linera_execution/

system.rs

// Copyright (c) Facebook, Inc. and its affiliates.
// Copyright (c) Zefchain Labs, Inc.
// SPDX-License-Identifier: Apache-2.0

#[cfg(test)]
#[path = "./unit_tests/system_tests.rs"]
mod tests;

use std::{
    collections::{BTreeMap, BTreeSet, HashSet},
    mem,
};

use custom_debug_derive::Debug;
use linera_base::{
    crypto::CryptoHash,
    data_types::{
        Amount, ApplicationPermissions, ArithmeticError, Blob, BlobContent, BlockHeight,
        ChainDescription, ChainOrigin, Epoch, InitialChainConfig, OracleResponse, Timestamp,
    },
    ensure, hex_debug,
    identifiers::{Account, AccountOwner, BlobId, BlobType, ChainId, EventId, ModuleId, StreamId},
    ownership::{ChainOwnership, TimeoutConfig},
};
use linera_views::{
    context::Context,
    map_view::{HashedMapView, MapView},
    register_view::HashedRegisterView,
    set_view::HashedSetView,
    views::{ClonableView, HashableView, View},
};
use serde::{Deserialize, Serialize};

#[cfg(test)]
use crate::test_utils::SystemExecutionState;
use crate::{
    committee::Committee, ApplicationDescription, ApplicationId, ExecutionError,
    ExecutionRuntimeContext, MessageContext, MessageKind, OperationContext, OutgoingMessage,
    QueryContext, QueryOutcome, ResourceController, TransactionTracker,
};

/// The event stream name for new epochs and committees.
pub static EPOCH_STREAM_NAME: &[u8] = &[0];
/// The event stream name for removed epochs.
pub static REMOVED_EPOCH_STREAM_NAME: &[u8] = &[1];

/// The number of times the [`SystemOperation::OpenChain`] was executed.
#[cfg(with_metrics)]
mod metrics {
    use std::sync::LazyLock;

    use linera_base::prometheus_util::register_int_counter_vec;
    use prometheus::IntCounterVec;

    pub static OPEN_CHAIN_COUNT: LazyLock<IntCounterVec> = LazyLock::new(|| {
        register_int_counter_vec(
            "open_chain_count",
            "The number of times the `OpenChain` operation was executed",
            &[],
        )
    });
}

/// A view accessing the execution state of the system of a chain.
#[derive(Debug, ClonableView, HashableView)]
pub struct SystemExecutionStateView<C> {
    /// How the chain was created. May be unknown for inactive chains.
    pub description: HashedRegisterView<C, Option<ChainDescription>>,
    /// The number identifying the current configuration.
    pub epoch: HashedRegisterView<C, Epoch>,
    /// The admin of the chain.
    pub admin_id: HashedRegisterView<C, Option<ChainId>>,
    /// The committees that we trust, indexed by epoch number.
    // Not using a `MapView` because the set active of committees is supposed to be
    // small. Plus, currently, we would create the `BTreeMap` anyway in various places
    // (e.g. the `OpenChain` operation).
    pub committees: HashedRegisterView<C, BTreeMap<Epoch, Committee>>,
    /// Ownership of the chain.
    pub ownership: HashedRegisterView<C, ChainOwnership>,
    /// Balance of the chain. (Available to any user able to create blocks in the chain.)
    pub balance: HashedRegisterView<C, Amount>,
    /// Balances attributed to a given owner.
    pub balances: HashedMapView<C, AccountOwner, Amount>,
    /// The timestamp of the most recent block.
    pub timestamp: HashedRegisterView<C, Timestamp>,
    /// Whether this chain has been closed.
    pub closed: HashedRegisterView<C, bool>,
    /// Permissions for applications on this chain.
    pub application_permissions: HashedRegisterView<C, ApplicationPermissions>,
    /// Blobs that have been used or published on this chain.
    pub used_blobs: HashedSetView<C, BlobId>,
    /// The event stream subscriptions of applications on this chain.
    pub event_subscriptions: MapView<C, (ChainId, StreamId), EventSubscriptions>,
}

/// The applications subscribing to a particular stream, and the next event index.
#[derive(Debug, Default, Clone, Serialize, Deserialize)]
pub struct EventSubscriptions {
    /// The next event index, i.e. the total number of events in this stream that have already
    /// been processed by this chain.
    pub next_index: u32,
    /// The applications that are subscribed to this stream.
    pub applications: BTreeSet<ApplicationId>,
}

