[rust-lightning] / lightning / src / ln / interactivetxs.rs

// This file is Copyright its original authors, visible in version control
// history.
//
// This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
// or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
// You may not use this file except in accordance with one or both of these
// licenses.

use crate::io_extras::sink;
use crate::prelude::*;
use core::ops::Deref;

use bitcoin::blockdata::constants::WITNESS_SCALE_FACTOR;
use bitcoin::consensus::Encodable;
use bitcoin::policy::MAX_STANDARD_TX_WEIGHT;
use bitcoin::{
        absolute::LockTime as AbsoluteLockTime, OutPoint, Sequence, Transaction, TxIn, TxOut,
};

use crate::chain::chaininterface::fee_for_weight;
use crate::events::bump_transaction::{BASE_INPUT_WEIGHT, EMPTY_SCRIPT_SIG_WEIGHT};
use crate::ln::channel::TOTAL_BITCOIN_SUPPLY_SATOSHIS;
use crate::ln::msgs::SerialId;
use crate::ln::{msgs, ChannelId};
use crate::sign::EntropySource;
use crate::util::ser::TransactionU16LenLimited;

/// The number of received `tx_add_input` messages during a negotiation at which point the
/// negotiation MUST be failed.
const MAX_RECEIVED_TX_ADD_INPUT_COUNT: u16 = 4096;

/// The number of received `tx_add_output` messages during a negotiation at which point the
/// negotiation MUST be failed.
const MAX_RECEIVED_TX_ADD_OUTPUT_COUNT: u16 = 4096;

/// The number of inputs or outputs that the state machine can have, before it MUST fail the
/// negotiation.
const MAX_INPUTS_OUTPUTS_COUNT: usize = 252;

trait SerialIdExt {
        fn is_for_initiator(&self) -> bool;
        fn is_for_non_initiator(&self) -> bool;
}

impl SerialIdExt for SerialId {
        fn is_for_initiator(&self) -> bool {
                self % 2 == 0
        }

        fn is_for_non_initiator(&self) -> bool {
                !self.is_for_initiator()
        }
}

#[derive(Debug, Clone, PartialEq)]
pub enum AbortReason {
        InvalidStateTransition,
        UnexpectedCounterpartyMessage,
        ReceivedTooManyTxAddInputs,
        ReceivedTooManyTxAddOutputs,
        IncorrectInputSequenceValue,
        IncorrectSerialIdParity,
        SerialIdUnknown,
        DuplicateSerialId,
        PrevTxOutInvalid,
        ExceededMaximumSatsAllowed,
        ExceededNumberOfInputsOrOutputs,
        TransactionTooLarge,
        BelowDustLimit,
        InvalidOutputScript,
        InsufficientFees,
        OutputsValueExceedsInputsValue,
        InvalidTx,
}

#[derive(Debug)]
pub struct TxInputWithPrevOutput {
        input: TxIn,
        prev_output: TxOut,
}

#[derive(Debug)]
struct NegotiationContext {
        holder_is_initiator: bool,
        received_tx_add_input_count: u16,
        received_tx_add_output_count: u16,
        inputs: HashMap<SerialId, TxInputWithPrevOutput>,
        prevtx_outpoints: HashSet<OutPoint>,
        outputs: HashMap<SerialId, TxOut>,
        tx_locktime: AbsoluteLockTime,
        feerate_sat_per_kw: u32,
}

impl NegotiationContext {
        fn is_serial_id_valid_for_counterparty(&self, serial_id: &SerialId) -> bool {
                // A received `SerialId`'s parity must match the role of the counterparty.
                self.holder_is_initiator == serial_id.is_for_non_initiator()
        }

        fn total_input_and_output_count(&self) -> usize {
                self.inputs.len().saturating_add(self.outputs.len())
        }

        fn counterparty_inputs_contributed(
                &self,
        ) -> impl Iterator<Item = &TxInputWithPrevOutput> + Clone {
                self.inputs
                        .iter()
                        .filter(move |(serial_id, _)| self.is_serial_id_valid_for_counterparty(serial_id))
                        .map(|(_, input_with_prevout)| input_with_prevout)
        }

        fn counterparty_outputs_contributed(&self) -> impl Iterator<Item = &TxOut> + Clone {
                self.outputs
                        .iter()
                        .filter(move |(serial_id, _)| self.is_serial_id_valid_for_counterparty(serial_id))
                        .map(|(_, output)| output)
        }

        fn received_tx_add_input(&mut self, msg: &msgs::TxAddInput) -> Result<(), AbortReason> {
                // The interactive-txs spec calls for us to fail negotiation if the `prevtx` we receive is
                // invalid. However, we would not need to account for this explicit negotiation failure
                // mode here since `PeerManager` would already disconnect the peer if the `prevtx` is
                // invalid; implicitly ending the negotiation.

                if !self.is_serial_id_valid_for_counterparty(&msg.serial_id) {
                        // The receiving node:
                        //  - MUST fail the negotiation if:
                        //     - the `serial_id` has the wrong parity
                        return Err(AbortReason::IncorrectSerialIdParity);
                }

                self.received_tx_add_input_count += 1;
                if self.received_tx_add_input_count > MAX_RECEIVED_TX_ADD_INPUT_COUNT {
                        // The receiving node:
                        //  - MUST fail the negotiation if:
                        //     - if has received 4096 `tx_add_input` messages during this negotiation
                        return Err(AbortReason::ReceivedTooManyTxAddInputs);
                }

                if msg.sequence >= 0xFFFFFFFE {
                        // The receiving node:
                        //  - MUST fail the negotiation if:
                        //    - `sequence` is set to `0xFFFFFFFE` or `0xFFFFFFFF`
                        return Err(AbortReason::IncorrectInputSequenceValue);
                }

                let transaction = msg.prevtx.as_transaction();
                let txid = transaction.txid();

                if let Some(tx_out) = transaction.output.get(msg.prevtx_out as usize) {
                        if !tx_out.script_pubkey.is_witness_program() {
                                // The receiving node:
                                //  - MUST fail the negotiation if:
                                //     - the `scriptPubKey` is not a witness program
                                return Err(AbortReason::PrevTxOutInvalid);
                        }

