maximum packet size.
            // 3. Large file flag is based on PDU header, so there should never be a missmatch.
            self.lost_segment_tracker
                .write_to_nak_segment_list(&mut nak_pdu_creator, acked_params.metadata_missing)
                .expect("unexpected lost segment write error");
            let written_len = nak_pdu_creator
                .finish(0, self.transaction_params.file_size)
                .map_err(PduError::from)?;
            self.pdu_sender.send_file_directive_pdu(
                FileDirectiveType::NakPdu,
                &self.pdu_and_cksum_buffer.borrow()[0..written_len],
            )?;
            sent_packets += 1;
        } else {
            sent_packets += self.write_multi_packet_nak_sequence(
                segments_to_send,
                max_segment_reqs,
                acked_params.metadata_missing,
            )?;
        }

        Ok(sent_packets)
    }

    #[cold]
    fn write_multi_packet_nak_sequence(
        &mut self,
        mut segments_to_send: usize,
        max_segments: usize,
        first_segment_metadata: bool,
    ) -> Result<u32, DestError> {
        let mut sent_packets = 0;
        let pdu_header = PduHeader::new_for_file_directive(self.transaction_params.pdu_conf, 0);
        let mut nak_pdu_creator = NakPduCreatorWithReservedSeqReqsBuf::new(
            self.pdu_and_cksum_buffer.get_mut(),
            pdu_header,
            max_segments,
        )
        .unwrap();

        let mut segment_index = 0;
        let mut buf_index = 0;
        let mut current_start_of_scope = 0;
        let mut current_end_of_scope = 0;
        let mut seg_buf = nak_pdu_creator.segment_request_buffer_mut();

        // First segment re-requests metadata PDU.
        if first_segment_metadata {
            if pdu_header.common_pdu_conf().file_flag == LargeFileFlag::Large {
                seg_buf[0..16].fill(0);
                buf_index = 16;
            } else {
                seg_buf[0..8].fill(0);
                buf_index = 8;
            }
            segment_index = 1; // account for the header gap entry
        }

        for (idx, (start, end)) in self.lost_segment_tracker.iter().enumerate() {
            // flush full PDU *before* writing the new entry
            if segment_index == max_segments {
                let written_len = nak_pdu_creator
                    .finish(current_start_of_scope, current_end_of_scope)
                    .map_err(PduError::from)?;
                self.pdu_sender.send_file_directive_pdu(
                    FileDirectiveType::NakPdu,
                    &self.pdu_and_cksum_buffer.borrow()[..written_len],
                )?;
                sent_packets += 1;
                segments_to_send = segments_to_send.saturating_sub(max_segments);

                // new PDU with the same capacity
                nak_pdu_creator = NakPduCreatorWithReservedSeqReqsBuf::new(
                    self.pdu_and_cksum_buffer.get_mut(),
                    pdu_header,
                    core::cmp::min(segments_to_send, max_segments),
                )
                .unwrap();
                seg_buf = nak_pdu_creator.segment_request_buffer_mut();
                segment_index = 0;
                current_start_of_scope = current_end_of_scope;
                buf_index = 0;
            }

            // write entry
            if pdu_header.common_pdu_conf().file_flag == LargeFileFlag::Large {
                seg_buf[buf_index..buf_index + 8].copy_from_slice(&start.to_be_bytes());
                buf_index += 8;
                seg_buf[buf_index..buf_index + 8].copy_from_slice(&end.to_be_bytes());
                buf_index += 8;
            } else {
                seg_buf[buf_index..buf_index + 4].copy_from_slice(&(start as u32).to_be_bytes());
                buf_index += 4;
                seg_buf[buf_index..buf_index + 4].copy_from_slice(&(end as u32).to_be_bytes());
                buf_index += 4;
            }
            if idx == max_segments {
                // Clamp the end of scope to the progress for the last NAK in the sequence.
                current_end_of_scope = self.transaction_params.progress;
            } else {
                current_end_of_scope = end;
            }
            segment_index += 1;
        }

        // send trailing PDU if anything was written
        if segment_index > 0 {
            let written_len = nak_pdu_creator
                .finish(current_start_of_scope, current_end_of_scope)
                .map_err(PduError::from)?;
            self.pdu_sender.send_file_directive_pdu(
                FileDirectiveType::NakPdu,
                &self.pdu_and_cksum_buffer.borrow()[..written_len],
            )?;
            sent_packets += 1;
        }
        Ok(sent_packets)
    }

    fn trigger_notice_of_completion_cancelled(
        &self,
        cond_code: ConditionCode,
        fault_location: EntityIdTlv,
    ) {
        self.transaction_params
            .completion_disposition
            .set(CompletionDisposition::Cancelled);
        self.transaction_params
            .finished_params
            .condition_code
            .set(cond_code);
        self.transaction_params
            .finished_params
            .fault_location_finished
            .set(Some(fault_location));
        // For anything larger than 0, we'd have to do a checksum check to verify whether
        // the delivery is really complete, and we need the EOF checksum for that..
        if self.transaction_params.progress == 0 {
            self.transaction_params
                .finished_params
                .delivery_code
                .set(DeliveryCode::Complete);
        }
    }

    /// Returns whether the transfer can be completed regularly.
    fn handle_eof_no_error(&mut self, eof_pdu: &EofPdu) -> Result<bool, DestError> {
        // CFDP 4.6.1.2.9: Declare file size error if progress exceeds file size
        if self.transaction_params.progress > eof_pdu.file_size() {
            match self.declare_fault(ConditionCode::FileSizeError) {
                FaultHandlerCode::IgnoreError => (),
                FaultHandlerCode::AbandonTransaction => {
                    self.abandon_transaction();
                    return Ok(false);
                }
                FaultHandlerCode::NoticeOfCancellation | FaultHandlerCode::NoticeOfSuspension => {
                    return Ok(false);
                }
            }
        } else if (self.transaction_params.progress < eof_pdu.file_size())
            && self.transaction_params.transmission_mode() == TransmissionMode::Acknowledged
        {
            // CFDP 4.6.4.3.1: The end offset of the last received file segment and the file
            // size as stated in the EOF PDU is not the same, so we need to add that segment to
            // the lost segments for the deferred lost segment detection procedure.
            self.lost_segment_tracker
                .add_lost_segment((self.transaction_params.progress, eof_pdu.file_size()))?;
        }

        self.transaction_params.file_size = eof_pdu.file_size();
        self.transaction_params.checksum = eof_pdu.file_checksum();
        if self.transaction_params.transmission_mode() == TransmissionMode::Unacknowledged
            && !self.checksum_verify(
                self.transaction_params.progress,
                self.transaction_params.checksum,
            )
        {
            self.start_check_limit_handling();
            return Ok(false);
        }
        self.transaction_params
            .finished_params
            .delivery_code
            .set(DeliveryCode::Complete);
        self.transaction_params
            .finished_params
            .condition_code
            .set(ConditionCode::NoError);
        Ok(true)
    }

    fn checksum_verify(&mut self, verify_len: u64, checksum: u32) -> bool {
        if self.transaction_params.metadata_params().checksum_type == ChecksumType::NullChecksum
            || self.transaction_params.metadata_only
        {
            return true;
        }
        match self.vfs.checksum_verify(
            checksum,
            // Safety: It was already verified that the path is valid during the transaction start.
            unsafe {
                from_utf8_unchecked(
                    &self.transaction_params.file_names.dest_path_buf
                        [0..self.transaction_params.file_names.dest_file_path_len],
                )
            },
            self.transaction_params.metadata_params().checksum_type,
            verify_len,
            self.pdu_and_cksum_buffer.get_mut(),
        ) {
            Ok(false) => {
                if self.declare_fault(ConditionCode::FileChecksumFailure)
                    == FaultHandlerCode::AbandonTransaction
                {
                    self.abandon_transaction();
                }
                false
            }
            Ok(true) => true,
            Err(e) => match e {
                FilestoreError::ChecksumTypeNotImplemented(_) => {
                    if self.declare_fault(ConditionCode::UnsupportedChecksumType)
                        == FaultHandlerCode::AbandonTransaction
                    {
                        self.abandon_transaction();
                    }
                    // For this case, the applicable algorithm shall be the the null checksum,
                    // which is always succesful.
                    true
                }
                _ => {
                    if self.declare_fault(ConditionCode::FilestoreRejection)
                        == FaultHandlerCode::AbandonTransaction
                    {
                        self.abandon_transaction();
                    }
                    // Treat this equivalent to a failed checksum procedure.
                    false
                }
            },
        }
    }

    fn start_check_limit_handling(&mut self) {
        self.set_step(TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling);
        self.transaction_params.current_check_timer = Some(
            self.check_timer_creator
                .create_countdown(TimerContext::CheckLimit {
                    local_id: self.local_cfg.id,
                    remote_id: self.transaction_params.remote_cfg.unwrap().entity_id,
                    entity_type: EntityType::Receiving,
                }),
        );
        self.transaction_params.current_check_count = 0;
    }

    fn check_limit_handling(&mut self) -> Result<(), DestError> {
        if self.transaction_params.current_check_timer.is_none() {
            return Ok(());
        }
        let check_timer = self
            .transaction_params
            .current_check_timer
            .as_ref()
            .unwrap();
        if check_timer.has_expired() {
            if self.checksum_verify(
                self.transaction_params.progress,
                self.transaction_params.checksum,
            ) {
                self.transaction_params
                    .finished_params
                    .condition_code
                    .set(ConditionCode::NoError);
                self.transaction_params
                    .finished_params
                    .delivery_code
                    .set(DeliveryCode::Complete);
                self.set_step(TransactionStep::TransferCompletion);
                return Ok(());
            }
            if self.transaction_params.current_check_count + 1
                >= self.transaction_params.remote_cfg.unwrap().check_limit
            {
                if self.declare_fault(ConditionCode::CheckLimitReached)
                    == FaultHandlerCode::AbandonTransaction
                {
                    self.abandon_transaction();
                }
            } else {
                self.transaction_params.current_check_count += 1;
                self.transaction_params
                    .current_check_timer
                    .as_mut()
                    .unwrap()
                    .reset();
            }
        }
        Ok(())
    }

    fn handle_prompt_pdu(&mut self, _raw_packet: &[u8]) -> Result<(), DestError> {
        Err(DestError::NotImplemented)
    }

