darkfi/src/runtime/vm_runtime.rs

/* This file is part of DarkFi (https://dark.fi)
 *
 * Copyright (C) 2020-2026 Dyne.org foundation
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

use std::{
    cell::{Cell, RefCell},
    collections::BTreeMap,
    sync::Arc,
};

use darkfi_sdk::{
    crypto::contract_id::{
        ContractId, SMART_CONTRACT_MONOTREE_DB_NAME, SMART_CONTRACT_ZKAS_DB_NAME,
    },
    tx::TransactionHash,
    wasm, AsHex,
};
use darkfi_serial::serialize;
use parking_lot::Mutex;
use tracing::{debug, error, info};
use wasmer::{
    imports, sys::CompilerConfig, wasmparser::Operator, AsStoreMut, AsStoreRef, Function,
    FunctionEnv, Instance, Memory, MemoryType, MemoryView, Module, Pages, Store, Value,
    WASM_PAGE_SIZE,
};
use wasmer_compiler_singlepass::Singlepass;
use wasmer_middlewares::{
    metering::{get_remaining_points, set_remaining_points, MeteringPoints},
    Metering,
};

use super::{import, import::db::DbHandle, memory::MemoryManipulation};
use crate::{blockchain::BlockchainOverlayPtr, Error, Result};

/// Name of the wasm linear memory in our guest module
const MEMORY: &str = "memory";

/// Gas limit for a single contract call (single WASM instance).
pub const CONTRACT_GAS_LIMIT: u64 = 800_000_000;

/// Gas limit for a single transaction (across all of its calls).
pub const TX_GAS_LIMIT: u64 = 1_000_000_000;

// ANCHOR: contract-section
#[derive(Clone, Copy, PartialEq)]
pub enum ContractSection {
    /// Setup function of a contract
    Deploy,
    /// Entrypoint function of a contract
    Exec,
    /// Apply function of a contract
    Update,
    /// Metadata
    Metadata,
    /// Placeholder state before any initialization
    Null,
}
// ANCHOR_END: contract-section

impl ContractSection {
    pub const fn name(&self) -> &str {
        match self {
            Self::Deploy => "__initialize",
            Self::Exec => "__entrypoint",
            Self::Update => "__update",
            Self::Metadata => "__metadata",
            Self::Null => unreachable!(),
        }
    }
}

/// Transaction-local state db.
///
/// This is an in-memory BTreeMap that works equivalently to the existing
/// blockchain DB in contracts, except its lifetime is during a single
/// transaction execution.
pub type TxLocalState = BTreeMap<ContractId, BTreeMap<[u8; 32], BTreeMap<Vec<u8>, Vec<u8>>>>;

/// The WASM VM runtime environment instantiated for every smart contract that runs.
pub struct Env {
    /// Blockchain overlay access
    pub blockchain: BlockchainOverlayPtr,
    /// Overlay tree handles used with `db_*`
    pub db_handles: RefCell<Vec<DbHandle>>,
    /// Transaction-local db handles used with `db_*_local`
    pub local_db_handles: RefCell<Vec<DbHandle>>,
    /// Transaction-local state
    pub tx_local: Arc<Mutex<TxLocalState>>,
    /// The contract ID being executed
    pub contract_id: ContractId,
    /// The compiled wasm bincode being executed,
    pub contract_bincode: Vec<u8>,
    /// The contract section being executed
    pub contract_section: ContractSection,
    /// State update produced by a smart contract function call
    pub contract_return_data: Cell<Option<Vec<u8>>>,
    /// Logs produced by the contract
    pub logs: RefCell<Vec<String>>,
    /// Direct memory access to the VM
    pub memory: Option<Memory>,
    /// Object store for transferring memory from the host to VM
    pub objects: RefCell<Vec<Vec<u8>>>,
    /// Block height number runtime verifies against.
    /// For unconfirmed txs, this will be the current max height in the chain.
    pub verifying_block_height: u32,
    /// Currently configured block time target, in seconds
    pub block_target: u32,
    /// The hash for this transaction the runtime is being run against.
    pub tx_hash: TransactionHash,
    /// The index for this call in the transaction
    pub call_idx: u8,
    /// Parent `Instance`
    pub instance: Option<Arc<Instance>>,
}

impl Env {
    /// Provide safe access to the memory
    /// (it must be initialized before it can be used)
    ///
    ///     // ctx: FunctionEnvMut<Env>
    ///     let env = ctx.data();
    ///     let memory = env.memory_view(&ctx);
    ///
    pub fn memory_view<'a>(&'a self, store: &'a impl AsStoreRef) -> MemoryView<'a> {
        self.memory().view(store)
    }