                        if !self.prevtx_outpoints.insert(OutPoint { txid, vout: msg.prevtx_out }) {
                                // The receiving node:
                                //  - MUST fail the negotiation if:
                                //     - the `prevtx` and `prevtx_vout` are identical to a previously added
                                //       (and not removed) input's
                                return Err(AbortReason::PrevTxOutInvalid);
                        }
                } else {
                        // The receiving node:
                        //  - MUST fail the negotiation if:
                        //     - `prevtx_vout` is greater or equal to the number of outputs on `prevtx`
                        return Err(AbortReason::PrevTxOutInvalid);
                }

                let prev_out = if let Some(prev_out) = transaction.output.get(msg.prevtx_out as usize) {
                        prev_out.clone()
                } else {
                        return Err(AbortReason::PrevTxOutInvalid);
                };
                match self.inputs.entry(msg.serial_id) {
                        hash_map::Entry::Occupied(_) => {
                                // The receiving node:
                                //  - MUST fail the negotiation if:
                                //    - the `serial_id` is already included in the transaction
                                Err(AbortReason::DuplicateSerialId)
                        },
                        hash_map::Entry::Vacant(entry) => {
                                let prev_outpoint = OutPoint { txid, vout: msg.prevtx_out };
                                entry.insert(TxInputWithPrevOutput {
                                        input: TxIn {
                                                previous_output: prev_outpoint,
                                                sequence: Sequence(msg.sequence),
                                                ..Default::default()
                                        },
                                        prev_output: prev_out,
                                });
                                self.prevtx_outpoints.insert(prev_outpoint);
                                Ok(())
                        },
                }
        }

        fn received_tx_remove_input(&mut self, msg: &msgs::TxRemoveInput) -> Result<(), AbortReason> {
                if !self.is_serial_id_valid_for_counterparty(&msg.serial_id) {
                        return Err(AbortReason::IncorrectSerialIdParity);
                }

                self.inputs
                        .remove(&msg.serial_id)
                        // The receiving node:
                        //  - MUST fail the negotiation if:
                        //    - the input or output identified by the `serial_id` was not added by the sender
                        //    - the `serial_id` does not correspond to a currently added input
                        .ok_or(AbortReason::SerialIdUnknown)
                        .map(|_| ())
        }

        fn received_tx_add_output(&mut self, msg: &msgs::TxAddOutput) -> Result<(), AbortReason> {
                // The receiving node:
                //  - MUST fail the negotiation if:
                //     - the serial_id has the wrong parity
                if !self.is_serial_id_valid_for_counterparty(&msg.serial_id) {
                        return Err(AbortReason::IncorrectSerialIdParity);
                }

                self.received_tx_add_output_count += 1;
                if self.received_tx_add_output_count > MAX_RECEIVED_TX_ADD_OUTPUT_COUNT {
                        // The receiving node:
                        //  - MUST fail the negotiation if:
                        //     - if has received 4096 `tx_add_output` messages during this negotiation
                        return Err(AbortReason::ReceivedTooManyTxAddOutputs);
                }

                if msg.sats < msg.script.dust_value().to_sat() {
                        // The receiving node:
                        // - MUST fail the negotiation if:
                        //              - the sats amount is less than the dust_limit
                        return Err(AbortReason::BelowDustLimit);
                }

                // Check that adding this output would not cause the total output value to exceed the total
                // bitcoin supply.
                let mut outputs_value: u64 = 0;
                for output in self.outputs.iter() {
                        outputs_value = outputs_value.saturating_add(output.1.value);
                }
                if outputs_value.saturating_add(msg.sats) > TOTAL_BITCOIN_SUPPLY_SATOSHIS {
                        // The receiving node:
                        // - MUST fail the negotiation if:
                        //              - the sats amount is greater than 2,100,000,000,000,000 (TOTAL_BITCOIN_SUPPLY_SATOSHIS)
                        return Err(AbortReason::ExceededMaximumSatsAllowed);
                }

                // The receiving node:
                //   - MUST accept P2WSH, P2WPKH, P2TR scripts
                //   - MAY fail the negotiation if script is non-standard
                //
                // We can actually be a bit looser than the above as only witness version 0 has special
                // length-based standardness constraints to match similar consensus rules. All witness scripts
                // with witness versions V1 and up are always considered standard. Yes, the scripts can be
                // anyone-can-spend-able, but if our counterparty wants to add an output like that then it's none
                // of our concern really ¯\_(ツ)_/¯
                //
                // TODO: The last check would be simplified when https://github.com/rust-bitcoin/rust-bitcoin/commit/1656e1a09a1959230e20af90d20789a4a8f0a31b
                // hits the next release of rust-bitcoin.
                if !(msg.script.is_v0_p2wpkh()
                        || msg.script.is_v0_p2wsh()
                        || (msg.script.is_witness_program()
                                && msg.script.witness_version().map(|v| v.to_num() >= 1).unwrap_or(false)))
                {
                        return Err(AbortReason::InvalidOutputScript);
                }

                match self.outputs.entry(msg.serial_id) {
                        hash_map::Entry::Occupied(_) => {
                                // The receiving node:
                                //  - MUST fail the negotiation if:
                                //    - the `serial_id` is already included in the transaction
                                Err(AbortReason::DuplicateSerialId)
                        },
                        hash_map::Entry::Vacant(entry) => {
                                entry.insert(TxOut { value: msg.sats, script_pubkey: msg.script.clone() });
                                Ok(())
                        },
                }
        }

        fn received_tx_remove_output(&mut self, msg: &msgs::TxRemoveOutput) -> Result<(), AbortReason> {
                if !self.is_serial_id_valid_for_counterparty(&msg.serial_id) {
                        return Err(AbortReason::IncorrectSerialIdParity);
                }
                if self.outputs.remove(&msg.serial_id).is_some() {
                        Ok(())
                } else {
                        // The receiving node:
                        //  - MUST fail the negotiation if:
                        //    - the input or output identified by the `serial_id` was not added by the sender
                        //    - the `serial_id` does not correspond to a currently added input
                        Err(AbortReason::SerialIdUnknown)
                }
        }

        fn sent_tx_add_input(&mut self, msg: &msgs::TxAddInput) -> Result<(), AbortReason> {
                let tx = msg.prevtx.as_transaction();
                let input = TxIn {
                        previous_output: OutPoint { txid: tx.txid(), vout: msg.prevtx_out },
                        sequence: Sequence(msg.sequence),
                        ..Default::default()
                };
                let prev_output =
                        tx.output.get(msg.prevtx_out as usize).ok_or(AbortReason::PrevTxOutInvalid)?.clone();
                if !self.prevtx_outpoints.insert(input.previous_output) {
                        // We have added an input that already exists
                        return Err(AbortReason::PrevTxOutInvalid);
                }
                self.inputs.insert(msg.serial_id, TxInputWithPrevOutput { input, prev_output });
                Ok(())
        }

        fn sent_tx_add_output(&mut self, msg: &msgs::TxAddOutput) -> Result<(), AbortReason> {
                self.outputs
                        .insert(msg.serial_id, TxOut { value: msg.sats, script_pubkey: msg.script.clone() });
                Ok(())
        }

        fn sent_tx_remove_input(&mut self, msg: &msgs::TxRemoveInput) -> Result<(), AbortReason> {
                self.inputs.remove(&msg.serial_id);
                Ok(())
        }

        fn sent_tx_remove_output(&mut self, msg: &msgs::TxRemoveOutput) -> Result<(), AbortReason> {
                self.outputs.remove(&msg.serial_id);
                Ok(())
        }

        fn build_transaction(self) -> Result<Transaction, AbortReason> {
                // The receiving node:
                // MUST fail the negotiation if:

                // - the peer's total input satoshis is less than their outputs
                let mut counterparty_inputs_value: u64 = 0;
                let mut counterparty_outputs_value: u64 = 0;
                for input in self.counterparty_inputs_contributed() {
                        counterparty_inputs_value =
                                counterparty_inputs_value.saturating_add(input.prev_output.value);
                }
                for output in self.counterparty_outputs_contributed() {
                        counterparty_outputs_value = counterparty_outputs_value.saturating_add(output.value);
                }
                if counterparty_inputs_value < counterparty_outputs_value {
                        return Err(AbortReason::OutputsValueExceedsInputsValue);
                }

                // - there are more than 252 inputs
                // - there are more than 252 outputs
                if self.inputs.len() > MAX_INPUTS_OUTPUTS_COUNT
                        || self.outputs.len() > MAX_INPUTS_OUTPUTS_COUNT
                {
                        return Err(AbortReason::ExceededNumberOfInputsOrOutputs);
                }

                // TODO: How do we enforce their fees cover the witness without knowing its expected length?
                const INPUT_WEIGHT: u64 = BASE_INPUT_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT;

                // - the peer's paid feerate does not meet or exceed the agreed feerate (based on the minimum fee).
                let mut counterparty_weight_contributed: u64 = self
                        .counterparty_outputs_contributed()
                        .map(|output| {
                                (8 /* value */ + output.script_pubkey.consensus_encode(&mut sink()).unwrap() as u64)
                                        * WITNESS_SCALE_FACTOR as u64
                        })
                        .sum();
                counterparty_weight_contributed +=
                        self.counterparty_inputs_contributed().count() as u64 * INPUT_WEIGHT;
                let counterparty_fees_contributed =
                        counterparty_inputs_value.saturating_sub(counterparty_outputs_value);
                let mut required_counterparty_contribution_fee =
                        fee_for_weight(self.feerate_sat_per_kw, counterparty_weight_contributed);
                if !self.holder_is_initiator {
                        // if is the non-initiator:
                        //      - the initiator's fees do not cover the common fields (version, segwit marker + flag,
                        //              input count, output count, locktime)
                        let tx_common_fields_weight =
                        (4 /* version */ + 4 /* locktime */ + 1 /* input count */ + 1 /* output count */) *
                            WITNESS_SCALE_FACTOR as u64 + 2 /* segwit marker + flag */;
                        let tx_common_fields_fee =
                                fee_for_weight(self.feerate_sat_per_kw, tx_common_fields_weight);
                        required_counterparty_contribution_fee += tx_common_fields_fee;
                }
                if counterparty_fees_contributed < required_counterparty_contribution_fee {
                        return Err(AbortReason::InsufficientFees);
                }

                // Inputs and outputs must be sorted by serial_id
                let mut inputs = self.inputs.into_iter().collect::<Vec<_>>();
                let mut outputs = self.outputs.into_iter().collect::<Vec<_>>();
                inputs.sort_unstable_by_key(|(serial_id, _)| *serial_id);
                outputs.sort_unstable_by_key(|(serial_id, _)| *serial_id);

                let tx_to_validate = Transaction {
                        version: 2,
                        lock_time: self.tx_locktime,
                        input: inputs.into_iter().map(|(_, input)| input.input).collect(),
                        output: outputs.into_iter().map(|(_, output)| output).collect(),
                };
                if tx_to_validate.weight().to_wu() > MAX_STANDARD_TX_WEIGHT as u64 {
                        return Err(AbortReason::TransactionTooLarge);
                }

                Ok(tx_to_validate)
        }
}

// The interactive transaction construction protocol allows two peers to collaboratively build a
// transaction for broadcast.
//
// The protocol is turn-based, so we define different states here that we store depending on whose
// turn it is to send the next message. The states are defined so that their types ensure we only
// perform actions (only send messages) via defined state transitions that do not violate the
// protocol.
//
// An example of a full negotiation and associated states follows:
//
//     +------------+                         +------------------+---- Holder state after message sent/received ----+
//     |            |--(1)- tx_add_input ---->|                  |                  SentChangeMsg                   +
//     |            |<-(2)- tx_complete ------|                  |                ReceivedTxComplete                +
//     |            |--(3)- tx_add_output --->|                  |                  SentChangeMsg                   +
//     |            |<-(4)- tx_complete ------|                  |                ReceivedTxComplete                +
//     |            |--(5)- tx_add_input ---->|                  |                  SentChangeMsg                   +
//     |   Holder   |<-(6)- tx_add_input -----|   Counterparty   |                ReceivedChangeMsg                 +
//     |            |--(7)- tx_remove_output >|                  |                  SentChangeMsg                   +
//     |            |<-(8)- tx_add_output ----|                  |                ReceivedChangeMsg                 +
//     |            |--(9)- tx_complete ----->|                  |                  SentTxComplete                  +
//     |            |<-(10) tx_complete ------|                  |                NegotiationComplete               +
//     +------------+                         +------------------+--------------------------------------------------+

/// Negotiation states that can send & receive `tx_(add|remove)_(input|output)` and `tx_complete`
trait State {}

/// Category of states where we have sent some message to the counterparty, and we are waiting for
/// a response.
trait SentMsgState: State {
        fn into_negotiation_context(self) -> NegotiationContext;
}

/// Category of states that our counterparty has put us in after we receive a message from them.
trait ReceivedMsgState: State {
        fn into_negotiation_context(self) -> NegotiationContext;
}

// This macro is a helper for implementing the above state traits for various states subsequently
// defined below the macro.
macro_rules! define_state {
        (SENT_MSG_STATE, $state: ident, $doc: expr) => {
                define_state!($state, NegotiationContext, $doc);
                impl SentMsgState for $state {
                        fn into_negotiation_context(self) -> NegotiationContext {
                                self.0
                        }
                }
        };
        (RECEIVED_MSG_STATE, $state: ident, $doc: expr) => {
                define_state!($state, NegotiationContext, $doc);
                impl ReceivedMsgState for $state {
                        fn into_negotiation_context(self) -> NegotiationContext {
                                self.0
                        }
                }
        };
        ($state: ident, $inner: ident, $doc: expr) => {
                #[doc = $doc]
                #[derive(Debug)]
                struct $state($inner);
                impl State for $state {}
        };
}

define_state!(
        SENT_MSG_STATE,
        SentChangeMsg,
        "We have sent a message to the counterparty that has affected our negotiation state."
);
define_state!(
        SENT_MSG_STATE,
        SentTxComplete,
        "We have sent a `tx_complete` message and are awaiting the counterparty's."
);
define_state!(
        RECEIVED_MSG_STATE,
        ReceivedChangeMsg,
        "We have received a message from the counterparty that has affected our negotiation state."
);
define_state!(
        RECEIVED_MSG_STATE,
        ReceivedTxComplete,
        "We have received a `tx_complete` message and the counterparty is awaiting ours."
);
define_state!(NegotiationComplete, Transaction, "We have exchanged consecutive `tx_complete` messages with the counterparty and the transaction negotiation is complete.");
define_state!(
        NegotiationAborted,
        AbortReason,
        "The negotiation has failed and cannot be continued."
);

type StateTransitionResult<S> = Result<S, AbortReason>;

trait StateTransition<NewState: State, TransitionData> {
        fn transition(self, data: TransitionData) -> StateTransitionResult<NewState>;
}