    fn fsm_busy(&mut self, cfdp_user: &mut impl CfdpUser) -> Result<u32, DestError> {
        let mut sent_packets = 0;
        if self.step() == TransactionStep::TransactionStart {
            let result = self.transaction_start(cfdp_user);
            if let Err(e) = result {
                // If we can not even start the transaction properly, reset the handler.
                // We could later let the user do this optionally, but for now this is the safer
                // approach to prevent inconsistent states of the handler.
                self.reset();
                return Err(e);
            }
        }
        if self.step() == TransactionStep::WaitingForMetadata
            || self.step() == TransactionStep::ReceivingFileDataPdus
        {
            if let Some(ack_params) = &mut self.transaction_params.acked_params {
                if ack_params.deferred_procedure_active {
                    sent_packets += self.deferred_lost_segment_handling()?;
                }
            }
        }
        if self.step() == TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling {
            self.check_limit_handling()?;
        }
        if self.step() == TransactionStep::TransferCompletion {
            sent_packets += self.transfer_completion(cfdp_user)?;
        }
        if self.step() == TransactionStep::WaitingForFinishedAck {
            sent_packets += self.handle_positive_ack_procedures()?;
            // Little hack because of the state machine handling order to ensure the transfer
            // is completed in one go.
            if self.step() == TransactionStep::TransferCompletion {
                sent_packets += self.transfer_completion(cfdp_user)?;
            }
        }
        Ok(sent_packets)
    }

    fn transaction_start(&mut self, cfdp_user: &mut impl CfdpUser) -> Result<(), DestError> {
        let id = self.transaction_id().unwrap();
        let dest_name = from_utf8(
            &self.transaction_params.file_names.dest_file_name
                [..self.transaction_params.file_names.dest_file_name_len],
        )?;
        self.transaction_params.file_names.dest_path_buf[0..dest_name.len()]
            .copy_from_slice(dest_name.as_bytes());
        self.transaction_params.file_names.dest_file_path_len = dest_name.len();
        let source_id = self.transaction_params.pdu_conf.source_id();
        let src_name = from_utf8(
            &self.transaction_params.file_names.src_file_name
                [0..self.transaction_params.file_names.src_file_name_len],
        )?;
        let mut msgs_to_user = SmallVec::<[MsgToUserTlv<'_>; 16]>::new();
        let mut num_msgs_to_user = 0;
        if self.transaction_params.msgs_to_user_size > 0 {
            let mut index = 0;
            while index < self.transaction_params.msgs_to_user_size {
                // This should never panic as the validity of the options was checked beforehand.
                let msgs_to_user_tlv =
                    MsgToUserTlv::from_bytes(&self.transaction_params.msgs_to_user_buf[index..])
                        .expect("message to user creation failed unexpectedly");
                msgs_to_user.push(msgs_to_user_tlv);
                index += msgs_to_user_tlv.len_full();
                num_msgs_to_user += 1;
            }
        }
        let metadata_recvd_params = MetadataReceivedParams {
            id,
            source_id,
            file_size: self.transaction_params.file_size(),
            src_file_name: src_name,
            dest_file_name: dest_name,
            msgs_to_user: &msgs_to_user[..num_msgs_to_user],
        };
        cfdp_user.metadata_recvd_indication(&metadata_recvd_params);

        if self.vfs.exists(dest_name)? && self.vfs.is_dir(dest_name)? {
            // Create new destination path by concatenating the last part of the source source
            // name and the destination folder. For example, for a source file of /tmp/hello.txt
            // and a destination name of /home/test, the resulting file name should be
            // /home/test/hello.txt
            // Safety: It was already verified that the path is valid during the transaction start.
            let source_file_name = self.vfs.file_name(src_name)?;
            if source_file_name.is_none() {
                return Err(DestError::PathConcat);
            }
            let source_name = source_file_name.unwrap();
            self.transaction_params.file_names.dest_path_buf[dest_name.len()] = b'/';
            self.transaction_params.file_names.dest_path_buf
                [dest_name.len() + 1..dest_name.len() + 1 + source_name.len()]
                .copy_from_slice(source_name.as_bytes());
            self.transaction_params.file_names.dest_file_path_len += 1 + source_name.len();
        }
        let dest_path_str = from_utf8(
            &self.transaction_params.file_names.dest_path_buf
                [0..self.transaction_params.file_names.dest_file_path_len],
        )?;
        if self.vfs.exists(dest_path_str)? {
            self.vfs.truncate_file(dest_path_str)?;
        } else {
            self.vfs.create_file(dest_path_str)?;
        }
        self.transaction_params.finished_params.file_status = FileStatus::Retained;
        drop(msgs_to_user);
        self.set_step(TransactionStep::ReceivingFileDataPdus);
        Ok(())
    }

    fn transfer_completion(&mut self, cfdp_user: &mut impl CfdpUser) -> Result<u32, DestError> {
        let mut sent_packets = 0;
        self.notice_of_completion(cfdp_user)?;
        if self.transaction_params.transmission_mode() == TransmissionMode::Acknowledged
            || self.transaction_params.metadata_params().closure_requested
        {
            sent_packets += self.send_finished_pdu()?;
            self.start_positive_ack_procedure();
            if self.transaction_params.transmission_mode() == TransmissionMode::Acknowledged {
                self.set_step(TransactionStep::WaitingForFinishedAck);
                return Ok(sent_packets);
            }
        }
        self.reset();
        Ok(sent_packets)
    }

    fn start_positive_ack_procedure(&mut self) {
        match &mut self.transaction_params.positive_ack_params {
            Some(current) => {
                current.ack_counter = 0;
            }
            None => {
                self.transaction_params.positive_ack_params = Some(PositiveAckParams {
                    ack_counter: 0,
                    positive_ack_of_cancellation: false,
                })
            }
        }
        self.transaction_params.ack_timer = Some(
            self.check_timer_creator
                .create_countdown(TimerContext::PositiveAck {
                    expiry_time: self
                        .transaction_params
                        .remote_cfg
                        .as_ref()
                        .unwrap()
                        .positive_ack_timer_interval,
                }),
        );
    }

    fn handle_positive_ack_procedures(&mut self) -> Result<u32, DestError> {
        // Do we have positive-ack params?
        if self.transaction_params.positive_ack_params.is_none() {
            return Ok(0);
        }
        let params = self.transaction_params.positive_ack_params.unwrap();

        // Has the timer expired?
        if !self
            .transaction_params
            .ack_timer
            .as_mut()
            .unwrap()
            .has_expired()
        {
            return Ok(0);
        }

        let expiration_limit = self
            .transaction_params
            .remote_cfg
            .as_ref()
            .unwrap()
            .positive_ack_timer_expiration_limit;
        // If bumping the counter would exceed the limit, fault & maybe abandon
        if params.ack_counter + 1 >= expiration_limit {
            if self.declare_fault(ConditionCode::PositiveAckLimitReached)
                == FaultHandlerCode::AbandonTransaction
            {
                self.abandon_transaction();
                return Ok(0);
            }
            self.transaction_params
                .positive_ack_params
                .as_mut()
                .unwrap()
                .positive_ack_of_cancellation = true;
            return Ok(0);
        }

        let params_mut = self
            .transaction_params
            .positive_ack_params
            .as_mut()
            .unwrap();
        params_mut.ack_counter += 1;
        self.transaction_params.ack_timer.as_mut().unwrap().reset();
        self.send_finished_pdu()
    }

    fn notice_of_completion(&mut self, cfdp_user: &mut impl CfdpUser) -> Result<(), DestError> {
        if self.transaction_params.completion_disposition.get() == CompletionDisposition::Completed
        {
            // TODO: Execute any filestore requests
        } else if self
            .transaction_params
            .remote_cfg
            .as_ref()
            .unwrap()
            .disposition_on_cancellation
            && self.transaction_params.finished_params.delivery_code.get()
                == DeliveryCode::Incomplete
        {
            // Safety: We already verified that the path is valid during the transaction start.
            let dest_path = unsafe {
                from_utf8_unchecked(
                    &self.transaction_params.file_names.dest_path_buf
                        [0..self.transaction_params.file_names.dest_file_path_len],
                )
            };
            if self.vfs.exists(dest_path)? && self.vfs.is_file(dest_path)? {
                self.vfs.remove_file(dest_path)?;
            }
            self.transaction_params.finished_params.file_status = FileStatus::DiscardDeliberately;
        }
        let tstate = &self.transaction_params;
        let transaction_finished_params = TransactionFinishedParams {
            id: tstate.transaction_id.unwrap(),
            condition_code: tstate.finished_params.condition_code.get(),
            delivery_code: tstate.finished_params.delivery_code.get(),
            file_status: tstate.finished_params.file_status,
        };
        cfdp_user.transaction_finished_indication(&transaction_finished_params);
        Ok(())
    }

    // When the fault handler code [FaultHandlerCode::AbandonTransaction] is returned, the caller
    // must call [Self::abandon_transaction] as soon as it is possible.
    fn declare_fault(&self, condition_code: ConditionCode) -> FaultHandlerCode {
        // Cache those, because they might be reset when abandoning the transaction.
        let transaction_id = self.transaction_id().unwrap();
        let progress = self.transaction_params.progress;
        let mut fh_code = self
            .local_cfg
            .fault_handler
            .get_fault_handler(condition_code);
        // CFDP 4.11.2.3.2: Any fault declared in the course of transferring the Finished (cancel)
        // PDU must result in abadonment of the transaction.
        if let Some(positive_ack) = &self.transaction_params.positive_ack_params {
            if positive_ack.positive_ack_of_cancellation {
                fh_code = FaultHandlerCode::AbandonTransaction;
            }
        }
        match fh_code {
            FaultHandlerCode::NoticeOfCancellation => {
                self.notice_of_cancellation(condition_code, EntityIdTlv::new(self.local_cfg().id));
            }
            FaultHandlerCode::NoticeOfSuspension => self.notice_of_suspension(),
            FaultHandlerCode::IgnoreError => (),
            FaultHandlerCode::AbandonTransaction => (),
        }
        self.local_cfg.fault_handler.report_fault(
            fh_code,
            FaultInfo::new(transaction_id, condition_code, progress),
        )
    }

    fn notice_of_cancellation(&self, condition_code: ConditionCode, fault_location: EntityIdTlv) {
        self.set_step_internal(TransactionStep::TransferCompletion);
        let tstate = &self.transaction_params;
        tstate.finished_params.condition_code.set(condition_code);
        tstate
            .completion_disposition
            .set(CompletionDisposition::Cancelled);
        tstate
            .finished_params
            .fault_location_finished
            .set(Some(fault_location));
    }

    fn notice_of_suspension(&self) {
        // TODO: Implement suspension handling.
    }

    fn abandon_transaction(&mut self) {
        self.reset();
    }

    #[inline]
    fn set_step_internal(&self, step: TransactionStep) {
        self.step.set(step);
    }