/// The initial configuration for a new chain.
#[derive(Debug, PartialEq, Eq, Hash, Clone, Serialize, Deserialize)]
pub struct OpenChainConfig {
    /// The ownership configuration of the new chain.
    pub ownership: ChainOwnership,
    /// The initial chain balance.
    pub balance: Amount,
    /// The initial application permissions.
    pub application_permissions: ApplicationPermissions,
}

impl OpenChainConfig {
    /// Creates an [`InitialChainConfig`] based on this [`OpenChainConfig`] and additional
    /// parameters.
    pub fn init_chain_config(
        &self,
        epoch: Epoch,
        committees: BTreeMap<Epoch, Vec<u8>>,
    ) -> InitialChainConfig {
        InitialChainConfig {
            application_permissions: self.application_permissions.clone(),
            balance: self.balance,
            committees,
            epoch,
            ownership: self.ownership.clone(),
        }
    }
}

/// A system operation.
#[derive(Debug, PartialEq, Eq, Hash, Clone, Serialize, Deserialize)]
pub enum SystemOperation {
    /// Transfers `amount` units of value from the given owner's account to the recipient.
    /// If no owner is given, try to take the units out of the unattributed account.
    Transfer {
        owner: AccountOwner,
        recipient: Recipient,
        amount: Amount,
    },
    /// Claims `amount` units of value from the given owner's account in the remote
    /// `target` chain. Depending on its configuration, the `target` chain may refuse to
    /// process the message.
    Claim {
        owner: AccountOwner,
        target_id: ChainId,
        recipient: Recipient,
        amount: Amount,
    },
    /// Creates (or activates) a new chain.
    /// This will automatically subscribe to the future committees created by `admin_id`.
    OpenChain(OpenChainConfig),
    /// Closes the chain.
    CloseChain,
    /// Changes the ownership of the chain.
    ChangeOwnership {
        /// Super owners can propose fast blocks in the first round, and regular blocks in any round.
        #[debug(skip_if = Vec::is_empty)]
        super_owners: Vec<AccountOwner>,
        /// The regular owners, with their weights that determine how often they are round leader.
        #[debug(skip_if = Vec::is_empty)]
        owners: Vec<(AccountOwner, u64)>,
        /// The number of initial rounds after 0 in which all owners are allowed to propose blocks.
        multi_leader_rounds: u32,
        /// Whether the multi-leader rounds are unrestricted, i.e. not limited to chain owners.
        /// This should only be `true` on chains with restrictive application permissions and an
        /// application-based mechanism to select block proposers.
        open_multi_leader_rounds: bool,
        /// The timeout configuration: how long fast, multi-leader and single-leader rounds last.
        timeout_config: TimeoutConfig,
    },
    /// Changes the application permissions configuration on this chain.
    ChangeApplicationPermissions(ApplicationPermissions),
    /// Publishes a new application module.
    PublishModule { module_id: ModuleId },
    /// Publishes a new data blob.
    PublishDataBlob { blob_hash: CryptoHash },
    /// Reads a blob and discards the result.
    // TODO(#2490): Consider removing this.
    ReadBlob { blob_id: BlobId },
    /// Creates a new application.
    CreateApplication {
        module_id: ModuleId,
        #[serde(with = "serde_bytes")]
        #[debug(with = "hex_debug")]
        parameters: Vec<u8>,
        #[serde(with = "serde_bytes")]
        #[debug(with = "hex_debug", skip_if = Vec::is_empty)]
        instantiation_argument: Vec<u8>,
        #[debug(skip_if = Vec::is_empty)]
        required_application_ids: Vec<ApplicationId>,
    },
    /// Operations that are only allowed on the admin chain.
    Admin(AdminOperation),
    /// Processes an event about a new epoch and committee.
    ProcessNewEpoch(Epoch),
    /// Processes an event about a removed epoch and committee.
    ProcessRemovedEpoch(Epoch),
    /// Updates the event stream trackers.
    UpdateStreams(Vec<(ChainId, StreamId, u32)>),
}

/// Operations that are only allowed on the admin chain.
#[derive(Debug, PartialEq, Eq, Hash, Clone, Serialize, Deserialize)]
pub enum AdminOperation {
    /// Publishes a new committee as a blob. This can be assigned to an epoch using
    /// [`AdminOperation::CreateCommittee`] in a later block.
    PublishCommitteeBlob { blob_hash: CryptoHash },
    /// Registers a new committee. Other chains can then migrate to the new epoch by executing
    /// [`SystemOperation::ProcessNewEpoch`].
    CreateCommittee { epoch: Epoch, blob_hash: CryptoHash },
    /// Removes a committee. Other chains should execute [`SystemOperation::ProcessRemovedEpoch`],
    /// so that blocks from the retired epoch will not be accepted until they are followed (hence
    /// re-certified) by a block certified by a recent committee.
    RemoveCommittee { epoch: Epoch },
}