// This macro helps define the legal transitions between the states above by implementing
// the `StateTransition` trait for each of the states that follow this declaration.
macro_rules! define_state_transitions {
        (SENT_MSG_STATE, [$(DATA $data: ty, TRANSITION $transition: ident),+]) => {
                $(
                        impl<S: SentMsgState> StateTransition<ReceivedChangeMsg, $data> for S {
                                fn transition(self, data: $data) -> StateTransitionResult<ReceivedChangeMsg> {
                                        let mut context = self.into_negotiation_context();
                                        context.$transition(data)?;
                                        Ok(ReceivedChangeMsg(context))
                                }
                        }
                 )*
        };
        (RECEIVED_MSG_STATE, [$(DATA $data: ty, TRANSITION $transition: ident),+]) => {
                $(
                        impl<S: ReceivedMsgState> StateTransition<SentChangeMsg, $data> for S {
                                fn transition(self, data: $data) -> StateTransitionResult<SentChangeMsg> {
                                        let mut context = self.into_negotiation_context();
                                        context.$transition(data)?;
                                        Ok(SentChangeMsg(context))
                                }
                        }
                 )*
        };
        (TX_COMPLETE, $from_state: ident, $tx_complete_state: ident) => {
                impl StateTransition<NegotiationComplete, &msgs::TxComplete> for $tx_complete_state {
                        fn transition(self, _data: &msgs::TxComplete) -> StateTransitionResult<NegotiationComplete> {
                                let context = self.into_negotiation_context();
                                let tx = context.build_transaction()?;
                                Ok(NegotiationComplete(tx))
                        }
                }

                impl StateTransition<$tx_complete_state, &msgs::TxComplete> for $from_state {
                        fn transition(self, _data: &msgs::TxComplete) -> StateTransitionResult<$tx_complete_state> {
                                Ok($tx_complete_state(self.into_negotiation_context()))
                        }
                }
        };
}

// State transitions when we have sent our counterparty some messages and are waiting for them
// to respond.
define_state_transitions!(SENT_MSG_STATE, [
        DATA &msgs::TxAddInput, TRANSITION received_tx_add_input,
        DATA &msgs::TxRemoveInput, TRANSITION received_tx_remove_input,
        DATA &msgs::TxAddOutput, TRANSITION received_tx_add_output,
        DATA &msgs::TxRemoveOutput, TRANSITION received_tx_remove_output
]);
// State transitions when we have received some messages from our counterparty and we should
// respond.
define_state_transitions!(RECEIVED_MSG_STATE, [
        DATA &msgs::TxAddInput, TRANSITION sent_tx_add_input,
        DATA &msgs::TxRemoveInput, TRANSITION sent_tx_remove_input,
        DATA &msgs::TxAddOutput, TRANSITION sent_tx_add_output,
        DATA &msgs::TxRemoveOutput, TRANSITION sent_tx_remove_output
]);
define_state_transitions!(TX_COMPLETE, SentChangeMsg, ReceivedTxComplete);
define_state_transitions!(TX_COMPLETE, ReceivedChangeMsg, SentTxComplete);

#[derive(Debug)]
enum StateMachine {
        Indeterminate,
        SentChangeMsg(SentChangeMsg),
        ReceivedChangeMsg(ReceivedChangeMsg),
        SentTxComplete(SentTxComplete),
        ReceivedTxComplete(ReceivedTxComplete),
        NegotiationComplete(NegotiationComplete),
        NegotiationAborted(NegotiationAborted),
}

impl Default for StateMachine {
        fn default() -> Self {
                Self::Indeterminate
        }
}

// The `StateMachine` internally executes the actual transition between two states and keeps
// track of the current state. This macro defines _how_ those state transitions happen to
// update the internal state.
macro_rules! define_state_machine_transitions {
        ($transition: ident, $msg: ty, [$(FROM $from_state: ident, TO $to_state: ident),+]) => {
                fn $transition(self, msg: $msg) -> StateMachine {
                        match self {
                                $(
                                        Self::$from_state(s) => match s.transition(msg) {
                                                Ok(new_state) => StateMachine::$to_state(new_state),
                                                Err(abort_reason) => StateMachine::NegotiationAborted(NegotiationAborted(abort_reason)),
                                        }
                                 )*
                                _ => StateMachine::NegotiationAborted(NegotiationAborted(AbortReason::UnexpectedCounterpartyMessage)),
                        }
                }
        };
}

impl StateMachine {
        fn new(feerate_sat_per_kw: u32, is_initiator: bool, tx_locktime: AbsoluteLockTime) -> Self {
                let context = NegotiationContext {
                        tx_locktime,
                        holder_is_initiator: is_initiator,
                        received_tx_add_input_count: 0,
                        received_tx_add_output_count: 0,
                        inputs: new_hash_map(),
                        prevtx_outpoints: new_hash_set(),
                        outputs: new_hash_map(),
                        feerate_sat_per_kw,
                };
                if is_initiator {
                        Self::ReceivedChangeMsg(ReceivedChangeMsg(context))
                } else {
                        Self::SentChangeMsg(SentChangeMsg(context))
                }
        }

        // TxAddInput
        define_state_machine_transitions!(sent_tx_add_input, &msgs::TxAddInput, [
                FROM ReceivedChangeMsg, TO SentChangeMsg,
                FROM ReceivedTxComplete, TO SentChangeMsg
        ]);
        define_state_machine_transitions!(received_tx_add_input, &msgs::TxAddInput, [
                FROM SentChangeMsg, TO ReceivedChangeMsg,
                FROM SentTxComplete, TO ReceivedChangeMsg
        ]);

        // TxAddOutput
        define_state_machine_transitions!(sent_tx_add_output, &msgs::TxAddOutput, [
                FROM ReceivedChangeMsg, TO SentChangeMsg,
                FROM ReceivedTxComplete, TO SentChangeMsg
        ]);
        define_state_machine_transitions!(received_tx_add_output, &msgs::TxAddOutput, [
                FROM SentChangeMsg, TO ReceivedChangeMsg,
                FROM SentTxComplete, TO ReceivedChangeMsg
        ]);

        // TxRemoveInput
        define_state_machine_transitions!(sent_tx_remove_input, &msgs::TxRemoveInput, [
                FROM ReceivedChangeMsg, TO SentChangeMsg,
                FROM ReceivedTxComplete, TO SentChangeMsg
        ]);
        define_state_machine_transitions!(received_tx_remove_input, &msgs::TxRemoveInput, [
                FROM SentChangeMsg, TO ReceivedChangeMsg,
                FROM SentTxComplete, TO ReceivedChangeMsg
        ]);