    #[inline]
    fn set_step(&mut self, step: TransactionStep) {
        self.set_step_internal(step);
    }

    fn send_finished_pdu(&mut self) -> Result<u32, DestError> {
        let tstate = &self.transaction_params;

        let pdu_header = PduHeader::new_for_file_directive(self.transaction_params.pdu_conf, 0);
        let finished_pdu = if tstate.finished_params.condition_code.get() == ConditionCode::NoError
            || tstate.finished_params.condition_code.get() == ConditionCode::UnsupportedChecksumType
        {
            FinishedPduCreator::new_no_error(
                pdu_header,
                tstate.finished_params.delivery_code.get(),
                tstate.finished_params.file_status,
            )
        } else {
            FinishedPduCreator::new(
                pdu_header,
                tstate.finished_params.condition_code.get(),
                tstate.finished_params.delivery_code.get(),
                tstate.finished_params.file_status,
                &[],
                tstate.finished_params.fault_location_finished.get(),
            )
        };
        finished_pdu.write_to_bytes(self.pdu_and_cksum_buffer.get_mut())?;
        self.pdu_sender.send_file_directive_pdu(
            FileDirectiveType::FinishedPdu,
            &self.pdu_and_cksum_buffer.borrow()[0..finished_pdu.len_written()],
        )?;
        Ok(1)
    }
}

#[cfg(test)]
mod tests {
    #[allow(unused_imports)]
    use std::println;
    use std::{
        fs,
        path::{Path, PathBuf},
        string::String,
    };

    use alloc::vec::Vec;
    use rand::Rng;
    use spacepackets::{
        cfdp::{
            ChecksumType, TransmissionMode,
            lv::Lv,
            pdu::{
                WritablePduPacket, finished::FinishedPduReader, metadata::MetadataPduCreator,
                nak::NakPduReader,
            },
        },
        util::UnsignedByteFieldU8,
    };

    use crate::{
        CRC_32, FaultHandler, IndicationConfig, PduRawWithInfo, RemoteConfigStoreStd,
        filestore::NativeFilestore,
        lost_segments::LostSegmentsList,
        tests::{
            LOCAL_ID, REMOTE_ID, SentPdu, TestCfdpSender, TestCfdpUser, TestCheckTimer,
            TestCheckTimerCreator, TestFaultHandler, TimerExpiryControl, basic_remote_cfg_table,
        },
    };

    use super::*;

    #[derive(Debug, Clone, Copy)]
    pub struct TransferInfo {
        id: TransactionId,
        header: PduHeader,
    }

    type TestDestHandler = DestinationHandler<
        TestCfdpSender,
        TestFaultHandler,
        NativeFilestore,
        RemoteConfigStoreStd,
        TestCheckTimerCreator,
        TestCheckTimer,
        LostSegmentsList,
    >;

    struct DestHandlerTestbench {
        expiry_control: TimerExpiryControl,
        handler: TestDestHandler,
        src_path: PathBuf,
        dest_path: PathBuf,
        check_dest_file: bool,
        check_handler_idle_at_drop: bool,
        closure_requested: bool,
        pdu_conf: CommonPduConfig,
        expected_full_data: Vec<u8>,
        expected_file_size: u64,
        buf: [u8; 512],
    }

    impl DestHandlerTestbench {
        fn new_with_fixed_paths(
            fault_handler: TestFaultHandler,

            transmission_mode: TransmissionMode,
            closure_requested: bool,
        ) -> Self {
            let (src_path, dest_path) = init_full_filepaths_textfile();
            assert!(!Path::exists(&dest_path));
            Self::new(
                fault_handler,
                transmission_mode,
                closure_requested,
                true,
                src_path,
                dest_path,
            )
        }

        fn new(
            fault_handler: TestFaultHandler,
            transmission_mode: TransmissionMode,
            closure_requested: bool,
            check_dest_file: bool,
            src_path: PathBuf,
            dest_path: PathBuf,
        ) -> Self {
            let expiry_control = TimerExpiryControl::default();
            let test_sender = TestCfdpSender::default();
            let dest_handler = default_dest_handler(fault_handler, test_sender, &expiry_control);
            let mut handler = Self {
                expiry_control,
                handler: dest_handler,
                src_path,
                closure_requested,
                dest_path,
                check_dest_file,
                check_handler_idle_at_drop: true,
                expected_file_size: 0,
                pdu_conf: CommonPduConfig::new_with_byte_fields(
                    LOCAL_ID,
                    REMOTE_ID,
                    UnsignedByteFieldU8::new(0),
                )
                .unwrap(),
                expected_full_data: Vec::new(),
                buf: [0; 512],
            };
            handler.pdu_conf.trans_mode = transmission_mode;
            handler.state_check(State::Idle, TransactionStep::Idle);
            handler
        }

        pub fn fault_handler(&self) -> &FaultHandler<TestFaultHandler> {
            &self.handler.local_cfg.fault_handler
        }

        #[inline]
        fn set_large_file_flag(&mut self, large_file: LargeFileFlag) {
            self.pdu_conf.file_flag = large_file;
        }

        fn dest_path(&self) -> &PathBuf {
            &self.dest_path
        }

        fn all_fault_queues_empty(&self) -> bool {
            self.handler
                .local_cfg
                .user_fault_hook()
                .borrow()
                .all_queues_empty()
        }

        #[allow(dead_code)]
        fn indication_cfg_mut(&mut self) -> &mut IndicationConfig {
            &mut self.handler.local_cfg.indication_cfg
        }

        fn remote_cfg_mut(&mut self) -> &mut RemoteEntityConfig {
            self.handler
                .remote_cfg_table
                .get_mut(LOCAL_ID.value())
                .unwrap()
        }

        fn pdu_queue_empty(&mut self) -> bool {
            self.handler.pdu_sender.queue_empty()
        }

        fn pdu_queue_len(&mut self) -> usize {
            self.handler.pdu_sender.queue_len()
        }

        fn get_next_pdu(&mut self) -> Option<SentPdu> {
            self.handler.pdu_sender.retrieve_next_pdu()
        }

        fn indication_cfg(&mut self) -> &IndicationConfig {
            &self.handler.local_cfg.indication_cfg
        }

        fn set_check_timer_expired(&mut self) {
            self.expiry_control.set_check_limit_expired();
        }

        fn set_nak_activity_timer_expired(&mut self) {
            self.expiry_control.set_nak_activity_expired();
        }

        fn set_positive_ack_expired(&mut self) {
            self.expiry_control.set_positive_ack_expired();
        }

        fn test_user_from_cached_paths(&self, expected_file_size: u64) -> TestCfdpUser {
            TestCfdpUser::new(
                0,
                self.src_path.to_string_lossy().into(),
                self.dest_path.to_string_lossy().into(),
                expected_file_size,
            )
        }

        fn generic_transfer_init(
            &mut self,
            user: &mut TestCfdpUser,
            file_size: u64,
        ) -> Result<TransferInfo, DestError> {
            self.expected_file_size = file_size;
            assert_eq!(user.transaction_indication_call_count, 0);
            assert_eq!(user.metadata_recv_queue.len(), 0);
            let pdu_header = PduHeader::new_for_file_directive(self.pdu_conf, 0);
            let metadata_pdu = create_metadata_pdu(
                &pdu_header,
                self.src_path.as_path(),
                self.dest_path.as_path(),
                file_size,
                self.closure_requested,
            );
            let packet_info = create_packet_info(&metadata_pdu, &mut self.buf);
            self.handler.state_machine(user, Some(&packet_info))?;
            assert_eq!(user.metadata_recv_queue.len(), 1);
            assert_eq!(
                self.handler.transmission_mode().unwrap(),
                self.pdu_conf.trans_mode
            );
            assert_eq!(user.transaction_indication_call_count, 0);
            assert_eq!(user.metadata_recv_queue.len(), 1);
            let metadata_recvd = user.metadata_recv_queue.pop_front().unwrap();
            assert_eq!(metadata_recvd.source_id, LOCAL_ID.into());
            assert_eq!(
                metadata_recvd.src_file_name,
                String::from(self.src_path.to_str().unwrap())
            );
            assert_eq!(
                metadata_recvd.dest_file_name,
                String::from(self.dest_path().to_str().unwrap())
            );
            assert_eq!(metadata_recvd.id, self.handler.transaction_id().unwrap());
            assert_eq!(metadata_recvd.file_size, file_size);
            assert!(metadata_recvd.msgs_to_user.is_empty());
            Ok(TransferInfo {
                id: self.handler.transaction_id().unwrap(),
                header: pdu_header,
            })
        }

        fn generic_file_data_insert(
            &mut self,
            user: &mut TestCfdpUser,
            offset: u64,
            file_data_chunk: &[u8],
        ) -> Result<u32, DestError> {
            let pdu_header = PduHeader::new_for_file_data_default(self.pdu_conf, 0);
            let filedata_pdu =
                FileDataPdu::new_no_seg_metadata(pdu_header, offset, file_data_chunk);
            filedata_pdu
                .write_to_bytes(&mut self.buf)
                .expect("writing file data PDU failed");
            let packet_info = PduRawWithInfo::new(&self.buf).expect("creating packet info failed");
            let result = self.handler.state_machine(user, Some(&packet_info));
            if self.indication_cfg().file_segment_recv {
                assert!(!user.file_seg_recvd_queue.is_empty());
                let file_seg = user.file_seg_recvd_queue.pop_front().unwrap();
                assert_eq!(file_seg.offset, offset);
                assert_eq!(file_seg.length, file_data_chunk.len());
            }
            result
        }

        fn generic_eof_no_error(
            &mut self,
            user: &mut TestCfdpUser,
            expected_full_data: Vec<u8>,
        ) -> Result<u32, DestError> {
            self.expected_full_data = expected_full_data;
            assert_eq!(user.finished_indic_queue.len(), 0);
            let pdu_header = PduHeader::new_for_file_directive(self.pdu_conf, 0);
            let eof_pdu = create_no_error_eof(&self.expected_full_data, &pdu_header);
            let packet_info = create_packet_info(&eof_pdu, &mut self.buf);
            self.check_handler_idle_at_drop = true;
            self.check_dest_file = true;
            let result = self.handler.state_machine(user, Some(&packet_info));
            if self.indication_cfg().eof_recv {
                assert_eq!(user.eof_recvd_call_count, 1);
            }
            result
        }

        fn check_completion_indication_success(&mut self, user: &mut TestCfdpUser) {
            assert_eq!(user.finished_indic_queue.len(), 1);
            let finished_indication = user.finished_indic_queue.pop_front().unwrap();
            assert_eq!(
                finished_indication.id,
                self.handler.transaction_id().unwrap()
            );
            assert_eq!(finished_indication.file_status, FileStatus::Retained);
            assert_eq!(finished_indication.delivery_code, DeliveryCode::Complete);
            assert_eq!(finished_indication.condition_code, ConditionCode::NoError);
        }

