component/func/host.rs

use crate::component::concurrent;
#[cfg(feature = "component-model-async")]
use crate::component::concurrent::{Accessor, Status};
use crate::component::func::{LiftContext, LowerContext};
use crate::component::matching::InstanceType;
use crate::component::storage::slice_to_storage_mut;
use crate::component::types::ComponentFunc;
use crate::component::{ComponentNamedList, ComponentType, Instance, Lift, Lower, Val};
use crate::prelude::*;
use crate::runtime::vm::component::{
    ComponentInstance, VMComponentContext, VMLowering, VMLoweringCallee,
};
use crate::runtime::vm::{SendSyncPtr, VMOpaqueContext, VMStore};
use crate::{AsContextMut, CallHook, StoreContextMut, ValRaw};
use alloc::sync::Arc;
use core::any::Any;
use core::future::Future;
use core::mem::{self, MaybeUninit};
use core::pin::Pin;
use core::ptr::NonNull;
use wasmtime_environ::component::{
    CanonicalAbiInfo, ComponentTypes, InterfaceType, MAX_FLAT_ASYNC_PARAMS, MAX_FLAT_PARAMS,
    MAX_FLAT_RESULTS, OptionsIndex, TypeFuncIndex, TypeTuple,
};

pub struct HostFunc {
    entrypoint: VMLoweringCallee,
    typecheck: Box<dyn (Fn(TypeFuncIndex, &InstanceType<'_>) -> Result<()>) + Send + Sync>,
    func: Box<dyn Any + Send + Sync>,
}

impl core::fmt::Debug for HostFunc {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_struct("HostFunc").finish_non_exhaustive()
    }
}

enum HostResult<T> {
    Done(Result<T>),
    #[cfg(feature = "component-model-async")]
    Future(Pin<Box<dyn Future<Output = Result<T>> + Send>>),
}

trait FunctionStyle {
    const ASYNC: bool;
}

struct SyncStyle;

impl FunctionStyle for SyncStyle {
    const ASYNC: bool = false;
}

#[cfg(feature = "component-model-async")]
struct AsyncStyle;

#[cfg(feature = "component-model-async")]
impl FunctionStyle for AsyncStyle {
    const ASYNC: bool = true;
}

impl HostFunc {
    fn from_canonical<T, F, P, R, S>(func: F) -> Arc<HostFunc>
    where
        F: Fn(StoreContextMut<'_, T>, P) -> HostResult<R> + Send + Sync + 'static,
        P: ComponentNamedList + Lift + 'static,
        R: ComponentNamedList + Lower + 'static,
        T: 'static,
        S: FunctionStyle + 'static,
    {
        let entrypoint = Self::entrypoint::<T, F, P, R, S>;
        Arc::new(HostFunc {
            entrypoint,
            typecheck: Box::new(typecheck::<P, R, S>),
            func: Box::new(func),
        })
    }

    pub(crate) fn from_closure<T, F, P, R>(func: F) -> Arc<HostFunc>
    where
        T: 'static,
        F: Fn(StoreContextMut<T>, P) -> Result<R> + Send + Sync + 'static,
        P: ComponentNamedList + Lift + 'static,
        R: ComponentNamedList + Lower + 'static,
    {
        Self::from_canonical::<T, _, _, _, SyncStyle>(move |store, params| {
            HostResult::Done(func(store, params))
        })
    }

    #[cfg(feature = "component-model-async")]
    pub(crate) fn from_concurrent<T, F, P, R>(func: F) -> Arc<HostFunc>
    where
        T: 'static,
        F: Fn(&Accessor<T>, P) -> Pin<Box<dyn Future<Output = Result<R>> + Send + '_>>
            + Send
            + Sync
            + 'static,
        P: ComponentNamedList + Lift + 'static,
        R: ComponentNamedList + Lower + 'static,
    {
        let func = Arc::new(func);
        Self::from_canonical::<T, _, _, _, AsyncStyle>(move |store, params| {
            let func = func.clone();
            HostResult::Future(Box::pin(
                store.wrap_call(move |accessor| func(accessor, params)),
            ))
        })
    }

    extern "C" fn entrypoint<T, F, P, R, S>(
        cx: NonNull<VMOpaqueContext>,
        data: NonNull<u8>,
        ty: u32,
        options: u32,
        storage: NonNull<MaybeUninit<ValRaw>>,
        storage_len: usize,
    ) -> bool
    where
        F: Fn(StoreContextMut<'_, T>, P) -> HostResult<R> + Send + Sync + 'static,
        P: ComponentNamedList + Lift,
        R: ComponentNamedList + Lower + 'static,
        T: 'static,
        S: FunctionStyle,
    {
        let data = SendSyncPtr::new(NonNull::new(data.as_ptr() as *mut F).unwrap());
        unsafe {
            call_host_and_handle_result::<T>(cx, |store, instance| {
                call_host::<_, _, _, _, S>(
                    store,
                    instance,
                    TypeFuncIndex::from_u32(ty),
                    OptionsIndex::from_u32(options),
                    NonNull::slice_from_raw_parts(storage, storage_len).as_mut(),
                    move |store, args| (*data.as_ptr())(store, args),
                )
            })
        }
    }

    fn new_dynamic_canonical<T, F, S>(func: F) -> Arc<HostFunc>
    where
        F: Fn(
                StoreContextMut<'_, T>,
                ComponentFunc,
                Vec<Val>,
                usize,
            ) -> Pin<Box<dyn Future<Output = Result<Vec<Val>>> + Send + 'static>>
            + Send
            + Sync
            + 'static,
        T: 'static,
        S: FunctionStyle,
    {
        Arc::new(HostFunc {
            entrypoint: dynamic_entrypoint::<T, F, S>,
            // This function performs dynamic type checks on its parameters and
            // results and subsequently does not need to perform up-front type
            // checks. However, we _do_ verify async-ness here.
            typecheck: Box::new(move |ty, types| {
                let ty = &types.types[ty];
                if S::ASYNC != ty.async_ {
                    bail!("type mismatch with async");
                }