        // TxRemoveOutput
        define_state_machine_transitions!(sent_tx_remove_output, &msgs::TxRemoveOutput, [
                FROM ReceivedChangeMsg, TO SentChangeMsg,
                FROM ReceivedTxComplete, TO SentChangeMsg
        ]);
        define_state_machine_transitions!(received_tx_remove_output, &msgs::TxRemoveOutput, [
                FROM SentChangeMsg, TO ReceivedChangeMsg,
                FROM SentTxComplete, TO ReceivedChangeMsg
        ]);

        // TxComplete
        define_state_machine_transitions!(sent_tx_complete, &msgs::TxComplete, [
                FROM ReceivedChangeMsg, TO SentTxComplete,
                FROM ReceivedTxComplete, TO NegotiationComplete
        ]);
        define_state_machine_transitions!(received_tx_complete, &msgs::TxComplete, [
                FROM SentChangeMsg, TO ReceivedTxComplete,
                FROM SentTxComplete, TO NegotiationComplete
        ]);
}

pub struct InteractiveTxConstructor {
        state_machine: StateMachine,
        channel_id: ChannelId,
        inputs_to_contribute: Vec<(SerialId, TxIn, TransactionU16LenLimited)>,
        outputs_to_contribute: Vec<(SerialId, TxOut)>,
}

pub enum InteractiveTxMessageSend {
        TxAddInput(msgs::TxAddInput),
        TxAddOutput(msgs::TxAddOutput),
        TxComplete(msgs::TxComplete),
}

// This macro executes a state machine transition based on a provided action.
macro_rules! do_state_transition {
        ($self: ident, $transition: ident, $msg: expr) => {{
                let state_machine = core::mem::take(&mut $self.state_machine);
                $self.state_machine = state_machine.$transition($msg);
                match &$self.state_machine {
                        StateMachine::NegotiationAborted(state) => Err(state.0.clone()),
                        _ => Ok(()),
                }
        }};
}

fn generate_holder_serial_id<ES: Deref>(entropy_source: &ES, is_initiator: bool) -> SerialId
where
        ES::Target: EntropySource,
{
        let rand_bytes = entropy_source.get_secure_random_bytes();
        let mut serial_id_bytes = [0u8; 8];
        serial_id_bytes.copy_from_slice(&rand_bytes[..8]);
        let mut serial_id = u64::from_be_bytes(serial_id_bytes);
        if serial_id.is_for_initiator() != is_initiator {
                serial_id ^= 1;
        }
        serial_id
}

pub enum HandleTxCompleteValue {
        SendTxMessage(InteractiveTxMessageSend),
        SendTxComplete(InteractiveTxMessageSend, Transaction),
        NegotiationComplete(Transaction),
}

impl InteractiveTxConstructor {
        /// Instantiates a new `InteractiveTxConstructor`.
        ///
        /// A tuple is returned containing the newly instantiate `InteractiveTxConstructor` and optionally
        /// an initial wrapped `Tx_` message which the holder needs to send to the counterparty.
        pub fn new<ES: Deref>(
                entropy_source: &ES, channel_id: ChannelId, feerate_sat_per_kw: u32, is_initiator: bool,
                funding_tx_locktime: AbsoluteLockTime,
                inputs_to_contribute: Vec<(TxIn, TransactionU16LenLimited)>,
                outputs_to_contribute: Vec<TxOut>,
        ) -> (Self, Option<InteractiveTxMessageSend>)
        where
                ES::Target: EntropySource,
        {
                let state_machine =
                        StateMachine::new(feerate_sat_per_kw, is_initiator, funding_tx_locktime);
                let mut inputs_to_contribute: Vec<(SerialId, TxIn, TransactionU16LenLimited)> =
                        inputs_to_contribute
                                .into_iter()
                                .map(|(input, tx)| {
                                        let serial_id = generate_holder_serial_id(entropy_source, is_initiator);
                                        (serial_id, input, tx)
                                })
                                .collect();
                // We'll sort by the randomly generated serial IDs, effectively shuffling the order of the inputs
                // as the user passed them to us to avoid leaking any potential categorization of transactions
                // before we pass any of the inputs to the counterparty.
                inputs_to_contribute.sort_unstable_by_key(|(serial_id, _, _)| *serial_id);
                let mut outputs_to_contribute: Vec<(SerialId, TxOut)> = outputs_to_contribute
                        .into_iter()
                        .map(|output| {
                                let serial_id = generate_holder_serial_id(entropy_source, is_initiator);
                                (serial_id, output)
                        })
                        .collect();
                // In the same manner and for the same rationale as the inputs above, we'll shuffle the outputs.
                outputs_to_contribute.sort_unstable_by_key(|(serial_id, _)| *serial_id);
                let mut constructor =
                        Self { state_machine, channel_id, inputs_to_contribute, outputs_to_contribute };
                let message_send = if is_initiator {
                        match constructor.maybe_send_message() {
                                Ok(msg_send) => Some(msg_send),
                                Err(_) => {
                                        debug_assert!(
                                                false,
                                                "We should always be able to start our state machine successfully"
                                        );
                                        None
                                },
                        }
                } else {
                        None
                };
                (constructor, message_send)
        }

        fn maybe_send_message(&mut self) -> Result<InteractiveTxMessageSend, AbortReason> {
                // We first attempt to send inputs we want to add, then outputs. Once we are done sending
                // them both, then we always send tx_complete.
                if let Some((serial_id, input, prevtx)) = self.inputs_to_contribute.pop() {
                        let msg = msgs::TxAddInput {
                                channel_id: self.channel_id,
                                serial_id,
                                prevtx,
                                prevtx_out: input.previous_output.vout,
                                sequence: input.sequence.to_consensus_u32(),
                        };
                        do_state_transition!(self, sent_tx_add_input, &msg)?;
                        Ok(InteractiveTxMessageSend::TxAddInput(msg))
                } else if let Some((serial_id, output)) = self.outputs_to_contribute.pop() {
                        let msg = msgs::TxAddOutput {
                                channel_id: self.channel_id,
                                serial_id,
                                sats: output.value,
                                script: output.script_pubkey,
                        };
                        do_state_transition!(self, sent_tx_add_output, &msg)?;
                        Ok(InteractiveTxMessageSend::TxAddOutput(msg))
                } else {
                        let msg = msgs::TxComplete { channel_id: self.channel_id };
                        do_state_transition!(self, sent_tx_complete, &msg)?;
                        Ok(InteractiveTxMessageSend::TxComplete(msg))
                }
        }

        pub fn handle_tx_add_input(
                &mut self, msg: &msgs::TxAddInput,
        ) -> Result<InteractiveTxMessageSend, AbortReason> {
                do_state_transition!(self, received_tx_add_input, msg)?;
                self.maybe_send_message()
        }

        pub fn handle_tx_remove_input(
                &mut self, msg: &msgs::TxRemoveInput,
        ) -> Result<InteractiveTxMessageSend, AbortReason> {
                do_state_transition!(self, received_tx_remove_input, msg)?;
                self.maybe_send_message()
        }