        fn check_completion_indication_failure(
            &mut self,
            user: &mut TestCfdpUser,
            cond_code: ConditionCode,
            file_status: FileStatus,
            delivery_code: DeliveryCode,
        ) {
            assert_eq!(user.finished_indic_queue.len(), 1);
            let finished_indication = user.finished_indic_queue.pop_front().unwrap();
            assert_eq!(
                finished_indication.id,
                self.handler.transaction_id().unwrap()
            );
            assert_eq!(finished_indication.file_status, file_status);
            assert_eq!(finished_indication.delivery_code, delivery_code);
            assert_eq!(finished_indication.condition_code, cond_code);
        }

        fn check_eof_ack_pdu(&mut self, cond_code: ConditionCode) {
            assert!(!self.pdu_queue_empty());
            let pdu = self.get_next_pdu().unwrap();
            assert_eq!(pdu.pdu_type, PduType::FileDirective);
            assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::AckPdu);
            let ack_pdu = AckPdu::from_bytes(&pdu.raw_pdu).unwrap();
            assert_eq!(ack_pdu.condition_code(), cond_code);
            assert_eq!(ack_pdu.transaction_status(), TransactionStatus::Active);
            assert_eq!(
                ack_pdu.directive_code_of_acked_pdu(),
                FileDirectiveType::EofPdu
            );
        }

        fn check_finished_pdu_success(&mut self) {
            let pdu = self.get_next_pdu().unwrap();
            assert_eq!(pdu.pdu_type, PduType::FileDirective);
            assert_eq!(
                pdu.file_directive_type.unwrap(),
                FileDirectiveType::FinishedPdu
            );
            let finished_pdu = FinishedPduReader::from_bytes(&pdu.raw_pdu).unwrap();
            assert_eq!(finished_pdu.delivery_code(), DeliveryCode::Complete);
            assert_eq!(finished_pdu.file_status(), FileStatus::Retained);
            assert_eq!(finished_pdu.condition_code(), ConditionCode::NoError);
            assert!(finished_pdu.fault_location().is_none());
        }

        fn check_finished_pdu_failure(
            &mut self,
            cond_code: ConditionCode,
            file_status: FileStatus,
            delivery_code: DeliveryCode,
        ) {
            let pdu = self.get_next_pdu().unwrap();
            assert_eq!(pdu.pdu_type, PduType::FileDirective);
            assert_eq!(
                pdu.file_directive_type.unwrap(),
                FileDirectiveType::FinishedPdu
            );
            let finished_pdu = FinishedPduReader::from_bytes(&pdu.raw_pdu).unwrap();
            assert_eq!(finished_pdu.delivery_code(), delivery_code);
            assert_eq!(finished_pdu.file_status(), file_status);
            assert_eq!(finished_pdu.condition_code(), cond_code);
        }

        fn acknowledge_finished_pdu(
            &mut self,
            user: &mut impl CfdpUser,
            transfer_info: &TransferInfo,
        ) {
            let ack_pdu = AckPdu::new_for_finished_pdu(
                transfer_info.header,
                ConditionCode::NoError,
                TransactionStatus::Active,
            );
            self.handler
                .state_machine(user, Some(&create_packet_info(&ack_pdu, &mut self.buf)))
                .expect("handling ack PDU failed");
        }

        fn state_check(&self, state: State, step: TransactionStep) {
            assert_eq!(self.handler.state(), state);
            assert_eq!(self.handler.step(), step);
        }
    }

    // Specifying some checks in the drop method avoids some boilerplate.
    impl Drop for DestHandlerTestbench {
        fn drop(&mut self) {
            if !self.all_fault_queues_empty() {
                let fh_queues = self.handler.local_cfg.user_fault_hook().borrow();
                println!(
                    "fault queues not empty. cancellation {}, suspension {}, ignored {}, abandon {}",
                    fh_queues.notice_of_cancellation_queue.len(),
                    fh_queues.notice_of_suspension_queue.len(),
                    fh_queues.ignored_queue.len(),
                    fh_queues.abandoned_queue.len()
                );
            }
            assert!(self.all_fault_queues_empty(), "fault queues not empty, ");
            assert!(self.pdu_queue_empty());
            if self.check_handler_idle_at_drop {
                self.state_check(State::Idle, TransactionStep::Idle);
            }
            if self.check_dest_file {
                assert!(Path::exists(&self.dest_path));
                let read_content = fs::read(&self.dest_path).expect("reading back string failed");
                assert_eq!(read_content.len() as u64, self.expected_file_size);
                assert_eq!(read_content, self.expected_full_data);
                assert!(fs::remove_file(self.dest_path.as_path()).is_ok());
            }
        }
    }

    fn init_full_filepaths_textfile() -> (PathBuf, PathBuf) {
        (
            tempfile::TempPath::from_path("/tmp/test.txt").to_path_buf(),
            tempfile::NamedTempFile::new()
                .unwrap()
                .into_temp_path()
                .to_path_buf(),
        )
    }

    fn default_dest_handler(
        test_fault_handler: TestFaultHandler,
        test_packet_sender: TestCfdpSender,
        expiry_control: &TimerExpiryControl,
    ) -> TestDestHandler {
        let local_entity_cfg = LocalEntityConfig {
            id: REMOTE_ID.into(),
            indication_cfg: IndicationConfig::default(),
            fault_handler: FaultHandler::new(test_fault_handler),
        };
        DestinationHandler::new(
            local_entity_cfg,
            test_packet_sender,
            NativeFilestore::default(),
            basic_remote_cfg_table(LOCAL_ID, 1024, true),
            TestCheckTimerCreator::new(expiry_control),
            LostSegmentsList::default(),
        )
    }

    fn create_metadata_pdu<'filename>(
        pdu_header: &PduHeader,
        src_name: &'filename Path,
        dest_name: &'filename Path,
        file_size: u64,
        closure_requested: bool,
    ) -> MetadataPduCreator<'filename, 'filename, 'static> {
        let checksum_type = if file_size == 0 {
            ChecksumType::NullChecksum
        } else {
            ChecksumType::Crc32
        };
        let metadata_params =
            MetadataGenericParams::new(closure_requested, checksum_type, file_size);
        MetadataPduCreator::new_no_opts(
            *pdu_header,
            metadata_params,
            Lv::new_from_str(src_name.as_os_str().to_str().unwrap()).unwrap(),
            Lv::new_from_str(dest_name.as_os_str().to_str().unwrap()).unwrap(),
        )
    }

    fn create_packet_info<'a>(
        pdu: &'a impl WritablePduPacket,
        buf: &'a mut [u8],
    ) -> PduRawWithInfo<'a> {
        let written_len = pdu
            .write_to_bytes(buf)
            .expect("writing metadata PDU failed");
        PduRawWithInfo::new(&buf[..written_len]).expect("generating packet info failed")
    }

    fn create_no_error_eof(file_data: &[u8], pdu_header: &PduHeader) -> EofPdu {
        let crc32 = if !file_data.is_empty() {
            let mut digest = CRC_32.digest();
            digest.update(file_data);
            digest.finalize()
        } else {
            0
        };
        EofPdu::new_no_error(*pdu_header, crc32, file_data.len() as u64)
    }

    #[test]
    fn test_basic() {
        let fault_handler = TestFaultHandler::default();
        let test_sender = TestCfdpSender::default();
        let dest_handler =
            default_dest_handler(fault_handler, test_sender, &TimerExpiryControl::default());
        assert!(dest_handler.transmission_mode().is_none());
        assert!(
            dest_handler
                .local_cfg
                .fault_handler
                .user_hook
                .borrow()
                .all_queues_empty()
        );
        assert!(dest_handler.pdu_sender.queue_empty());
        assert_eq!(dest_handler.state(), State::Idle);
        assert_eq!(dest_handler.step(), TransactionStep::Idle);
    }

    #[test]
    fn test_cancelling_idle_fsm() {
        let fault_handler = TestFaultHandler::default();
        let test_sender = TestCfdpSender::default();
        let mut dest_handler =
            default_dest_handler(fault_handler, test_sender, &TimerExpiryControl::default());
        assert!(!dest_handler.cancel_request(&TransactionId::new(
            UnsignedByteFieldU8::new(0).into(),
            UnsignedByteFieldU8::new(0).into()
        )));
    }

    #[test]
    fn test_empty_file_transfer_not_acked_no_closure() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        let mut test_user = tb.test_user_from_cached_paths(0);
        tb.generic_transfer_init(&mut test_user, 0)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.generic_eof_no_error(&mut test_user, Vec::new())
            .expect("EOF no error insertion failed");
        tb.check_completion_indication_success(&mut test_user);
    }

    #[test]
    fn test_empty_file_transfer_invalid_remote_id() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        let mut test_user = tb.test_user_from_cached_paths(0);
        let mut conf = tb.pdu_conf;
        // Just swap them..
        conf.set_source_and_dest_id(REMOTE_ID, LOCAL_ID).unwrap();
        let pdu_header = PduHeader::new_for_file_directive(conf, 0);
        let metadata_pdu = create_metadata_pdu(
            &pdu_header,
            tb.src_path.as_path(),
            tb.dest_path.as_path(),
            0,
            false,
        );
        let packet_info = create_packet_info(&metadata_pdu, &mut tb.buf);
        if let Err(DestError::NoRemoteConfigFound(id)) =
            tb.handler.state_machine(&mut test_user, Some(&packet_info))
        {
            assert_eq!(id, REMOTE_ID.into());
        } else {
            panic!("expected no remote config found error");
        }
        tb.check_dest_file = false;
    }

    #[test]
    fn test_empty_file_transfer_acked() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        let mut user = tb.test_user_from_cached_paths(0);
        let transfer_info = tb
            .generic_transfer_init(&mut user, 0)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.generic_eof_no_error(&mut user, Vec::new())
            .expect("EOF no error insertion failed");
        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    #[test]
    fn test_small_file_transfer_not_acked() {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        let mut user = tb.test_user_from_cached_paths(file_size);
        let _transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 0, file_data)
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_eof_no_error(&mut user, file_data.to_vec())
                .expect("EOF no error insertion failed"),
            0
        );
        tb.check_completion_indication_success(&mut user);
    }