/// A system message meant to be executed on a remote chain.
#[derive(Debug, PartialEq, Eq, Hash, Clone, Serialize, Deserialize)]
pub enum SystemMessage {
    /// Credits `amount` units of value to the account `target` -- unless the message is
    /// bouncing, in which case `source` is credited instead.
    Credit {
        target: AccountOwner,
        amount: Amount,
        source: AccountOwner,
    },
    /// Withdraws `amount` units of value from the account and starts a transfer to credit
    /// the recipient. The message must be properly authenticated. Receiver chains may
    /// refuse it depending on their configuration.
    Withdraw {
        owner: AccountOwner,
        amount: Amount,
        recipient: Recipient,
    },
    /// Notifies that a new application was created.
    ApplicationCreated,
}

/// A query to the system state.
#[derive(Debug, PartialEq, Eq, Hash, Clone, Serialize, Deserialize)]
pub struct SystemQuery;

/// The response to a system query.
#[derive(Debug, PartialEq, Eq, Hash, Clone, Serialize, Deserialize)]
pub struct SystemResponse {
    pub chain_id: ChainId,
    pub balance: Amount,
}

/// The recipient of a transfer.
#[derive(Debug, PartialEq, Eq, Hash, Copy, Clone, Serialize, Deserialize)]
pub enum Recipient {
    /// This is mainly a placeholder for future extensions.
    Burn,
    /// Transfers to the balance of the given account.
    Account(Account),
}

impl Recipient {
    /// Returns the default recipient for the given chain (no owner).
    pub fn chain(chain_id: ChainId) -> Recipient {
        Recipient::Account(Account::chain(chain_id))
    }
}

impl From<ChainId> for Recipient {
    fn from(chain_id: ChainId) -> Self {
        Recipient::chain(chain_id)
    }
}

impl From<Account> for Recipient {
    fn from(account: Account) -> Self {
        Recipient::Account(account)
    }
}

/// Optional user message attached to a transfer.
#[derive(Eq, PartialEq, Ord, PartialOrd, Clone, Hash, Default, Debug, Serialize, Deserialize)]
pub struct UserData(pub Option<[u8; 32]>);

impl UserData {
    pub fn from_option_string(opt_str: Option<String>) -> Result<Self, usize> {
        // Convert the Option<String> to Option<[u8; 32]>
        let option_array = match opt_str {
            Some(s) => {
                // Convert the String to a Vec<u8>
                let vec = s.into_bytes();
                if vec.len() <= 32 {
                    // Create an array from the Vec<u8>
                    let mut array = [b' '; 32];

                    // Copy bytes from the vector into the array
                    let len = vec.len().min(32);
                    array[..len].copy_from_slice(&vec[..len]);

                    Some(array)
                } else {
                    return Err(vec.len());
                }
            }
            None => None,
        };

        // Return the UserData with the converted Option<[u8; 32]>
        Ok(UserData(option_array))
    }
}

#[derive(Debug)]
pub struct CreateApplicationResult {
    pub app_id: ApplicationId,
    pub txn_tracker: TransactionTracker,
}

impl<C> SystemExecutionStateView<C>
where
    C: Context + Clone + Send + Sync + 'static,
    C::Extra: ExecutionRuntimeContext,
{
    /// Invariant for the states of active chains.
    pub fn is_active(&self) -> bool {
        self.description.get().is_some()
            && self.ownership.get().is_active()
            && self.current_committee().is_some()
            && self.admin_id.get().is_some()
    }

    /// Returns the current committee, if any.
    pub fn current_committee(&self) -> Option<(Epoch, &Committee)> {
        let epoch = self.epoch.get();
        let committee = self.committees.get().get(epoch)?;
        Some((*epoch, committee))
    }