        pub fn handle_tx_add_output(
                &mut self, msg: &msgs::TxAddOutput,
        ) -> Result<InteractiveTxMessageSend, AbortReason> {
                do_state_transition!(self, received_tx_add_output, msg)?;
                self.maybe_send_message()
        }

        pub fn handle_tx_remove_output(
                &mut self, msg: &msgs::TxRemoveOutput,
        ) -> Result<InteractiveTxMessageSend, AbortReason> {
                do_state_transition!(self, received_tx_remove_output, msg)?;
                self.maybe_send_message()
        }

        pub fn handle_tx_complete(
                &mut self, msg: &msgs::TxComplete,
        ) -> Result<HandleTxCompleteValue, AbortReason> {
                do_state_transition!(self, received_tx_complete, msg)?;
                match &self.state_machine {
                        StateMachine::ReceivedTxComplete(_) => {
                                let msg_send = self.maybe_send_message()?;
                                match &self.state_machine {
                                        StateMachine::NegotiationComplete(s) => {
                                                Ok(HandleTxCompleteValue::SendTxComplete(msg_send, s.0.clone()))
                                        },
                                        StateMachine::SentChangeMsg(_) => {
                                                Ok(HandleTxCompleteValue::SendTxMessage(msg_send))
                                        }, // We either had an input or output to contribute.
                                        _ => {
                                                debug_assert!(false, "We cannot transition to any other states after receiving `tx_complete` and responding");
                                                Err(AbortReason::InvalidStateTransition)
                                        },
                                }
                        },
                        StateMachine::NegotiationComplete(s) => {
                                Ok(HandleTxCompleteValue::NegotiationComplete(s.0.clone()))
                        },
                        _ => {
                                debug_assert!(
                                        false,
                                        "We cannot transition to any other states after receiving `tx_complete`"
                                );
                                Err(AbortReason::InvalidStateTransition)
                        },
                }
        }
}

#[cfg(test)]
mod tests {
        use crate::chain::chaininterface::FEERATE_FLOOR_SATS_PER_KW;
        use crate::ln::channel::TOTAL_BITCOIN_SUPPLY_SATOSHIS;
        use crate::ln::interactivetxs::{
                generate_holder_serial_id, AbortReason, HandleTxCompleteValue, InteractiveTxConstructor,
                InteractiveTxMessageSend, MAX_INPUTS_OUTPUTS_COUNT, MAX_RECEIVED_TX_ADD_INPUT_COUNT,
                MAX_RECEIVED_TX_ADD_OUTPUT_COUNT,
        };
        use crate::ln::ChannelId;
        use crate::sign::EntropySource;
        use crate::util::atomic_counter::AtomicCounter;
        use crate::util::ser::TransactionU16LenLimited;
        use bitcoin::blockdata::opcodes;
        use bitcoin::blockdata::script::Builder;
        use bitcoin::{
                absolute::LockTime as AbsoluteLockTime, OutPoint, Sequence, Transaction, TxIn, TxOut,
        };
        use core::ops::Deref;

        // A simple entropy source that works based on an atomic counter.
        struct TestEntropySource(AtomicCounter);
        impl EntropySource for TestEntropySource {
                fn get_secure_random_bytes(&self) -> [u8; 32] {
                        let mut res = [0u8; 32];
                        let increment = self.0.get_increment();
                        for i in 0..32 {
                                // Rotate the increment value by 'i' bits to the right, to avoid clashes
                                // when `generate_local_serial_id` does a parity flip on consecutive calls for the
                                // same party.
                                let rotated_increment = increment.rotate_right(i as u32);
                                res[i] = (rotated_increment & 0xff) as u8;
                        }
                        res
                }
        }

        // An entropy source that deliberately returns you the same seed every time. We use this
        // to test if the constructor would catch inputs/outputs that are attempting to be added
        // with duplicate serial ids.
        struct DuplicateEntropySource;
        impl EntropySource for DuplicateEntropySource {
                fn get_secure_random_bytes(&self) -> [u8; 32] {
                        let mut res = [0u8; 32];
                        let count = 1u64;
                        res[0..8].copy_from_slice(&count.to_be_bytes());
                        res
                }
        }

        #[derive(Debug, PartialEq, Eq)]
        enum ErrorCulprit {
                NodeA,
                NodeB,
                // Some error values are only checked at the end of the negotiation and are not easy to attribute
                // to a particular party. Both parties would indicate an `AbortReason` in this case.
                // e.g. Exceeded max inputs and outputs after negotiation.
                Indeterminate,
        }

        struct TestSession {
                inputs_a: Vec<(TxIn, TransactionU16LenLimited)>,
                outputs_a: Vec<TxOut>,
                inputs_b: Vec<(TxIn, TransactionU16LenLimited)>,
                outputs_b: Vec<TxOut>,
                expect_error: Option<(AbortReason, ErrorCulprit)>,
        }

        fn do_test_interactive_tx_constructor(session: TestSession) {
                let entropy_source = TestEntropySource(AtomicCounter::new());
                do_test_interactive_tx_constructor_internal(session, &&entropy_source);
        }

        fn do_test_interactive_tx_constructor_with_entropy_source<ES: Deref>(
                session: TestSession, entropy_source: ES,
        ) where
                ES::Target: EntropySource,
        {
                do_test_interactive_tx_constructor_internal(session, &entropy_source);
        }

        fn do_test_interactive_tx_constructor_internal<ES: Deref>(
                session: TestSession, entropy_source: &ES,
        ) where
                ES::Target: EntropySource,
        {
                let channel_id = ChannelId(entropy_source.get_secure_random_bytes());
                let tx_locktime = AbsoluteLockTime::from_height(1337).unwrap();

                let (mut constructor_a, first_message_a) = InteractiveTxConstructor::new(
                        entropy_source,
                        channel_id,
                        FEERATE_FLOOR_SATS_PER_KW * 10,
                        true,
                        tx_locktime,
                        session.inputs_a,
                        session.outputs_a,
                );
                let (mut constructor_b, first_message_b) = InteractiveTxConstructor::new(
                        entropy_source,
                        channel_id,
                        FEERATE_FLOOR_SATS_PER_KW * 10,
                        false,
                        tx_locktime,
                        session.inputs_b,
                        session.outputs_b,
                );

                let handle_message_send =
                        |msg: InteractiveTxMessageSend, for_constructor: &mut InteractiveTxConstructor| {
                                match msg {
                                        InteractiveTxMessageSend::TxAddInput(msg) => for_constructor
                                                .handle_tx_add_input(&msg)
                                                .map(|msg_send| (Some(msg_send), None)),
                                        InteractiveTxMessageSend::TxAddOutput(msg) => for_constructor
                                                .handle_tx_add_output(&msg)
                                                .map(|msg_send| (Some(msg_send), None)),
                                        InteractiveTxMessageSend::TxComplete(msg) => {
                                                for_constructor.handle_tx_complete(&msg).map(|value| match value {
                                                        HandleTxCompleteValue::SendTxMessage(msg_send) => {
                                                                (Some(msg_send), None)
                                                        },
                                                        HandleTxCompleteValue::SendTxComplete(msg_send, tx) => {
                                                                (Some(msg_send), Some(tx))
                                                        },
                                                        HandleTxCompleteValue::NegotiationComplete(tx) => (None, Some(tx)),
                                                })
                                        },
                                }
                        };