    #[test]
    fn test_small_file_transfer_acked() {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 0, file_data)
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_eof_no_error(&mut user, file_data.to_vec())
                .expect("EOF no error insertion failed"),
            2
        );
        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    #[test]
    fn test_segmented_file_transfer_not_acked() {
        let mut rng = rand::rng();
        let mut random_data = [0u8; 512];
        rng.fill(&mut random_data);
        let file_size = random_data.len() as u64;
        let segment_len = 256;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        let mut test_user = tb.test_user_from_cached_paths(file_size);
        tb.generic_transfer_init(&mut test_user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.generic_file_data_insert(&mut test_user, 0, &random_data[0..segment_len])
            .expect("file data insertion failed");
        tb.generic_file_data_insert(
            &mut test_user,
            segment_len as u64,
            &random_data[segment_len..],
        )
        .expect("file data insertion failed");
        tb.generic_eof_no_error(&mut test_user, random_data.to_vec())
            .expect("EOF no error insertion failed");
        tb.check_completion_indication_success(&mut test_user);
    }

    #[test]
    fn test_segmented_file_transfer_acked() {
        let mut rng = rand::rng();
        let mut random_data = [0u8; 512];
        rng.fill(&mut random_data);
        let file_size = random_data.len() as u64;
        let segment_len = 256;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.generic_file_data_insert(&mut user, 0, &random_data[0..segment_len])
            .expect("file data insertion failed");
        tb.generic_file_data_insert(&mut user, segment_len as u64, &random_data[segment_len..])
            .expect("file data insertion failed");
        tb.generic_eof_no_error(&mut user, random_data.to_vec())
            .expect("EOF no error insertion failed");
        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    #[test]
    fn test_check_limit_handling_transfer_success() {
        let mut rng = rand::rng();
        let mut random_data = [0u8; 512];
        rng.fill(&mut random_data);
        let file_size = random_data.len() as u64;
        let segment_len = 256;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        let mut test_user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut test_user, file_size)
            .expect("transfer init failed");

        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.generic_file_data_insert(&mut test_user, 0, &random_data[0..segment_len])
            .expect("file data insertion 0 failed");
        tb.generic_eof_no_error(&mut test_user, random_data.to_vec())
            .expect("EOF no error insertion failed");

        // Checksum failure.
        let mut fault_handler = tb.handler.local_cfg.fault_handler.user_hook.borrow_mut();
        assert_eq!(fault_handler.ignored_queue.len(), 1);
        let cancelled = fault_handler.ignored_queue.pop_front().unwrap();
        assert_eq!(cancelled.transaction_id(), transfer_info.id);
        assert_eq!(
            cancelled.condition_code(),
            ConditionCode::FileChecksumFailure
        );
        assert_eq!(cancelled.progress(), segment_len as u64);

        drop(fault_handler);
        tb.state_check(
            State::Busy,
            TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling,
        );
        tb.set_check_timer_expired();

        tb.generic_file_data_insert(
            &mut test_user,
            segment_len as u64,
            &random_data[segment_len..],
        )
        .expect("file data insertion 1 failed");
        tb.handler
            .state_machine_no_packet(&mut test_user)
            .expect("fsm failure");

        tb.check_completion_indication_success(&mut test_user);
        assert!(test_user.indication_queues_empty());
    }

    #[test]
    fn test_check_limit_handling_limit_reached() {
        let mut rng = rand::rng();
        let mut random_data = [0u8; 512];
        rng.fill(&mut random_data);
        let file_size = random_data.len() as u64;
        let segment_len: usize = 256;
        let check_checksum_failure =
            |testbench: &mut DestHandlerTestbench, transaction_id: TransactionId| {
                let mut fault_hook = testbench.fault_handler().user_hook.borrow_mut();
                assert!(fault_hook.notice_of_suspension_queue.is_empty());
                let ignored_queue = &mut fault_hook.ignored_queue;
                assert_eq!(ignored_queue.len(), 1);
                let ignored = ignored_queue.pop_front().unwrap();
                assert_eq!(ignored.transaction_id(), transaction_id);
                assert_eq!(ignored.condition_code(), ConditionCode::FileChecksumFailure);
                assert_eq!(ignored.progress(), segment_len as u64);
            };

        let fault_handler = TestFaultHandler::default();
        let mut testbench = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        let mut test_user = testbench.test_user_from_cached_paths(file_size);
        let transfer_info = testbench
            .generic_transfer_init(&mut test_user, file_size)
            .expect("transfer init failed");

        testbench.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        testbench
            .generic_file_data_insert(&mut test_user, 0, &random_data[0..segment_len])
            .expect("file data insertion 0 failed");
        testbench
            .generic_eof_no_error(&mut test_user, random_data.to_vec())
            .expect("EOF no error insertion failed");
        check_checksum_failure(&mut testbench, transfer_info.id);

        testbench.state_check(
            State::Busy,
            TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling,
        );

        testbench.set_check_timer_expired();
        testbench
            .handler
            .state_machine_no_packet(&mut test_user)
            .expect("fsm error");
        testbench.state_check(
            State::Busy,
            TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling,
        );
        check_checksum_failure(&mut testbench, transfer_info.id);

        testbench.set_check_timer_expired();
        testbench
            .handler
            .state_machine_no_packet(&mut test_user)
            .expect("fsm error");
        check_checksum_failure(&mut testbench, transfer_info.id);

        testbench.state_check(State::Idle, TransactionStep::Idle);

        {
            let mut fault_hook = testbench.fault_handler().user_hook.borrow_mut();
            let cancelled_queue = &mut fault_hook.notice_of_cancellation_queue;
            assert_eq!(cancelled_queue.len(), 1);
            let cancelled = cancelled_queue.pop_front().unwrap();
            assert_eq!(cancelled.transaction_id(), transfer_info.id);
            assert_eq!(cancelled.condition_code(), ConditionCode::CheckLimitReached);
            assert_eq!(cancelled.progress(), segment_len as u64);
        }

        assert_eq!(test_user.finished_indic_queue.len(), 1);
        let finished_indication = test_user.finished_indic_queue.pop_front().unwrap();
        assert_eq!(finished_indication.id, transfer_info.id);
        assert_eq!(
            finished_indication.condition_code,
            ConditionCode::CheckLimitReached
        );
        assert_eq!(finished_indication.delivery_code, DeliveryCode::Incomplete);
        assert_eq!(finished_indication.file_status, FileStatus::Retained);

        assert!(testbench.handler.pdu_sender.queue_empty());

        // Check that the broken file exists.
        testbench.check_dest_file = false;
        assert!(Path::exists(testbench.dest_path()));
        let read_content = fs::read(testbench.dest_path()).expect("reading back string failed");
        assert_eq!(read_content.len(), segment_len);
        assert_eq!(read_content, &random_data[0..segment_len]);
        assert!(fs::remove_file(testbench.dest_path().as_path()).is_ok());
        assert!(test_user.indication_queues_empty());
    }

    fn check_finished_pdu_success(sent_pdu: &SentPdu) {
        assert_eq!(sent_pdu.pdu_type, PduType::FileDirective);
        assert_eq!(
            sent_pdu.file_directive_type,
            Some(FileDirectiveType::FinishedPdu)
        );
        let finished_pdu = FinishedPduReader::from_bytes(&sent_pdu.raw_pdu).unwrap();
        assert_eq!(finished_pdu.file_status(), FileStatus::Retained);
        assert_eq!(finished_pdu.condition_code(), ConditionCode::NoError);
        assert_eq!(finished_pdu.delivery_code(), DeliveryCode::Complete);
        assert!(finished_pdu.fault_location().is_none());
        assert_eq!(finished_pdu.fs_responses_raw(), &[]);
    }

    #[test]
    fn test_file_transfer_with_closure() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            true,
        );
        let mut test_user = tb.test_user_from_cached_paths(0);
        tb.generic_transfer_init(&mut test_user, 0)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        let sent_packets = tb
            .generic_eof_no_error(&mut test_user, Vec::new())
            .expect("EOF no error insertion failed");
        assert_eq!(sent_packets, 1);
        assert!(tb.all_fault_queues_empty());
        // The Finished PDU was sent, so the state machine is done.
        tb.state_check(State::Idle, TransactionStep::Idle);
        assert!(!tb.handler.pdu_sender.queue_empty());
        let sent_pdu = tb.handler.pdu_sender.retrieve_next_pdu().unwrap();
        check_finished_pdu_success(&sent_pdu);
        tb.check_completion_indication_success(&mut test_user);
        assert!(test_user.indication_queues_empty());
    }

    #[test]
    fn test_finished_pdu_insertion_rejected() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        tb.check_dest_file = false;
        let mut user = tb.test_user_from_cached_paths(0);
        let finished_pdu = FinishedPduCreator::new_no_error(
            PduHeader::new_for_file_directive(CommonPduConfig::default(), 0),
            DeliveryCode::Complete,
            FileStatus::Retained,
        );
        let finished_pdu_raw = finished_pdu.to_vec().unwrap();
        let packet_info = PduRawWithInfo::new(&finished_pdu_raw).unwrap();
        let error = tb.handler.state_machine(&mut user, Some(&packet_info));
        assert!(error.is_err());
        let error = error.unwrap_err();
        if let DestError::CantProcessPacketType {
            pdu_type,
            directive_type,
        } = error
        {
            assert_eq!(pdu_type, PduType::FileDirective);
            assert_eq!(directive_type, Some(FileDirectiveType::FinishedPdu));
        }
    }

    #[test]
    fn test_metadata_insertion_twice_fails() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            true,
        );
        let mut user = tb.test_user_from_cached_paths(0);
        tb.generic_transfer_init(&mut user, 0)
            .expect("transfer init failed");
        tb.check_handler_idle_at_drop = false;
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        let pdu_header = PduHeader::new_for_file_directive(tb.pdu_conf, 0);
        let metadata_pdu = create_metadata_pdu(
            &pdu_header,
            tb.src_path.as_path(),
            tb.dest_path.as_path(),
            0,
            tb.closure_requested,
        );
        let packet_info = create_packet_info(&metadata_pdu, &mut tb.buf);
        let error = tb.handler.state_machine(&mut user, Some(&packet_info));
        assert!(error.is_err());
        let error = error.unwrap_err();
        if let DestError::RecvdMetadataButIsBusy = error {
        } else {
            panic!("unexpected error: {:?}", error);
        }
    }

    #[test]
    fn test_checksum_failure_not_acked() {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            true,
        );
        let mut user = tb.test_user_from_cached_paths(file_size);
        tb.generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        let faulty_file_data = b"Hemlo World!";
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 0, faulty_file_data)
                .expect("file data insertion failed"),
            0
        );
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        let sent_packets = tb
            .generic_eof_no_error(&mut user, file_data.into())
            .expect("EOF no error insertion failed");
        // FSM enters check limit algorithm here, so no finished PDU is created.
        assert_eq!(sent_packets, 0);

        let transaction_id = tb.handler.transaction_id().unwrap();
        let mut fault_hook = tb.handler.local_cfg.user_fault_hook().borrow_mut();
        assert!(fault_hook.notice_of_suspension_queue.is_empty());