    /// Returns a map of epochs to serialized_committees.
    pub fn get_committees(&self) -> BTreeMap<Epoch, Vec<u8>> {
        self.committees
            .get()
            .iter()
            .map(|(epoch, committee)| {
                let serialized_committee =
                    bcs::to_bytes(committee).expect("Serializing a committee should not fail!");
                (*epoch, serialized_committee)
            })
            .collect()
    }

    async fn get_event(&self, event_id: EventId) -> Result<Vec<u8>, ExecutionError> {
        match self.context().extra().get_event(event_id.clone()).await? {
            None => Err(ExecutionError::EventsNotFound(vec![event_id])),
            Some(vec) => Ok(vec),
        }
    }

    /// Executes the sender's side of an operation and returns a list of actions to be
    /// taken.
    pub async fn execute_operation(
        &mut self,
        context: OperationContext,
        operation: SystemOperation,
        txn_tracker: &mut TransactionTracker,
        resource_controller: &mut ResourceController<Option<AccountOwner>>,
    ) -> Result<Option<(ApplicationId, Vec<u8>)>, ExecutionError> {
        use SystemOperation::*;
        let mut new_application = None;
        match operation {
            OpenChain(config) => {
                let _chain_id = self
                    .open_chain(
                        config,
                        context.chain_id,
                        context.height,
                        context.timestamp,
                        txn_tracker,
                    )
                    .await?;
                #[cfg(with_metrics)]
                metrics::OPEN_CHAIN_COUNT.with_label_values(&[]).inc();
            }
            ChangeOwnership {
                super_owners,
                owners,
                multi_leader_rounds,
                open_multi_leader_rounds,
                timeout_config,
            } => {
                self.ownership.set(ChainOwnership {
                    super_owners: super_owners.into_iter().collect(),
                    owners: owners.into_iter().collect(),
                    multi_leader_rounds,
                    open_multi_leader_rounds,
                    timeout_config,
                });
            }
            ChangeApplicationPermissions(application_permissions) => {
                self.application_permissions.set(application_permissions);
            }
            CloseChain => self.close_chain().await?,
            Transfer {
                owner,
                amount,
                recipient,
            } => {
                let maybe_message = self
                    .transfer(context.authenticated_signer, None, owner, recipient, amount)
                    .await?;
                txn_tracker.add_outgoing_messages(maybe_message)?;
            }
            Claim {
                owner,
                target_id,
                recipient,
                amount,
            } => {
                let message = self
                    .claim(
                        context.authenticated_signer,
                        None,
                        owner,
                        target_id,
                        recipient,
                        amount,
                    )
                    .await?;
                txn_tracker.add_outgoing_message(message)?;
            }
            Admin(admin_operation) => {
                ensure!(
                    *self.admin_id.get() == Some(context.chain_id),
                    ExecutionError::AdminOperationOnNonAdminChain
                );
                match admin_operation {
                    AdminOperation::PublishCommitteeBlob { blob_hash } => {
                        self.blob_published(
                            &BlobId::new(blob_hash, BlobType::Committee),
                            txn_tracker,
                        )?;
                    }
                    AdminOperation::CreateCommittee { epoch, blob_hash } => {
                        self.check_next_epoch(epoch)?;
                        let blob_id = BlobId::new(blob_hash, BlobType::Committee);
                        let committee =
                            bcs::from_bytes(self.read_blob_content(blob_id).await?.bytes())?;
                        self.blob_used(txn_tracker, blob_id).await?;
                        self.committees.get_mut().insert(epoch, committee);
                        self.epoch.set(epoch);
                        txn_tracker.add_event(
                            StreamId::system(EPOCH_STREAM_NAME),
                            epoch.0,
                            bcs::to_bytes(&blob_hash)?,
                        );
                    }
                    AdminOperation::RemoveCommittee { epoch } => {
                        ensure!(
                            self.committees.get_mut().remove(&epoch).is_some(),
                            ExecutionError::InvalidCommitteeRemoval
                        );
                        txn_tracker.add_event(
                            StreamId::system(REMOVED_EPOCH_STREAM_NAME),
                            epoch.0,
                            vec![],
                        );
                    }
                }
            }
            PublishModule { module_id } => {
                for blob_id in module_id.bytecode_blob_ids() {
                    self.blob_published(&blob_id, txn_tracker)?;
                }
            }
            CreateApplication {
                module_id,