                assert!(first_message_b.is_none());
                let mut message_send_a = first_message_a;
                let mut message_send_b = None;
                let mut final_tx_a = None;
                let mut final_tx_b = None;
                while final_tx_a.is_none() || final_tx_b.is_none() {
                        if let Some(message_send_a) = message_send_a.take() {
                                match handle_message_send(message_send_a, &mut constructor_b) {
                                        Ok((msg_send, final_tx)) => {
                                                message_send_b = msg_send;
                                                final_tx_b = final_tx;
                                        },
                                        Err(abort_reason) => {
                                                let error_culprit = match abort_reason {
                                                        AbortReason::ExceededNumberOfInputsOrOutputs => {
                                                                ErrorCulprit::Indeterminate
                                                        },
                                                        _ => ErrorCulprit::NodeA,
                                                };
                                                assert_eq!(Some((abort_reason, error_culprit)), session.expect_error);
                                                assert!(message_send_b.is_none());
                                                return;
                                        },
                                }
                        }
                        if let Some(message_send_b) = message_send_b.take() {
                                match handle_message_send(message_send_b, &mut constructor_a) {
                                        Ok((msg_send, final_tx)) => {
                                                message_send_a = msg_send;
                                                final_tx_a = final_tx;
                                        },
                                        Err(abort_reason) => {
                                                let error_culprit = match abort_reason {
                                                        AbortReason::ExceededNumberOfInputsOrOutputs => {
                                                                ErrorCulprit::Indeterminate
                                                        },
                                                        _ => ErrorCulprit::NodeB,
                                                };
                                                assert_eq!(Some((abort_reason, error_culprit)), session.expect_error);
                                                assert!(message_send_a.is_none());
                                                return;
                                        },
                                }
                        }
                }
                assert!(message_send_a.is_none());
                assert!(message_send_b.is_none());
                assert_eq!(final_tx_a, final_tx_b);
                assert!(session.expect_error.is_none());
        }

        fn generate_tx(values: &[u64]) -> Transaction {
                generate_tx_with_locktime(values, 1337)
        }

        fn generate_tx_with_locktime(values: &[u64], locktime: u32) -> Transaction {
                Transaction {
                        version: 2,
                        lock_time: AbsoluteLockTime::from_height(locktime).unwrap(),
                        input: vec![TxIn { ..Default::default() }],
                        output: values
                                .iter()
                                .map(|value| TxOut {
                                        value: *value,
                                        script_pubkey: Builder::new()
                                                .push_opcode(opcodes::OP_TRUE)
                                                .into_script()
                                                .to_v0_p2wsh(),
                                })
                                .collect(),
                }
        }

        fn generate_inputs(values: &[u64]) -> Vec<(TxIn, TransactionU16LenLimited)> {
                let tx = generate_tx(values);
                let txid = tx.txid();
                tx.output
                        .iter()
                        .enumerate()
                        .map(|(idx, _)| {
                                let input = TxIn {
                                        previous_output: OutPoint { txid, vout: idx as u32 },
                                        script_sig: Default::default(),
                                        sequence: Sequence::ENABLE_RBF_NO_LOCKTIME,
                                        witness: Default::default(),
                                };
                                (input, TransactionU16LenLimited::new(tx.clone()).unwrap())
                        })
                        .collect()
        }

        fn generate_outputs(values: &[u64]) -> Vec<TxOut> {
                values
                        .iter()
                        .map(|value| TxOut {
                                value: *value,
                                script_pubkey: Builder::new()
                                        .push_opcode(opcodes::OP_TRUE)
                                        .into_script()
                                        .to_v0_p2wsh(),
                        })
                        .collect()
        }

        fn generate_fixed_number_of_inputs(count: u16) -> Vec<(TxIn, TransactionU16LenLimited)> {
                // Generate transactions with a total `count` number of outputs such that no transaction has a
                // serialized length greater than u16::MAX.
                let max_outputs_per_prevtx = 1_500;
                let mut remaining = count;
                let mut inputs: Vec<(TxIn, TransactionU16LenLimited)> = Vec::with_capacity(count as usize);

                while remaining > 0 {
                        let tx_output_count = remaining.min(max_outputs_per_prevtx);
                        remaining -= tx_output_count;

                        // Use unique locktime for each tx so outpoints are different across transactions
                        let tx = generate_tx_with_locktime(
                                &vec![1_000_000; tx_output_count as usize],
                                (1337 + remaining).into(),
                        );
                        let txid = tx.txid();

                        let mut temp: Vec<(TxIn, TransactionU16LenLimited)> = tx
                                .output
                                .iter()
                                .enumerate()
                                .map(|(idx, _)| {
                                        let input = TxIn {
                                                previous_output: OutPoint { txid, vout: idx as u32 },
                                                script_sig: Default::default(),
                                                sequence: Sequence::ENABLE_RBF_NO_LOCKTIME,
                                                witness: Default::default(),
                                        };
                                        (input, TransactionU16LenLimited::new(tx.clone()).unwrap())
                                })
                                .collect();

                        inputs.append(&mut temp);
                }

                inputs
        }

        fn generate_fixed_number_of_outputs(count: u16) -> Vec<TxOut> {
                // Set a constant value for each TxOut
                generate_outputs(&vec![1_000_000; count as usize])
        }

        fn generate_non_witness_output(value: u64) -> TxOut {
                TxOut {
                        value,
                        script_pubkey: Builder::new().push_opcode(opcodes::OP_TRUE).into_script().to_p2sh(),
                }
        }

        #[test]
        fn test_interactive_tx_constructor() {
                // No contributions.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: vec![],
                        outputs_a: vec![],
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::InsufficientFees, ErrorCulprit::NodeA)),
                });
                // Single contribution, no initiator inputs.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: vec![],
                        outputs_a: generate_outputs(&[1_000_000]),
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::OutputsValueExceedsInputsValue, ErrorCulprit::NodeA)),
                });
                // Single contribution, no initiator outputs.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: generate_inputs(&[1_000_000]),
                        outputs_a: vec![],
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: None,
                });
                // Single contribution, insufficient fees.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: generate_inputs(&[1_000_000]),
                        outputs_a: generate_outputs(&[1_000_000]),
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::InsufficientFees, ErrorCulprit::NodeA)),
                });
                // Initiator contributes sufficient fees, but non-initiator does not.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: generate_inputs(&[1_000_000]),
                        outputs_a: vec![],
                        inputs_b: generate_inputs(&[100_000]),
                        outputs_b: generate_outputs(&[100_000]),
                        expect_error: Some((AbortReason::InsufficientFees, ErrorCulprit::NodeB)),
                });
                // Multi-input-output contributions from both sides.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: generate_inputs(&[1_000_000, 1_000_000]),
                        outputs_a: generate_outputs(&[1_000_000, 200_000]),
                        inputs_b: generate_inputs(&[1_000_000, 500_000]),
                        outputs_b: generate_outputs(&[1_000_000, 400_000]),
                        expect_error: None,
                });