    /// Get memory, that needs to have been set fist
    pub fn memory(&self) -> &Memory {
        self.memory.as_ref().unwrap()
    }

    /// Subtract given gas cost from remaining gas in the current runtime
    pub fn subtract_gas(&mut self, ctx: &mut impl AsStoreMut, gas: u64) {
        match get_remaining_points(ctx, self.instance.as_ref().unwrap()) {
            MeteringPoints::Remaining(rem) => {
                if gas > rem {
                    set_remaining_points(ctx, self.instance.as_ref().unwrap(), 0);
                } else {
                    set_remaining_points(ctx, self.instance.as_ref().unwrap(), rem - gas);
                }
            }
            MeteringPoints::Exhausted => {
                set_remaining_points(ctx, self.instance.as_ref().unwrap(), 0);
            }
        }
    }
}

/// Define a wasm runtime.
pub struct Runtime {
    /// A wasm instance
    pub instance: Arc<Instance>,
    /// A wasm store (global state)
    pub store: Store,
    // Wrapper for [`Env`], defined above.
    pub ctx: FunctionEnv<Env>,
}

impl Runtime {
    /// Create a new wasm runtime instance that contains the given wasm module.
    #[allow(clippy::too_many_arguments)]
    pub fn new(
        wasm_bytes: &[u8],
        blockchain: BlockchainOverlayPtr,
        tx_local: Arc<Mutex<TxLocalState>>,
        contract_id: ContractId,
        verifying_block_height: u32,
        block_target: u32,
        tx_hash: TransactionHash,
        call_idx: u8,
    ) -> Result<Self> {
        info!(target: "runtime::vm_runtime", "[WASM] Instantiating a new runtime");
        // This function will be called for each `Operator` encountered during
        // the wasm module execution. It should return the cost of the operator
        // that it received as its first argument. For now, every wasm opcode
        // has a cost of `1`.
        // https://docs.rs/wasmparser/latest/wasmparser/enum.Operator.html
        let cost_function = |_operator: &Operator| -> u64 { 1 };

        // `Metering` needs to be configured with a limit and a cost function.
        // For each `Operator`, the metering middleware will call the cost
        // function and subtract the cost from the remaining points.
        let metering = Arc::new(Metering::new(CONTRACT_GAS_LIMIT, cost_function));

        // Define the compiler and middleware, engine, and store
        let mut compiler_config = Singlepass::new();
        compiler_config.push_middleware(metering);
        let mut store = Store::new(compiler_config);

        // Create a larger Memory for the instance
        let memory_type = MemoryType::new(
            Pages(256),        // init: 16 MB (256 * 64KB)
            Some(Pages(4096)), // max: 256 MB
            false,
        );
        let memory = Memory::new(&mut store, memory_type)?;

        debug!(target: "runtime::vm_runtime", "Compiling module");
        let module = Module::new(&store, wasm_bytes)?;

        // Initialize data
        let db_handles = RefCell::new(vec![]);
        let local_db_handles = RefCell::new(vec![]);
        let logs = RefCell::new(vec![]);

        // Initialize a tx-local db for the calling contract
        tx_local.lock().entry(contract_id).or_default();

        debug!(target: "runtime::vm_runtime", "Importing functions");

        let ctx = FunctionEnv::new(
            &mut store,
            Env {
                blockchain,
                db_handles,
                local_db_handles,
                tx_local,
                contract_id,
                contract_bincode: wasm_bytes.to_vec(),
                contract_section: ContractSection::Null,
                contract_return_data: Cell::new(None),
                logs,
                memory: Some(memory.clone()),
                objects: RefCell::new(vec![]),
                verifying_block_height,
                block_target,
                tx_hash,
                call_idx,
                instance: None,
            },
        );

        let imports = imports! {
            "env" => {
                "memory" => memory,

                "drk_log_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::drk_log,
                ),

                "set_return_data_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::set_return_data,
                ),

                "db_init_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_init,
                ),

                "db_lookup_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_lookup,
                ),

                "db_lookup_local_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_lookup_local,
                ),

                "db_get_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_get,
                ),

                "db_get_local_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_get_local,
                ),

                "db_contains_key_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_contains_key,
                ),

                "db_contains_key_local_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_contains_key_local,
                ),

                "db_set_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_set,
                ),

                "db_set_local_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_set_local,
                ),

                "db_del_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_del,
                ),

                "db_del_local_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::db_del_local,
                ),

                "zkas_db_set_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::db::zkas_db_set,
                ),