                parameters,
                instantiation_argument,
                required_application_ids,
            } => {
                let txn_tracker_moved = mem::take(txn_tracker);
                let CreateApplicationResult {
                    app_id,
                    txn_tracker: txn_tracker_moved,
                } = self
                    .create_application(
                        context.chain_id,
                        context.height,
                        module_id,
                        parameters,
                        required_application_ids,
                        txn_tracker_moved,
                    )
                    .await?;
                *txn_tracker = txn_tracker_moved;
                new_application = Some((app_id, instantiation_argument));
            }
            PublishDataBlob { blob_hash } => {
                self.blob_published(&BlobId::new(blob_hash, BlobType::Data), txn_tracker)?;
            }
            ReadBlob { blob_id } => {
                let content = self.read_blob_content(blob_id).await?;
                if blob_id.blob_type == BlobType::Data {
                    resource_controller
                        .with_state(self)
                        .await?
                        .track_blob_read(content.bytes().len() as u64)?;
                }
                self.blob_used(txn_tracker, blob_id).await?;
            }
            ProcessNewEpoch(epoch) => {
                self.check_next_epoch(epoch)?;
                let admin_id = self
                    .admin_id
                    .get()
                    .ok_or_else(|| ExecutionError::InactiveChain(context.chain_id))?;
                let event_id = EventId {
                    chain_id: admin_id,
                    stream_id: StreamId::system(EPOCH_STREAM_NAME),
                    index: epoch.0,
                };
                let bytes = match txn_tracker.next_replayed_oracle_response()? {
                    None => self.get_event(event_id.clone()).await?,
                    Some(OracleResponse::Event(recorded_event_id, bytes))
                        if recorded_event_id == event_id =>
                    {
                        bytes
                    }
                    Some(_) => return Err(ExecutionError::OracleResponseMismatch),
                };
                let blob_id = BlobId::new(bcs::from_bytes(&bytes)?, BlobType::Committee);
                txn_tracker.add_oracle_response(OracleResponse::Event(event_id, bytes));
                let committee = bcs::from_bytes(self.read_blob_content(blob_id).await?.bytes())?;
                self.blob_used(txn_tracker, blob_id).await?;
                self.committees.get_mut().insert(epoch, committee);
                self.epoch.set(epoch);
            }
            ProcessRemovedEpoch(epoch) => {
                ensure!(
                    self.committees.get_mut().remove(&epoch).is_some(),
                    ExecutionError::InvalidCommitteeRemoval
                );
                let admin_id = self
                    .admin_id
                    .get()
                    .ok_or_else(|| ExecutionError::InactiveChain(context.chain_id))?;
                let event_id = EventId {
                    chain_id: admin_id,
                    stream_id: StreamId::system(REMOVED_EPOCH_STREAM_NAME),
                    index: epoch.0,
                };
                let bytes = match txn_tracker.next_replayed_oracle_response()? {
                    None => self.get_event(event_id.clone()).await?,
                    Some(OracleResponse::Event(recorded_event_id, bytes))
                        if recorded_event_id == event_id =>
                    {
                        bytes
                    }
                    Some(_) => return Err(ExecutionError::OracleResponseMismatch),
                };
                txn_tracker.add_oracle_response(OracleResponse::Event(event_id, bytes));
            }
            UpdateStreams(streams) => {
                for (chain_id, stream_id, next_index) in streams {
                    let subscriptions = self
                        .event_subscriptions
                        .get_mut_or_default(&(chain_id, stream_id.clone()))
                        .await?;
                    ensure!(
                        subscriptions.next_index < next_index,
                        ExecutionError::OutdatedUpdateStreams
                    );
                    for application_id in &subscriptions.applications {
                        txn_tracker.add_stream_to_process(
                            *application_id,
                            chain_id,
                            stream_id.clone(),
                            subscriptions.next_index,
                            next_index,
                        );
                    }
                    subscriptions.next_index = next_index;
                    let index = next_index
                        .checked_sub(1)
                        .ok_or(ArithmeticError::Underflow)?;
                    let event_id = EventId {
                        chain_id,
                        stream_id,
                        index,
                    };
                    ensure!(
                        self.context()
                            .extra()
                            .contains_event(event_id.clone())
                            .await?,
                        ExecutionError::EventNotFound(event_id)
                    );
                }
            }
        }