                // Prevout from initiator is not a witness program
                let non_segwit_output_tx = {
                        let mut tx = generate_tx(&[1_000_000]);
                        tx.output.push(TxOut {
                                script_pubkey: Builder::new()
                                        .push_opcode(opcodes::all::OP_RETURN)
                                        .into_script()
                                        .to_p2sh(),
                                ..Default::default()
                        });

                        TransactionU16LenLimited::new(tx).unwrap()
                };
                let non_segwit_input = TxIn {
                        previous_output: OutPoint {
                                txid: non_segwit_output_tx.as_transaction().txid(),
                                vout: 1,
                        },
                        sequence: Sequence::ENABLE_RBF_NO_LOCKTIME,
                        ..Default::default()
                };
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: vec![(non_segwit_input, non_segwit_output_tx)],
                        outputs_a: vec![],
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::PrevTxOutInvalid, ErrorCulprit::NodeA)),
                });

                // Invalid input sequence from initiator.
                let tx = TransactionU16LenLimited::new(generate_tx(&[1_000_000])).unwrap();
                let invalid_sequence_input = TxIn {
                        previous_output: OutPoint { txid: tx.as_transaction().txid(), vout: 0 },
                        ..Default::default()
                };
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: vec![(invalid_sequence_input, tx.clone())],
                        outputs_a: generate_outputs(&[1_000_000]),
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::IncorrectInputSequenceValue, ErrorCulprit::NodeA)),
                });
                // Duplicate prevout from initiator.
                let duplicate_input = TxIn {
                        previous_output: OutPoint { txid: tx.as_transaction().txid(), vout: 0 },
                        sequence: Sequence::ENABLE_RBF_NO_LOCKTIME,
                        ..Default::default()
                };
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: vec![(duplicate_input.clone(), tx.clone()), (duplicate_input, tx.clone())],
                        outputs_a: generate_outputs(&[1_000_000]),
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::PrevTxOutInvalid, ErrorCulprit::NodeB)),
                });
                // Non-initiator uses same prevout as initiator.
                let duplicate_input = TxIn {
                        previous_output: OutPoint { txid: tx.as_transaction().txid(), vout: 0 },
                        sequence: Sequence::ENABLE_RBF_NO_LOCKTIME,
                        ..Default::default()
                };
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: vec![(duplicate_input.clone(), tx.clone())],
                        outputs_a: generate_outputs(&[1_000_000]),
                        inputs_b: vec![(duplicate_input.clone(), tx.clone())],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::PrevTxOutInvalid, ErrorCulprit::NodeA)),
                });
                // Initiator sends too many TxAddInputs
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: generate_fixed_number_of_inputs(MAX_RECEIVED_TX_ADD_INPUT_COUNT + 1),
                        outputs_a: vec![],
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::ReceivedTooManyTxAddInputs, ErrorCulprit::NodeA)),
                });
                // Attempt to queue up two inputs with duplicate serial ids. We use a deliberately bad
                // entropy source, `DuplicateEntropySource` to simulate this.
                do_test_interactive_tx_constructor_with_entropy_source(
                        TestSession {
                                inputs_a: generate_fixed_number_of_inputs(2),
                                outputs_a: vec![],
                                inputs_b: vec![],
                                outputs_b: vec![],
                                expect_error: Some((AbortReason::DuplicateSerialId, ErrorCulprit::NodeA)),
                        },
                        &DuplicateEntropySource,
                );
                // Initiator sends too many TxAddOutputs.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: vec![],
                        outputs_a: generate_fixed_number_of_outputs(MAX_RECEIVED_TX_ADD_OUTPUT_COUNT + 1),
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::ReceivedTooManyTxAddOutputs, ErrorCulprit::NodeA)),
                });
                // Initiator sends an output below dust value.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: vec![],
                        outputs_a: generate_outputs(&[1]),
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::BelowDustLimit, ErrorCulprit::NodeA)),
                });
                // Initiator sends an output above maximum sats allowed.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: vec![],
                        outputs_a: generate_outputs(&[TOTAL_BITCOIN_SUPPLY_SATOSHIS + 1]),
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::ExceededMaximumSatsAllowed, ErrorCulprit::NodeA)),
                });
                // Initiator sends an output without a witness program.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: vec![],
                        outputs_a: vec![generate_non_witness_output(1_000_000)],
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::InvalidOutputScript, ErrorCulprit::NodeA)),
                });
                // Attempt to queue up two outputs with duplicate serial ids. We use a deliberately bad
                // entropy source, `DuplicateEntropySource` to simulate this.
                do_test_interactive_tx_constructor_with_entropy_source(
                        TestSession {
                                inputs_a: vec![],
                                outputs_a: generate_fixed_number_of_outputs(2),
                                inputs_b: vec![],
                                outputs_b: vec![],
                                expect_error: Some((AbortReason::DuplicateSerialId, ErrorCulprit::NodeA)),
                        },
                        &DuplicateEntropySource,
                );

                // Peer contributed more output value than inputs
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: generate_inputs(&[100_000]),
                        outputs_a: generate_outputs(&[1_000_000]),
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((AbortReason::OutputsValueExceedsInputsValue, ErrorCulprit::NodeA)),
                });

                // Peer contributed more than allowed number of inputs.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: generate_fixed_number_of_inputs(MAX_INPUTS_OUTPUTS_COUNT as u16 + 1),
                        outputs_a: vec![],
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((
                                AbortReason::ExceededNumberOfInputsOrOutputs,
                                ErrorCulprit::Indeterminate,
                        )),
                });
                // Peer contributed more than allowed number of outputs.
                do_test_interactive_tx_constructor(TestSession {
                        inputs_a: generate_inputs(&[TOTAL_BITCOIN_SUPPLY_SATOSHIS]),
                        outputs_a: generate_fixed_number_of_outputs(MAX_INPUTS_OUTPUTS_COUNT as u16 + 1),
                        inputs_b: vec![],
                        outputs_b: vec![],
                        expect_error: Some((
                                AbortReason::ExceededNumberOfInputsOrOutputs,
                                ErrorCulprit::Indeterminate,
                        )),
                });
        }

        #[test]
        fn test_generate_local_serial_id() {
                let entropy_source = TestEntropySource(AtomicCounter::new());

                // Initiators should have even serial id, non-initiators should have odd serial id.
                assert_eq!(generate_holder_serial_id(&&entropy_source, true) % 2, 0);
                assert_eq!(generate_holder_serial_id(&&entropy_source, false) % 2, 1)
        }
}