                "get_object_bytes_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::get_object_bytes,
                ),

                "get_object_size_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::get_object_size,
                ),

                "merkle_add_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::merkle::merkle_add,
                ),

                "merkle_add_local_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::merkle::merkle_add_local,
                ),

                "sparse_merkle_insert_batch_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::smt::sparse_merkle_insert_batch,
                ),

                "get_verifying_block_height_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::get_verifying_block_height,
                ),

                "get_block_target_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::get_block_target,
                ),

                "get_tx_hash_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::get_tx_hash,
                ),

                "get_call_index_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::get_call_index,
                ),

                "get_blockchain_time_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::get_blockchain_time,
                ),

                "get_last_block_height_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::get_last_block_height,
                ),

                "get_tx_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::get_tx,
                ),

                "get_tx_location_" => Function::new_typed_with_env(
                    &mut store,
                    &ctx,
                    import::util::get_tx_location,
                ),
            }
        };

        debug!(target: "runtime::vm_runtime", "Instantiating module");
        let instance = Arc::new(Instance::new(&mut store, &module, &imports)?);

        let env_mut = ctx.as_mut(&mut store);
        env_mut.memory = Some(instance.exports.get_with_generics(MEMORY)?);
        env_mut.instance = Some(Arc::clone(&instance));

        Ok(Self { instance, store, ctx })
    }

    /// Call a contract method defined by a [`ContractSection`] using a supplied
    /// payload. Returns a `Vec<u8>` corresponding to the result data of the call.
    /// For calls that do not return any data, an empty `Vec<u8>` is returned.
    fn call(&mut self, section: ContractSection, payload: &[u8]) -> Result<Vec<u8>> {
        debug!(target: "runtime::vm_runtime", "Calling {} method", section.name());

        let env_mut = self.ctx.as_mut(&mut self.store);
        env_mut.contract_section = section;
        // Verify contract's return data is empty, or quit.
        assert!(env_mut.contract_return_data.take().is_none());

        // Clear the logs
        let _ = env_mut.logs.take();

        // Serialize the payload for the format the wasm runtime is expecting.
        let payload = Self::serialize_payload(&env_mut.contract_id, payload);

        // Allocate enough memory for the payload and copy it into the memory.
        let pages_required = payload.len() / WASM_PAGE_SIZE + 1;
        self.set_memory_page_size(pages_required as u32)?;
        self.copy_to_memory(&payload)?;

        debug!(target: "runtime::vm_runtime", "Getting {} function", section.name());
        let entrypoint = self.instance.exports.get_function(section.name())?;

        // Call the entrypoint. On success, `call` returns a WASM [`Value`]. (The
        // value may be empty.) This value functions similarly to a UNIX exit code.
        // The following section is intended to unwrap the exit code and handle fatal
        // errors in the Wasmer runtime. The value itself and the return data of the
        // contract are processed later.
        debug!(target: "runtime::vm_runtime", "Executing wasm");
        let ret = match entrypoint.call(&mut self.store, &[Value::I32(0_i32)]) {
            Ok(retvals) => {
                self.print_logs();
                info!(target: "runtime::vm_runtime", "[WASM] {}", self.gas_info());
                retvals
            }
            Err(e) => {
                self.print_logs();
                info!(target: "runtime::vm_runtime", "[WASM] {}", self.gas_info());
                // WasmerRuntimeError panics are handled here. Return from run() immediately.
                error!(target: "runtime::vm_runtime", "[WASM] Wasmer Runtime Error: {e:#?}");
                return Err(e.into())
            }
        };

        debug!(target: "runtime::vm_runtime", "wasm executed successfully");

        // Move the contract's return data into `retdata`.
        let env_mut = self.ctx.as_mut(&mut self.store);
        env_mut.contract_section = ContractSection::Null;
        let retdata = env_mut.contract_return_data.take().unwrap_or_default();

        // Determine the return value of the contract call. If `ret` is empty,
        // assumed that the contract call was successful.
        let retval: i64 = match ret.len() {
            0 => {
                // Return a success value if there is no return value from
                // the contract.
                debug!(target: "runtime::vm_runtime", "Contract has no return value (expected)");
                wasm::entrypoint::SUCCESS
            }
            _ => {
                match ret[0] {
                    Value::I64(v) => {
                        debug!(target: "runtime::vm_runtime", "Contract returned: {:?}", ret[0]);
                        v
                    }
                    // The only supported return type is i64, so panic if another
                    // value is returned.
                    _ => unreachable!("Got unexpected result return value: {ret:?}"),
                }
            }
        };