        Ok(new_application)
    }

    /// Returns an error if the `provided` epoch is not exactly one higher than the chain's current
    /// epoch.
    fn check_next_epoch(&self, provided: Epoch) -> Result<(), ExecutionError> {
        let expected = self.epoch.get().try_add_one()?;
        ensure!(
            provided == expected,
            ExecutionError::InvalidCommitteeEpoch { provided, expected }
        );
        Ok(())
    }

    pub async fn transfer(
        &mut self,
        authenticated_signer: Option<AccountOwner>,
        authenticated_application_id: Option<ApplicationId>,
        source: AccountOwner,
        recipient: Recipient,
        amount: Amount,
    ) -> Result<Option<OutgoingMessage>, ExecutionError> {
        if source == AccountOwner::CHAIN {
            ensure!(
                authenticated_signer.is_some()
                    && self
                        .ownership
                        .get()
                        .verify_owner(&authenticated_signer.unwrap()),
                ExecutionError::UnauthenticatedTransferOwner
            );
        } else {
            ensure!(
                authenticated_signer == Some(source)
                    || authenticated_application_id.map(AccountOwner::from) == Some(source),
                ExecutionError::UnauthenticatedTransferOwner
            );
        }
        ensure!(
            amount > Amount::ZERO,
            ExecutionError::IncorrectTransferAmount
        );
        self.debit(&source, amount).await?;
        match recipient {
            Recipient::Account(account) => {
                let message = SystemMessage::Credit {
                    amount,
                    source,
                    target: account.owner,
                };
                Ok(Some(
                    OutgoingMessage::new(account.chain_id, message).with_kind(MessageKind::Tracked),
                ))
            }
            Recipient::Burn => Ok(None),
        }
    }

    pub async fn claim(
        &self,
        authenticated_signer: Option<AccountOwner>,
        authenticated_application_id: Option<ApplicationId>,
        source: AccountOwner,
        target_id: ChainId,
        recipient: Recipient,
        amount: Amount,
    ) -> Result<OutgoingMessage, ExecutionError> {
        ensure!(
            authenticated_signer == Some(source)
                || authenticated_application_id.map(AccountOwner::from) == Some(source),
            ExecutionError::UnauthenticatedClaimOwner
        );
        ensure!(amount > Amount::ZERO, ExecutionError::IncorrectClaimAmount);

        let message = SystemMessage::Withdraw {
            amount,
            owner: source,
            recipient,
        };
        Ok(
            OutgoingMessage::new(target_id, message)
                .with_authenticated_signer(authenticated_signer),
        )
    }

    /// Debits an [`Amount`] of tokens from an account's balance.
    async fn debit(
        &mut self,
        account: &AccountOwner,
        amount: Amount,
    ) -> Result<(), ExecutionError> {
        let balance = if account == &AccountOwner::CHAIN {
            self.balance.get_mut()
        } else {
            self.balances.get_mut(account).await?.ok_or_else(|| {
                ExecutionError::InsufficientBalance {
                    balance: Amount::ZERO,
                    account: *account,
                }
            })?
        };

        balance
            .try_sub_assign(amount)
            .map_err(|_| ExecutionError::InsufficientBalance {
                balance: *balance,
                account: *account,
            })?;

        if account != &AccountOwner::CHAIN && balance.is_zero() {
            self.balances.remove(account)?;
        }

        Ok(())
    }

    /// Executes a cross-chain message that represents the recipient's side of an operation.
    pub async fn execute_message(
        &mut self,
        context: MessageContext,
        message: SystemMessage,
    ) -> Result<Vec<OutgoingMessage>, ExecutionError> {
        let mut outcome = Vec::new();
        use SystemMessage::*;
        match message {
            Credit {
                amount,
                source,
                target,
            } => {
                let receiver = if context.is_bouncing { source } else { target };
                if receiver == AccountOwner::CHAIN {
                    let new_balance = self.balance.get().saturating_add(amount);
                    self.balance.set(new_balance);
                } else {
                    let balance = self.balances.get_mut_or_default(&receiver).await?;
                    *balance = balance.saturating_add(amount);
                }
            }
            Withdraw {
                amount,
                owner,
                recipient,
            } => {
                self.debit(&owner, amount).await?;
                match recipient {
                    Recipient::Account(account) => {
                        let message = SystemMessage::Credit {
                            amount,
                            source: owner,
                            target: account.owner,
                        };
                        outcome.push(
                            OutgoingMessage::new(account.chain_id, message)
                                .with_kind(MessageKind::Tracked),
                        );
                    }
                    Recipient::Burn => (),
                }
            }
            // This message is only a placeholder: Its ID is part of the application ID.
            ApplicationCreated => {}
        }
        Ok(outcome)
    }