        // The file checksum failure is ignored by default and check limit handling is now
        // performed.
        let ignored_queue = &mut fault_hook.ignored_queue;
        assert_eq!(ignored_queue.len(), 1);
        let cancelled = ignored_queue.pop_front().unwrap();
        assert_eq!(cancelled.transaction_id(), transaction_id);
        assert_eq!(
            cancelled.condition_code(),
            ConditionCode::FileChecksumFailure
        );
        assert_eq!(cancelled.progress(), file_size);
        drop(fault_hook);

        tb.state_check(
            State::Busy,
            TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling,
        );
        tb.set_check_timer_expired();
        tb.handler
            .state_machine_no_packet(&mut user)
            .expect("fsm error");
        tb.state_check(
            State::Busy,
            TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling,
        );
        tb.set_check_timer_expired();
        tb.handler
            .state_machine_no_packet(&mut user)
            .expect("fsm error");
        tb.state_check(State::Idle, TransactionStep::Idle);

        // Transaction is cancelled because the check limit is reached.
        let mut fault_hook = tb.handler.local_cfg.user_fault_hook().borrow_mut();
        let cancelled_queue = &mut fault_hook.notice_of_cancellation_queue;
        assert_eq!(cancelled_queue.len(), 1);
        let cancelled = cancelled_queue.pop_front().unwrap();
        assert_eq!(cancelled.transaction_id(), transaction_id);
        assert_eq!(cancelled.condition_code(), ConditionCode::CheckLimitReached);
        assert_eq!(cancelled.progress(), file_size);

        drop(fault_hook);

        let sent_pdu = tb.handler.pdu_sender.retrieve_next_pdu().unwrap();
        assert_eq!(sent_pdu.pdu_type, PduType::FileDirective);
        assert_eq!(
            sent_pdu.file_directive_type,
            Some(FileDirectiveType::FinishedPdu)
        );
        let finished_pdu = FinishedPduReader::from_bytes(&sent_pdu.raw_pdu).unwrap();
        assert_eq!(finished_pdu.file_status(), FileStatus::Retained);
        assert_eq!(
            finished_pdu.condition_code(),
            ConditionCode::CheckLimitReached
        );

        let mut fault_hook = tb.handler.local_cfg.user_fault_hook().borrow_mut();
        let ignored_queue = &mut fault_hook.ignored_queue;
        assert_eq!(ignored_queue.len(), 2);
        let mut ignored = ignored_queue.pop_front().unwrap();
        assert_eq!(ignored.transaction_id(), transaction_id);
        assert_eq!(ignored.condition_code(), ConditionCode::FileChecksumFailure);
        assert_eq!(ignored.progress(), file_size);
        ignored = ignored_queue.pop_front().unwrap();
        assert_eq!(ignored.transaction_id(), transaction_id);
        assert_eq!(ignored.condition_code(), ConditionCode::FileChecksumFailure);
        assert_eq!(ignored.progress(), file_size);

        assert_eq!(finished_pdu.delivery_code(), DeliveryCode::Incomplete);
        assert!(finished_pdu.fault_location().is_some());
        assert_eq!(
            *finished_pdu.fault_location().unwrap().entity_id(),
            REMOTE_ID.into()
        );
        assert_eq!(finished_pdu.fs_responses_raw(), &[]);
        assert!(tb.handler.pdu_sender.queue_empty());
        user.verify_finished_indication_retained(
            DeliveryCode::Incomplete,
            ConditionCode::CheckLimitReached,
            transaction_id,
        );
        tb.expected_full_data = faulty_file_data.to_vec();
    }

    #[test]
    fn test_file_copy_to_directory() {
        let fault_handler = TestFaultHandler::default();
        let src_path = tempfile::TempPath::from_path("/tmp/test.txt").to_path_buf();
        let dest_path = tempfile::TempDir::new().unwrap();
        let mut dest_path_buf = dest_path.keep();
        let mut tb = DestHandlerTestbench::new(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
            false,
            src_path.clone(),
            dest_path_buf.clone(),
        );
        dest_path_buf.push(src_path.file_name().unwrap());
        tb.dest_path = dest_path_buf;
        let mut user = tb.test_user_from_cached_paths(0);
        tb.generic_transfer_init(&mut user, 0)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.generic_eof_no_error(&mut user, Vec::new())
            .expect("EOF no error insertion failed");
        tb.check_completion_indication_success(&mut user);
    }

    #[test]
    fn test_tranfer_cancellation_empty_file_with_eof_pdu() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        let mut user = tb.test_user_from_cached_paths(0);
        let transfer_info = tb
            .generic_transfer_init(&mut user, 0)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        let pdu_header = PduHeader::new_for_file_directive(tb.pdu_conf, 0);
        let cancel_eof = EofPdu::new(
            pdu_header,
            ConditionCode::CancelRequestReceived,
            0,
            0,
            Some(EntityIdTlv::new(LOCAL_ID.into())),
        );
        let packets = tb
            .handler
            .state_machine(
                &mut user,
                Some(&PduRawWithInfo::new(&cancel_eof.to_vec().unwrap()).unwrap()),
            )
            .expect("state machine call with EOF insertion failed");
        assert_eq!(packets, 0);
        user.verify_finished_indication_retained(
            DeliveryCode::Complete,
            ConditionCode::CancelRequestReceived,
            transfer_info.id,
        );
    }

    fn generic_tranfer_cancellation_partial_file_with_eof_pdu(
        with_closure: bool,
        insert_packet: bool,
    ) {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = 5;

        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            with_closure,
        );
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        if insert_packet {
            tb.generic_file_data_insert(&mut user, 0, &file_data[0..5])
                .expect("file data insertion failed");
        }
        let mut digest = CRC_32.digest();
        digest.update(&file_data[0..5]);
        let checksum = digest.finalize();
        let pdu_header = PduHeader::new_for_file_directive(tb.pdu_conf, 0);
        let cancel_eof = EofPdu::new(
            pdu_header,
            ConditionCode::CancelRequestReceived,
            checksum,
            5,
            Some(EntityIdTlv::new(LOCAL_ID.into())),
        );
        let packets = tb
            .handler
            .state_machine(
                &mut user,
                Some(&PduRawWithInfo::new(&cancel_eof.to_vec().unwrap()).unwrap()),
            )
            .expect("state machine call with EOF insertion failed");
        if with_closure {
            assert_eq!(packets, 1);
            let finished_pdu = tb.handler.pdu_sender.retrieve_next_pdu().unwrap();
            assert_eq!(finished_pdu.pdu_type, PduType::FileDirective);
            assert_eq!(
                finished_pdu.file_directive_type.unwrap(),
                FileDirectiveType::FinishedPdu
            );
            let finished_pdu = FinishedPduReader::from_bytes(&finished_pdu.raw_pdu).unwrap();
            assert_eq!(
                finished_pdu.condition_code(),
                ConditionCode::CancelRequestReceived
            );
            if insert_packet {
                // Checksum success, so data is complete.
                assert_eq!(finished_pdu.delivery_code(), DeliveryCode::Complete);
                user.verify_finished_indication_retained(
                    DeliveryCode::Complete,
                    ConditionCode::CancelRequestReceived,
                    transfer_info.id,
                );
            } else {
                // Checksum failure, so data is incomplete.
                assert_eq!(finished_pdu.delivery_code(), DeliveryCode::Incomplete);
                tb.check_dest_file = false;
                let mut fault_hook = tb.handler.local_cfg.user_fault_hook().borrow_mut();
                let ignored_queue = &mut fault_hook.ignored_queue;
                let ignored = ignored_queue.pop_front().unwrap();
                assert_eq!(ignored.transaction_id(), transfer_info.id);
                assert_eq!(ignored.condition_code(), ConditionCode::FileChecksumFailure);
                assert_eq!(ignored.progress(), 5);
                user.verify_finished_indication_retained(
                    DeliveryCode::Incomplete,
                    ConditionCode::CancelRequestReceived,
                    transfer_info.id,
                );
            }
            assert_eq!(finished_pdu.file_status(), FileStatus::Retained);
            assert_eq!(
                finished_pdu
                    .fault_location()
                    .expect("no fault location set"),
                EntityIdTlv::new(LOCAL_ID.into())
            );
        } else {
            user.verify_finished_indication_retained(
                DeliveryCode::Complete,
                ConditionCode::CancelRequestReceived,
                transfer_info.id,
            );
            assert_eq!(packets, 0);
        }
        tb.expected_file_size = file_size;
        tb.expected_full_data = file_data[0..file_size as usize].to_vec();
    }

    #[test]
    fn test_tranfer_cancellation_partial_file_with_eof_pdu_no_closure_complete() {
        generic_tranfer_cancellation_partial_file_with_eof_pdu(false, true);
    }

    #[test]
    fn test_tranfer_cancellation_partial_file_with_eof_pdu_with_closure_complete() {
        generic_tranfer_cancellation_partial_file_with_eof_pdu(true, true);
    }

    #[test]
    fn test_tranfer_cancellation_partial_file_with_eof_pdu_with_closure_incomplete() {
        generic_tranfer_cancellation_partial_file_with_eof_pdu(true, false);
    }

    #[test]
    fn test_tranfer_cancellation_empty_file_with_cancel_api() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        let mut user = tb.test_user_from_cached_paths(0);
        let transfer_info = tb
            .generic_transfer_init(&mut user, 0)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.handler.cancel_request(&transfer_info.id);
        let packets = tb
            .handler
            .state_machine_no_packet(&mut user)
            .expect("state machine call with EOF insertion failed");
        user.verify_finished_indication_retained(
            DeliveryCode::Complete,
            ConditionCode::CancelRequestReceived,
            transfer_info.id,
        );
        assert_eq!(packets, 0);
    }

    #[test]
    fn test_tranfer_cancellation_empty_file_with_cancel_api_and_closure() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            true,
        );
        let mut user = tb.test_user_from_cached_paths(0);
        let transfer_info = tb
            .generic_transfer_init(&mut user, 0)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.handler.cancel_request(&transfer_info.id);
        let packets = tb
            .handler
            .state_machine_no_packet(&mut user)
            .expect("state machine call with EOF insertion failed");
        assert_eq!(packets, 1);
        let next_pdu = tb.get_next_pdu().unwrap();
        assert_eq!(next_pdu.pdu_type, PduType::FileDirective);
        assert_eq!(
            next_pdu.file_directive_type.unwrap(),
            FileDirectiveType::FinishedPdu
        );
        let finished_pdu =
            FinishedPduReader::new(&next_pdu.raw_pdu).expect("finished pdu read failed");
        assert_eq!(
            finished_pdu.condition_code(),
            ConditionCode::CancelRequestReceived
        );
        assert_eq!(finished_pdu.file_status(), FileStatus::Retained);
        // Empty file, so still complete.
        assert_eq!(finished_pdu.delivery_code(), DeliveryCode::Complete);
        assert_eq!(
            finished_pdu.fault_location(),
            Some(EntityIdTlv::new(REMOTE_ID.into()))
        );
        user.verify_finished_indication_retained(
            DeliveryCode::Complete,
            ConditionCode::CancelRequestReceived,
            transfer_info.id,
        );
    }