        // Check the integer return value of the call. A value of `entrypoint::SUCCESS` (i.e. zero)
        // corresponds to a successful contract call; in this case, we return the contract's
        // result data. Otherwise, map the integer return value to a [`ContractError`].
        match retval {
            wasm::entrypoint::SUCCESS => Ok(retdata),
            _ => {
                let err = darkfi_sdk::error::ContractError::from(retval);
                error!(target: "runtime::vm_runtime", "[WASM] Contract returned: {err:?}");
                Err(Error::ContractError(err))
            }
        }
    }

    /// This function runs when a smart contract is initially deployed, or re-deployed.
    ///
    /// The runtime will look for an `__initialize` symbol in the wasm code, and execute
    /// it if found. Optionally, it is possible to pass in a payload for any kind of special
    /// instructions the developer wants to manage in the initialize function.
    ///
    /// This process is supposed to set up the overlay trees for storing the smart contract
    /// state, and it can create, delete, modify, read, and write to databases it's allowed to.
    /// The permissions for this are handled by the `ContractId` in the overlay db API so we
    /// assume that the contract is only able to do write operations on its own overlay trees.
    pub fn deploy(&mut self, payload: &[u8]) -> Result<()> {
        let cid = self.ctx.as_ref(&self.store).contract_id;
        info!(target: "runtime::vm_runtime", "[WASM] Running deploy() for ContractID: {cid}");

        // Scoped for borrows
        {
            let env_mut = self.ctx.as_mut(&mut self.store);
            // We always want to have the zkas db as index 0 in db handles and batches when
            // deploying.
            let contracts = &env_mut.blockchain.lock().unwrap().contracts;

            // Open or create the zkas db tree for this contract
            let zkas_tree_handle =
                match contracts.lookup(&env_mut.contract_id, SMART_CONTRACT_ZKAS_DB_NAME) {
                    Ok(v) => v,
                    Err(_) => contracts.init(&env_mut.contract_id, SMART_CONTRACT_ZKAS_DB_NAME)?,
                };

            // Create the monotree db tree for this contract,
            // if it doesn't exists.
            if contracts.lookup(&env_mut.contract_id, SMART_CONTRACT_MONOTREE_DB_NAME).is_err() {
                contracts.init(&env_mut.contract_id, SMART_CONTRACT_MONOTREE_DB_NAME)?;
            }

            let mut db_handles = env_mut.db_handles.borrow_mut();
            db_handles.push(DbHandle::new(env_mut.contract_id, zkas_tree_handle));
        }

        //debug!(target: "runtime::vm_runtime", "[WASM] payload: {payload:?}");
        let _ = self.call(ContractSection::Deploy, payload)?;

        // Update the wasm bincode in the ContractStore wasm tree if the deploy exec passed successfully.
        let env_mut = self.ctx.as_mut(&mut self.store);
        env_mut
            .blockchain
            .lock()
            .unwrap()
            .contracts
            .insert(env_mut.contract_id, &env_mut.contract_bincode)?;

        info!(target: "runtime::vm_runtime", "[WASM] Successfully deployed ContractID: {cid}");
        Ok(())
    }

    /// This function runs first in the entire scheme of executing a smart contract.
    ///
    /// The runtime will look for a `__metadata` symbol in the wasm code and execute it.
    /// It is supposed to correctly extract public inputs for any ZK proofs included
    /// in the contract calls, and also extract the public keys used to verify the
    /// call/transaction signatures.
    pub fn metadata(&mut self, payload: &[u8]) -> Result<Vec<u8>> {
        let cid = self.ctx.as_ref(&self.store).contract_id;
        info!(target: "runtime::vm_runtime", "[WASM] Running metadata() for ContractID: {cid}");

        debug!(target: "runtime::vm_runtime", "metadata payload: {}", payload.hex());
        let ret = self.call(ContractSection::Metadata, payload)?;
        debug!(target: "runtime::vm_runtime", "metadata returned: {:?}", ret.hex());

        info!(target: "runtime::vm_runtime", "[WASM] Successfully got metadata ContractID: {cid}");
        Ok(ret)
    }

    /// This function runs when someone wants to execute a smart contract.
    ///
    /// The runtime will look for an `__entrypoint` symbol in the wasm code, and
    /// execute it if found. A payload is also passed as an instruction that can
    /// be used inside the vm by the runtime.
    pub fn exec(&mut self, payload: &[u8]) -> Result<Vec<u8>> {
        let cid = self.ctx.as_ref(&self.store).contract_id;
        info!(target: "runtime::vm_runtime", "[WASM] Running exec() for ContractID: {cid}");

        debug!(target: "runtime::vm_runtime", "exec payload: {}", payload.hex());
        let ret = self.call(ContractSection::Exec, payload)?;
        debug!(target: "runtime::vm_runtime", "exec returned: {:?}", ret.hex());

        info!(target: "runtime::vm_runtime", "[WASM] Successfully executed ContractID: {cid}");
        Ok(ret)
    }