    /// Initializes the system application state on a newly opened chain.
    /// Returns `Ok(true)` if the chain was already initialized, `Ok(false)` if it wasn't.
    pub async fn initialize_chain(&mut self, chain_id: ChainId) -> Result<bool, ExecutionError> {
        if self.description.get().is_some() {
            // already initialized
            return Ok(true);
        }
        let description_blob = self
            .read_blob_content(BlobId::new(chain_id.0, BlobType::ChainDescription))
            .await?;
        let description: ChainDescription = bcs::from_bytes(description_blob.bytes())?;
        let InitialChainConfig {
            ownership,
            epoch,
            committees,
            balance,
            application_permissions,
        } = description.config().clone();
        self.timestamp.set(description.timestamp());
        self.description.set(Some(description));
        self.epoch.set(epoch);
        let committees = committees
            .into_iter()
            .map(|(epoch, serialized_committee)| {
                let committee = bcs::from_bytes(&serialized_committee)
                    .expect("Deserializing a committee shouldn't fail");
                (epoch, committee)
            })
            .collect();
        self.committees.set(committees);
        // If `admin_id` is `None`, this chain is its own admin chain.
        let admin_id = self
            .context()
            .extra()
            .get_network_description()
            .await?
            .ok_or(ExecutionError::NoNetworkDescriptionFound)?
            .admin_chain_id;
        self.admin_id.set(Some(admin_id));
        self.ownership.set(ownership);
        self.balance.set(balance);
        self.application_permissions.set(application_permissions);
        Ok(false)
    }

    pub async fn handle_query(
        &mut self,
        context: QueryContext,
        _query: SystemQuery,
    ) -> Result<QueryOutcome<SystemResponse>, ExecutionError> {
        let response = SystemResponse {
            chain_id: context.chain_id,
            balance: *self.balance.get(),
        };
        Ok(QueryOutcome {
            response,
            operations: vec![],
        })
    }

    /// Returns the messages to open a new chain, and subtracts the new chain's balance
    /// from this chain's.
    pub async fn open_chain(
        &mut self,
        config: OpenChainConfig,
        parent: ChainId,
        block_height: BlockHeight,
        timestamp: Timestamp,
        txn_tracker: &mut TransactionTracker,
    ) -> Result<ChainId, ExecutionError> {
        let chain_index = txn_tracker.next_chain_index();
        let chain_origin = ChainOrigin::Child {
            parent,
            block_height,
            chain_index,
        };
        let committees = self.get_committees();
        let init_chain_config = config.init_chain_config(*self.epoch.get(), committees);
        let chain_description = ChainDescription::new(chain_origin, init_chain_config, timestamp);
        let child_id = chain_description.id();
        self.debit(&AccountOwner::CHAIN, config.balance).await?;
        let blob = Blob::new_chain_description(&chain_description);
        txn_tracker.add_created_blob(blob);
        Ok(child_id)
    }

    pub async fn close_chain(&mut self) -> Result<(), ExecutionError> {
        self.closed.set(true);
        Ok(())
    }

    pub async fn create_application(
        &mut self,
        chain_id: ChainId,
        block_height: BlockHeight,
        module_id: ModuleId,
        parameters: Vec<u8>,
        required_application_ids: Vec<ApplicationId>,
        mut txn_tracker: TransactionTracker,
    ) -> Result<CreateApplicationResult, ExecutionError> {
        let application_index = txn_tracker.next_application_index();

        let blob_ids = self.check_bytecode_blobs(&module_id).await?;
        // We only remember to register the blobs that aren't recorded in `used_blobs`
        // already.
        for blob_id in blob_ids {
            self.blob_used(&mut txn_tracker, blob_id).await?;
        }

        let application_description = ApplicationDescription {
            module_id,
            creator_chain_id: chain_id,
            block_height,
            application_index,
            parameters,
            required_application_ids,
        };
        self.check_required_applications(&application_description, &mut txn_tracker)
            .await?;

        let blob = Blob::new_application_description(&application_description);
        self.used_blobs.insert(&blob.id())?;
        txn_tracker.add_created_blob(blob);

        Ok(CreateApplicationResult {
            app_id: ApplicationId::from(&application_description),
            txn_tracker,
        })
    }

    async fn check_required_applications(
        &mut self,
        application_description: &ApplicationDescription,
        txn_tracker: &mut TransactionTracker,
    ) -> Result<(), ExecutionError> {
        // Make sure that referenced applications IDs have been registered.
        for required_id in &application_description.required_application_ids {
            Box::pin(self.describe_application(*required_id, txn_tracker)).await?;
        }
        Ok(())
    }