    #[test]
    fn test_tranfer_cancellation_partial_file_with_cancel_api_and_closure() {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = 5;

        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            true,
        );
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.generic_file_data_insert(&mut user, 0, &file_data[0..5])
            .expect("file data insertion failed");

        tb.handler.cancel_request(&transfer_info.id);
        let packets = tb
            .handler
            .state_machine_no_packet(&mut user)
            .expect("state machine call with EOF insertion failed");
        assert_eq!(packets, 1);
        let next_pdu = tb.get_next_pdu().unwrap();
        assert_eq!(next_pdu.pdu_type, PduType::FileDirective);
        assert_eq!(
            next_pdu.file_directive_type.unwrap(),
            FileDirectiveType::FinishedPdu
        );
        let finished_pdu =
            FinishedPduReader::new(&next_pdu.raw_pdu).expect("finished pdu read failed");
        assert_eq!(
            finished_pdu.condition_code(),
            ConditionCode::CancelRequestReceived
        );
        assert_eq!(finished_pdu.file_status(), FileStatus::Retained);
        assert_eq!(finished_pdu.delivery_code(), DeliveryCode::Incomplete);
        assert_eq!(
            finished_pdu.fault_location(),
            Some(EntityIdTlv::new(REMOTE_ID.into()))
        );
        tb.expected_file_size = file_size;
        tb.expected_full_data = file_data[0..file_size as usize].to_vec();
        user.verify_finished_indication_retained(
            DeliveryCode::Incomplete,
            ConditionCode::CancelRequestReceived,
            transfer_info.id,
        );
    }

    // Only incomplete received files will be removed.
    #[test]
    fn test_tranfer_cancellation_file_disposition_not_done_for_empty_file() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        let remote_cfg_mut = tb
            .handler
            .remote_cfg_table
            .get_mut(LOCAL_ID.value())
            .unwrap();
        remote_cfg_mut.disposition_on_cancellation = true;
        let mut user = tb.test_user_from_cached_paths(0);
        let transfer_info = tb
            .generic_transfer_init(&mut user, 0)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);

        tb.handler.cancel_request(&transfer_info.id);
        let packets = tb
            .handler
            .state_machine_no_packet(&mut user)
            .expect("state machine call with EOF insertion failed");
        assert_eq!(packets, 0);
        user.verify_finished_indication_retained(
            DeliveryCode::Complete,
            ConditionCode::CancelRequestReceived,
            transfer_info.id,
        );
    }

    #[test]
    fn test_tranfer_cancellation_file_disposition_not_done_for_incomplete_file() {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = file_data.len() as u64;

        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Unacknowledged,
            false,
        );
        tb.check_dest_file = false;
        let remote_cfg_mut = tb
            .handler
            .remote_cfg_table
            .get_mut(LOCAL_ID.value())
            .unwrap();
        remote_cfg_mut.disposition_on_cancellation = true;
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.generic_file_data_insert(&mut user, 0, &file_data[0..5])
            .expect("file data insertion failed");

        tb.handler.cancel_request(&transfer_info.id);
        let packets = tb
            .handler
            .state_machine_no_packet(&mut user)
            .expect("state machine call with EOF insertion failed");
        assert_eq!(packets, 0);
        // File was disposed.
        assert!(!Path::exists(tb.dest_path()));
        assert_eq!(user.finished_indic_queue.len(), 1);
        user.verify_finished_indication(
            DeliveryCode::Incomplete,
            ConditionCode::CancelRequestReceived,
            transfer_info.id,
            FileStatus::DiscardDeliberately,
        );
    }

    fn generic_test_immediate_nak_request(file_flag: LargeFileFlag) {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        tb.set_large_file_flag(file_flag);
        tb.remote_cfg_mut().immediate_nak_mode = true;
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);

        // This should immediately trigger a NAK request for file segment 0..4
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 4, &file_data[4..])
                .expect("file data insertion failed"),
            1
        );
        assert_eq!(tb.pdu_queue_len(), 1);
        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.pdu_header().common_pdu_conf().file_flag, file_flag);
        assert_eq!(nak_pdu.start_of_scope(), 0);
        assert_eq!(nak_pdu.end_of_scope(), file_size);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs.len(), 1);
        assert_eq!(seg_reqs[0], (0, 4));
        // We simulate the reply by re-inserting the missing file segment.
        tb.generic_file_data_insert(&mut user, 0, &file_data[0..4])
            .expect("file data insertion failed");

        tb.generic_eof_no_error(&mut user, file_data.to_vec())
            .expect("EOF no error insertion failed");
        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    #[test]
    fn test_immediate_nak_request() {
        generic_test_immediate_nak_request(LargeFileFlag::Normal);
    }

    #[test]
    fn test_immediate_nak_request_large_file() {
        generic_test_immediate_nak_request(LargeFileFlag::Large);
    }

    fn generic_test_deferred_nak_request(file_flag: LargeFileFlag) {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        tb.set_large_file_flag(file_flag);
        // Disable this, we only want to check the deferred procedure.
        tb.remote_cfg_mut().immediate_nak_mode = false;
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);

        assert_eq!(
            tb.generic_file_data_insert(&mut user, 4, &file_data[4..])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_eof_no_error(&mut user, file_data.to_vec())
                .expect("EOF no error insertion failed"),
            2
        );
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        assert_eq!(tb.pdu_queue_len(), 1);
        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.start_of_scope(), 0);
        assert_eq!(nak_pdu.end_of_scope(), file_size);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs.len(), 1);
        assert_eq!(seg_reqs[0], (0, 4));

        // We simulate the reply by re-inserting the missing file segment.
        tb.generic_file_data_insert(&mut user, 0, &file_data[0..4])
            .expect("file data insertion failed");
        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 1);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    #[test]
    fn test_deferred_nak_request() {
        generic_test_deferred_nak_request(LargeFileFlag::Normal);
    }

    #[test]
    fn test_deferred_nak_request_large() {
        generic_test_deferred_nak_request(LargeFileFlag::Large);
    }

    #[test]
    fn test_file_data_before_metadata() {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        tb.remote_cfg_mut().immediate_nak_mode = true;
        let mut user = tb.test_user_from_cached_paths(file_size);
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 0, file_data)
                .expect("file data insertion failed"),
            1
        );
        tb.state_check(State::Busy, TransactionStep::WaitingForMetadata);
        assert_eq!(tb.pdu_queue_len(), 1);
        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.start_of_scope(), 0);
        assert_eq!(nak_pdu.end_of_scope(), file_size);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs.len(), 2);
        // Metadata is re-requested.
        assert_eq!(seg_reqs[0], (0, 0));
        // File data is re-requested. The destination handler can not do anything with the file
        // data as long as no metadata has been received. Buffering of FD PDUs is not supported.
        assert_eq!(seg_reqs[1], (0, 12));

        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        // Re-insert missing file data.
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 0, file_data)
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_eof_no_error(&mut user, file_data.to_vec())
                .expect("EOF no error insertion failed"),
            2
        );
        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    #[test]
    fn test_eof_before_metadata() {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        tb.remote_cfg_mut().immediate_nak_mode = true;
        let mut user = tb.test_user_from_cached_paths(file_size);
        // We expect the ACK for the EOF and a NAK packet re-requesting the metadata and file data.
        assert_eq!(
            tb.generic_eof_no_error(&mut user, file_data.to_vec())
                .expect("file data insertion failed"),
            2
        );
        tb.state_check(State::Busy, TransactionStep::WaitingForMetadata);
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.start_of_scope(), 0);
        assert_eq!(nak_pdu.end_of_scope(), file_size);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs.len(), 2);
        // Metadata is re-requested.
        assert_eq!(seg_reqs[0], (0, 0));
        // File data is re-requested. The destination handler can not do anything with the file
        // data as long as no metadata has been received. Buffering of FD PDUs is not supported.
        assert_eq!(seg_reqs[1], (0, 12));

        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        // Re-insert missing file data.
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 0, file_data)
                .expect("file data insertion failed"),
            1
        );
        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 1);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    #[test]
    fn test_nak_limit_reached() {
        let file_data_str = "Hello World!";
        let file_data = file_data_str.as_bytes();
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        // Disable this, we only want to check the deferred procedure.
        tb.remote_cfg_mut().immediate_nak_mode = false;
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);

        assert_eq!(
            tb.generic_file_data_insert(&mut user, 4, &file_data[4..])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_eof_no_error(&mut user, file_data.to_vec())
                .expect("EOF no error insertion failed"),
            2
        );
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        assert_eq!(tb.pdu_queue_len(), 1);
        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.start_of_scope(), 0);
        assert_eq!(nak_pdu.end_of_scope(), file_size);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs.len(), 1);
        assert_eq!(seg_reqs[0], (0, 4));

        // Let the NAK timer expire
        tb.set_nak_activity_timer_expired();
        assert_eq!(tb.handler.state_machine_no_packet(&mut user).unwrap(), 1);
        assert_eq!(tb.pdu_queue_len(), 1);
        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.start_of_scope(), 0);
        assert_eq!(nak_pdu.end_of_scope(), file_size);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs.len(), 1);
        assert_eq!(seg_reqs[0], (0, 4));

        // Let the NAK timer expire again.
        tb.set_nak_activity_timer_expired();
        assert_eq!(tb.handler.state_machine_no_packet(&mut user).unwrap(), 1);
        assert_eq!(tb.pdu_queue_len(), 1);
        tb.check_completion_indication_failure(
            &mut user,
            ConditionCode::NakLimitReached,
            FileStatus::Retained,
            DeliveryCode::Incomplete,
        );
        tb.check_finished_pdu_failure(
            ConditionCode::NakLimitReached,
            FileStatus::Retained,
            DeliveryCode::Incomplete,
        );

        {
            let mut fault_hook = tb.fault_handler().user_hook.borrow_mut();
            assert_eq!(fault_hook.notice_of_cancellation_queue.len(), 1);
            let cancellation = fault_hook.notice_of_cancellation_queue.pop_front().unwrap();
            assert_eq!(cancellation.transaction_id(), transfer_info.id);
            assert_eq!(
                cancellation.condition_code(),
                ConditionCode::NakLimitReached
            );
            assert_eq!(cancellation.progress(), file_size);
        }
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
        tb.check_dest_file = false;
    }