    /// This function runs after successful execution of `exec` and tries to
    /// apply the state change to the overlay databases.
    ///
    /// The runtime will lok for an `__update` symbol in the wasm code, and execute
    /// it if found. The function does not take an arbitrary payload, but just takes
    /// a state update from `env` and passes it into the wasm runtime.
    pub fn apply(&mut self, update: &[u8]) -> Result<()> {
        let cid = self.ctx.as_ref(&self.store).contract_id;
        info!(target: "runtime::vm_runtime", "[WASM] Running apply() for ContractID: {cid}");

        debug!(target: "runtime::vm_runtime", "apply payload: {:?}", update.hex());
        let ret = self.call(ContractSection::Update, update)?;
        debug!(target: "runtime::vm_runtime", "apply returned: {:?}", ret.hex());

        info!(target: "runtime::vm_runtime", "[WASM] Successfully applied ContractID: {cid}");
        Ok(())
    }

    /// Prints the wasm contract logs.
    fn print_logs(&self) {
        let logs = self.ctx.as_ref(&self.store).logs.borrow();
        for msg in logs.iter() {
            info!(target: "runtime::vm_runtime", "[WASM] Contract log: {msg}");
        }
    }

    /// Calculate the remaining gas using wasm's concept
    /// of metering points.
    pub fn gas_used(&mut self) -> u64 {
        let remaining_points = get_remaining_points(&mut self.store, &self.instance);

        match remaining_points {
            MeteringPoints::Remaining(rem) => {
                if rem > CONTRACT_GAS_LIMIT {
                    // This should never occur, but catch it explicitly to avoid
                    // potential underflow issues when calculating `remaining_points`.
                    unreachable!("Remaining wasm points exceed CONTRACT_GAS_LIMIT");
                }
                CONTRACT_GAS_LIMIT - rem
            }
            MeteringPoints::Exhausted => CONTRACT_GAS_LIMIT + 1,
        }
    }

    // Return a message informing the user whether there is any
    // gas remaining. Values equal to CONTRACT_GAS_LIMIT are not considered
    // to be exhausted. e.g. Using 100/100 gas should not give a
    // 'gas exhausted' message.
    fn gas_info(&mut self) -> String {
        let gas_used = self.gas_used();

        if gas_used > CONTRACT_GAS_LIMIT {
            format!("Gas fully exhausted: {gas_used}/{CONTRACT_GAS_LIMIT}")
        } else {
            format!("Gas used: {gas_used}/{CONTRACT_GAS_LIMIT}")
        }
    }

    /// Set the memory page size. Returns the previous memory size.
    fn set_memory_page_size(&mut self, pages: u32) -> Result<Pages> {
        // Grab memory by value
        let memory = self.take_memory();
        // Modify the memory
        let ret = memory.grow(&mut self.store, Pages(pages))?;
        // Replace the memory back again
        self.ctx.as_mut(&mut self.store).memory = Some(memory);
        Ok(ret)
    }

    /// Take Memory by value. Needed to modify the Memory object
    /// Will panic if memory isn't set.
    fn take_memory(&mut self) -> Memory {
        let env_memory = &mut self.ctx.as_mut(&mut self.store).memory;
        let memory = env_memory.take();
        memory.expect("memory should be set")
    }

    /// Copy payload to the start of the memory
    fn copy_to_memory(&self, payload: &[u8]) -> Result<()> {
        // Payload is copied to index 0.
        // Get the memory view
        let env = self.ctx.as_ref(&self.store);
        let memory_view = env.memory_view(&self.store);
        memory_view.write_slice(payload, 0)
    }

    /// Serialize contract payload to the format accepted by the runtime functions.
    /// We keep the same payload as a slice of bytes, and prepend it with a [`ContractId`],
    /// and then a little-endian u64 to tell the payload's length.
    fn serialize_payload(cid: &ContractId, payload: &[u8]) -> Vec<u8> {
        let ser_cid = serialize(cid);
        let payload_len = payload.len();
        let mut out = Vec::with_capacity(ser_cid.len() + 8 + payload_len);
        out.extend_from_slice(&ser_cid);
        out.extend_from_slice(&(payload_len as u64).to_le_bytes());
        out.extend_from_slice(payload);
        out
    }
}