    /// Retrieves an application's description.
    pub async fn describe_application(
        &mut self,
        id: ApplicationId,
        txn_tracker: &mut TransactionTracker,
    ) -> Result<ApplicationDescription, ExecutionError> {
        let blob_id = id.description_blob_id();
        let blob_content = match txn_tracker.created_blobs().get(&blob_id) {
            Some(blob) => blob.content().clone(),
            None => self.read_blob_content(blob_id).await?,
        };
        self.blob_used(txn_tracker, blob_id).await?;
        let description: ApplicationDescription = bcs::from_bytes(blob_content.bytes())?;

        let blob_ids = self.check_bytecode_blobs(&description.module_id).await?;
        // We only remember to register the blobs that aren't recorded in `used_blobs`
        // already.
        for blob_id in blob_ids {
            self.blob_used(txn_tracker, blob_id).await?;
        }

        self.check_required_applications(&description, txn_tracker)
            .await?;

        Ok(description)
    }

    /// Retrieves the recursive dependencies of applications and applies a topological sort.
    pub async fn find_dependencies(
        &mut self,
        mut stack: Vec<ApplicationId>,
        txn_tracker: &mut TransactionTracker,
    ) -> Result<Vec<ApplicationId>, ExecutionError> {
        // What we return at the end.
        let mut result = Vec::new();
        // The entries already inserted in `result`.
        let mut sorted = HashSet::new();
        // The entries for which dependencies have already been pushed once to the stack.
        let mut seen = HashSet::new();

        while let Some(id) = stack.pop() {
            if sorted.contains(&id) {
                continue;
            }
            if seen.contains(&id) {
                // Second time we see this entry. It was last pushed just before its
                // dependencies -- which are now fully sorted.
                sorted.insert(id);
                result.push(id);
                continue;
            }
            // First time we see this entry:
            // 1. Mark it so that its dependencies are no longer pushed to the stack.
            seen.insert(id);
            // 2. Schedule all the (yet unseen) dependencies, then this entry for a second visit.
            stack.push(id);
            let app = self.describe_application(id, txn_tracker).await?;
            for child in app.required_application_ids.iter().rev() {
                if !seen.contains(child) {
                    stack.push(*child);
                }
            }
        }
        Ok(result)
    }

    /// Records a blob that is used in this block. If this is the first use on this chain, creates
    /// an oracle response for it.
    pub(crate) async fn blob_used(
        &mut self,
        txn_tracker: &mut TransactionTracker,
        blob_id: BlobId,
    ) -> Result<bool, ExecutionError> {
        if self.used_blobs.contains(&blob_id).await? {
            return Ok(false); // Nothing to do.
        }
        self.used_blobs.insert(&blob_id)?;
        txn_tracker.replay_oracle_response(OracleResponse::Blob(blob_id))?;
        Ok(true)
    }

    /// Records a blob that is published in this block. This does not create an oracle entry, and
    /// the blob can be used without using an oracle in the future on this chain.
    fn blob_published(
        &mut self,
        blob_id: &BlobId,
        txn_tracker: &mut TransactionTracker,
    ) -> Result<(), ExecutionError> {
        self.used_blobs.insert(blob_id)?;
        txn_tracker.add_published_blob(*blob_id);
        Ok(())
    }

    pub async fn read_blob_content(&self, blob_id: BlobId) -> Result<BlobContent, ExecutionError> {
        match self.context().extra().get_blob(blob_id).await {
            Ok(Some(blob)) => Ok(blob.into()),
            Ok(None) => Err(ExecutionError::BlobsNotFound(vec![blob_id])),
            Err(error) => Err(error.into()),
        }
    }

    pub async fn assert_blob_exists(&mut self, blob_id: BlobId) -> Result<(), ExecutionError> {
        if self.context().extra().contains_blob(blob_id).await? {
            Ok(())
        } else {
            Err(ExecutionError::BlobsNotFound(vec![blob_id]))
        }
    }

    async fn check_bytecode_blobs(
        &mut self,
        module_id: &ModuleId,
    ) -> Result<Vec<BlobId>, ExecutionError> {
        let blob_ids = module_id.bytecode_blob_ids();

        let mut missing_blobs = Vec::new();
        for blob_id in &blob_ids {
            if !self.context().extra().contains_blob(*blob_id).await? {
                missing_blobs.push(*blob_id);
            }
        }
        ensure!(
            missing_blobs.is_empty(),
            ExecutionError::BlobsNotFound(missing_blobs)
        );

        Ok(blob_ids)
    }
}