    #[test]
    fn test_positive_ack_procedure() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        let mut user = tb.test_user_from_cached_paths(0);
        let transfer_info = tb
            .generic_transfer_init(&mut user, 0)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.generic_eof_no_error(&mut user, Vec::new())
            .expect("EOF no error insertion failed");
        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        tb.check_finished_pdu_success();

        tb.set_positive_ack_expired();
        // This should cause the PDU to be sent again.
        assert_eq!(tb.handler.state_machine_no_packet(&mut user).unwrap(), 1);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    fn generic_positive_ack_test(
        tb: &mut DestHandlerTestbench,
        user: &mut TestCfdpUser,
    ) -> TransferInfo {
        let transfer_info = tb
            .generic_transfer_init(user, 0)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);
        tb.generic_eof_no_error(user, Vec::new())
            .expect("EOF no error insertion failed");
        tb.check_completion_indication_success(user);
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        tb.check_finished_pdu_success();

        // This should cause the PDU to be sent again.
        tb.set_positive_ack_expired();
        assert_eq!(tb.handler.state_machine_no_packet(user).unwrap(), 1);
        tb.check_finished_pdu_success();

        // Positive ACK limit reached.
        tb.set_positive_ack_expired();
        assert_eq!(tb.handler.state_machine_no_packet(user).unwrap(), 1);

        tb.check_finished_pdu_failure(
            ConditionCode::PositiveAckLimitReached,
            FileStatus::Retained,
            DeliveryCode::Complete,
        );
        tb.check_completion_indication_failure(
            user,
            ConditionCode::PositiveAckLimitReached,
            FileStatus::Retained,
            DeliveryCode::Complete,
        );
        {
            let mut fault_handler = tb.fault_handler().user_hook.borrow_mut();
            assert!(!fault_handler.cancellation_queue_empty());
            let cancellation = fault_handler
                .notice_of_cancellation_queue
                .pop_front()
                .unwrap();
            assert_eq!(cancellation.transaction_id(), transfer_info.id);
            assert_eq!(
                cancellation.condition_code(),
                ConditionCode::PositiveAckLimitReached
            );
            assert_eq!(cancellation.progress(), 0);
        }
        transfer_info
    }

    #[test]
    fn test_positive_ack_limit_reached() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        let mut user = tb.test_user_from_cached_paths(0);
        let transfer_info = generic_positive_ack_test(&mut tb, &mut user);
        // Chances are that this one won't work either leading to transfer abandonment, but we
        // acknowledge it here
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    #[test]
    fn test_positive_ack_limit_reached_with_subsequent_abandonment() {
        let fault_handler = TestFaultHandler::default();
        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        let mut user = tb.test_user_from_cached_paths(0);
        let transfer_info = generic_positive_ack_test(&mut tb, &mut user);

        // This should cause the PDU to be sent again.
        tb.set_positive_ack_expired();
        assert_eq!(tb.handler.state_machine_no_packet(&mut user).unwrap(), 1);
        tb.check_finished_pdu_failure(
            ConditionCode::PositiveAckLimitReached,
            FileStatus::Retained,
            DeliveryCode::Complete,
        );

        // Postive ACK limit reached which leads to abandonment.
        tb.set_positive_ack_expired();
        assert_eq!(tb.handler.state_machine_no_packet(&mut user).unwrap(), 0);
        {
            let mut fault_handler = tb.fault_handler().user_hook.borrow_mut();
            assert!(!fault_handler.abandoned_queue_empty());
            let cancellation = fault_handler.abandoned_queue.pop_front().unwrap();
            assert_eq!(cancellation.transaction_id(), transfer_info.id);
            assert_eq!(
                cancellation.condition_code(),
                ConditionCode::PositiveAckLimitReached
            );
            assert_eq!(cancellation.progress(), 0);
        }
    }

    #[test]
    fn test_multi_segment_nak() {
        let file_data_str = "Hello Wooorld!";
        let file_data = file_data_str.as_bytes();
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        // Disable this, we only want to check the deferred procedure.
        tb.remote_cfg_mut().immediate_nak_mode = false;
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);

        assert_eq!(
            tb.generic_file_data_insert(&mut user, 2, &file_data[2..4])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 6, &file_data[6..8])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 10, &file_data[10..])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_eof_no_error(&mut user, file_data.to_vec())
                .expect("EOF no error insertion failed"),
            2
        );
        assert_eq!(tb.pdu_queue_len(), 2);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        assert_eq!(tb.pdu_queue_len(), 1);
        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.start_of_scope(), 0);
        assert_eq!(nak_pdu.end_of_scope(), file_size);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs.len(), 3);
        assert_eq!(seg_reqs[0], (0, 2));
        assert_eq!(seg_reqs[1], (4, 6));
        assert_eq!(seg_reqs[2], (8, 10));

        // We simulate the reply by re-inserting the missing file segment.
        tb.generic_file_data_insert(&mut user, 0, &file_data[0..2])
            .expect("file data insertion failed");
        tb.generic_file_data_insert(&mut user, 4, &file_data[4..6])
            .expect("file data insertion failed");
        tb.generic_file_data_insert(&mut user, 8, &file_data[8..10])
            .expect("file data insertion failed");
        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 1);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    #[test]
    fn test_multi_packet_nak_sequence_large_file_flag() {
        let file_data = [0_u8; 64];
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        // Disable this, we only want to check the deferred procedure.
        tb.remote_cfg_mut().immediate_nak_mode = false;
        let max_packet_len = 80;
        tb.remote_cfg_mut().max_packet_len = max_packet_len;
        tb.set_large_file_flag(LargeFileFlag::Large);
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);

        assert_eq!(
            NakPduCreatorWithReservedSeqReqsBuf::calculate_max_segment_requests(
                max_packet_len,
                &PduHeader::new_for_file_directive(tb.pdu_conf, 0),
            )
            .unwrap(),
            3
        );
        let missing_segs_0: &[(u64, u64)] = &[(0, 2), (4, 6), (8, 10)];
        let missing_segs_1: &[(u64, u64)] = &[(12, 14)];
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 2, &file_data[2..4])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 6, &file_data[6..8])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 10, &file_data[10..12])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 14, &file_data[14..file_size as usize])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_eof_no_error(&mut user, file_data.to_vec())
                .expect("EOF no error insertion failed"),
            3
        );
        assert_eq!(tb.pdu_queue_len(), 3);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.start_of_scope(), 0);
        assert_eq!(nak_pdu.end_of_scope(), 10);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs, missing_segs_0);

        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.start_of_scope(), 10);
        assert_eq!(nak_pdu.end_of_scope(), file_size);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs, missing_segs_1);

        for missing_seg in missing_segs_0 {
            assert_eq!(
                tb.generic_file_data_insert(
                    &mut user,
                    missing_seg.0,
                    &file_data[missing_seg.0 as usize..missing_seg.1 as usize]
                )
                .expect("file data insertion failed"),
                0
            );
        }
        for missing_seg in missing_segs_1 {
            assert_eq!(
                tb.generic_file_data_insert(
                    &mut user,
                    missing_seg.0,
                    &file_data[missing_seg.0 as usize..missing_seg.1 as usize]
                )
                .expect("file data insertion failed"),
                1
            );
        }

        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 1);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }

    #[test]
    fn test_multi_packet_nak_sequence_normal_file_flag() {
        let file_data = [0_u8; 64];
        let file_size = file_data.len() as u64;
        let fault_handler = TestFaultHandler::default();

        let mut tb = DestHandlerTestbench::new_with_fixed_paths(
            fault_handler,
            TransmissionMode::Acknowledged,
            false,
        );
        // Disable this, we only want to check the deferred procedure.
        tb.remote_cfg_mut().immediate_nak_mode = false;
        let max_packet_len = 50;
        tb.remote_cfg_mut().max_packet_len = max_packet_len;
        let mut user = tb.test_user_from_cached_paths(file_size);
        let transfer_info = tb
            .generic_transfer_init(&mut user, file_size)
            .expect("transfer init failed");
        tb.state_check(State::Busy, TransactionStep::ReceivingFileDataPdus);

        assert_eq!(
            NakPduCreatorWithReservedSeqReqsBuf::calculate_max_segment_requests(
                max_packet_len,
                &PduHeader::new_for_file_directive(tb.pdu_conf, 0),
            )
            .unwrap(),
            4
        );
        let missing_segs_0: &[(u64, u64)] = &[(0, 2), (4, 6), (8, 10), (12, 14)];
        let missing_segs_1: &[(u64, u64)] = &[(16, 18)];
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 2, &file_data[2..4])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 6, &file_data[6..8])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 10, &file_data[10..12])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 14, &file_data[14..16])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_file_data_insert(&mut user, 18, &file_data[18..file_size as usize])
                .expect("file data insertion failed"),
            0
        );
        assert_eq!(
            tb.generic_eof_no_error(&mut user, file_data.to_vec())
                .expect("EOF no error insertion failed"),
            3
        );
        assert_eq!(tb.pdu_queue_len(), 3);
        tb.check_eof_ack_pdu(ConditionCode::NoError);
        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.start_of_scope(), 0);
        assert_eq!(nak_pdu.end_of_scope(), 14);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs, missing_segs_0);

        let pdu = tb.get_next_pdu().unwrap();
        assert_eq!(pdu.pdu_type, PduType::FileDirective);
        assert_eq!(pdu.file_directive_type.unwrap(), FileDirectiveType::NakPdu);
        let nak_pdu = NakPduReader::new(&pdu.raw_pdu).unwrap();
        assert_eq!(nak_pdu.start_of_scope(), 14);
        assert_eq!(nak_pdu.end_of_scope(), file_size);
        let seg_reqs: Vec<(u64, u64)> = nak_pdu.get_segment_requests_iterator().unwrap().collect();
        assert_eq!(seg_reqs, missing_segs_1);

        for missing_seg in missing_segs_0 {
            assert_eq!(
                tb.generic_file_data_insert(
                    &mut user,
                    missing_seg.0,
                    &file_data[missing_seg.0 as usize..missing_seg.1 as usize]
                )
                .expect("file data insertion failed"),
                0
            );
        }
        for missing_seg in missing_segs_1 {
            assert_eq!(
                tb.generic_file_data_insert(
                    &mut user,
                    missing_seg.0,
                    &file_data[missing_seg.0 as usize..missing_seg.1 as usize]
                )
                .expect("file data insertion failed"),
                1
            );
        }

        tb.check_completion_indication_success(&mut user);
        assert_eq!(tb.pdu_queue_len(), 1);
        tb.check_finished_pdu_success();
        tb.acknowledge_finished_pdu(&mut user, &transfer_info);
    }
}