                Ok(())
            }),
            func: Box::new(func),
        })
    }

    pub(crate) fn new_dynamic<T: 'static, F>(func: F) -> Arc<HostFunc>
    where
        F: Fn(StoreContextMut<'_, T>, ComponentFunc, &[Val], &mut [Val]) -> Result<()>
            + Send
            + Sync
            + 'static,
    {
        Self::new_dynamic_canonical::<T, _, SyncStyle>(
            move |store, ty, mut params_and_results, result_start| {
                let (params, results) = params_and_results.split_at_mut(result_start);
                let result = func(store, ty, params, results).map(move |()| params_and_results);
                Box::pin(async move { result })
            },
        )
    }

    #[cfg(feature = "component-model-async")]
    pub(crate) fn new_dynamic_concurrent<T, F>(func: F) -> Arc<HostFunc>
    where
        T: 'static,
        F: for<'a> Fn(
                &'a Accessor<T>,
                ComponentFunc,
                &'a [Val],
                &'a mut [Val],
            ) -> Pin<Box<dyn Future<Output = Result<()>> + Send + 'a>>
            + Send
            + Sync
            + 'static,
    {
        let func = Arc::new(func);
        Self::new_dynamic_canonical::<T, _, AsyncStyle>(
            move |store, ty, mut params_and_results, result_start| {
                let func = func.clone();
                Box::pin(store.wrap_call(move |accessor| {
                    Box::pin(async move {
                        let (params, results) = params_and_results.split_at_mut(result_start);
                        func(accessor, ty, params, results).await?;
                        Ok(params_and_results)
                    })
                }))
            },
        )
    }

    pub fn typecheck(&self, ty: TypeFuncIndex, types: &InstanceType<'_>) -> Result<()> {
        (self.typecheck)(ty, types)
    }

    pub fn lowering(&self) -> VMLowering {
        let data = NonNull::from(&*self.func).cast();
        VMLowering {
            callee: NonNull::new(self.entrypoint as *mut _).unwrap().into(),
            data: data.into(),
        }
    }
}

fn typecheck<P, R, S>(ty: TypeFuncIndex, types: &InstanceType<'_>) -> Result<()>
where
    P: ComponentNamedList + Lift,
    R: ComponentNamedList + Lower,
    S: FunctionStyle,
{
    let ty = &types.types[ty];
    if S::ASYNC != ty.async_ {
        bail!("type mismatch with async");
    }
    P::typecheck(&InterfaceType::Tuple(ty.params), types)
        .context("type mismatch with parameters")?;
    R::typecheck(&InterfaceType::Tuple(ty.results), types).context("type mismatch with results")?;
    Ok(())
}

/// The "meat" of calling a host function from wasm.
///
/// This function is delegated to from implementations of
/// `HostFunc::from_closure`. Most of the arguments from the `entrypoint` are
/// forwarded here except for the `data` pointer which is encapsulated in the
/// `closure` argument here.
///
/// This function is parameterized over:
///
/// * `T` - the type of store this function works with (an unsafe assertion)
/// * `Params` - the parameters to the host function, viewed as a tuple
/// * `Return` - the result of the host function
/// * `F` - the `closure` to actually receive the `Params` and return the
///   `Return`
/// * `S` - the expected `FunctionStyle`
///
/// It's expected that `F` will "un-tuple" the arguments to pass to a host
/// closure.
///
/// This function is in general `unsafe` as the validity of all the parameters
/// must be upheld. Generally that's done by ensuring this is only called from
/// the select few places it's intended to be called from.
unsafe fn call_host<T, Params, Return, F, S>(
    store: StoreContextMut<'_, T>,
    instance: Instance,
    ty: TypeFuncIndex,
    options: OptionsIndex,
    storage: &mut [MaybeUninit<ValRaw>],
    closure: F,
) -> Result<()>
where
    F: Fn(StoreContextMut<'_, T>, Params) -> HostResult<Return> + Send + Sync + 'static,
    Params: Lift,
    Return: Lower + 'static,
    S: FunctionStyle,
{
    let (component, store) = instance.component_and_store_mut(store.0);
    let mut store = StoreContextMut(store);
    let vminstance = instance.id().get(store.0);
    let opts = &vminstance.component().env_component().options[options];
    let async_lower = opts.async_;
    let caller_instance = opts.instance;
    let mut flags = vminstance.instance_flags(caller_instance);

    // Perform a dynamic check that this instance can indeed be left. Exiting
    // the component is disallowed, for example, when the `realloc` function
    // calls a canonical import.
    if unsafe { !flags.may_leave() } {
        return Err(anyhow!(crate::Trap::CannotLeaveComponent));
    }

    let types = component.types();
    let ty = &types[ty];
    let param_tys = InterfaceType::Tuple(ty.params);
    let result_tys = InterfaceType::Tuple(ty.results);

    if async_lower {
        #[cfg(feature = "component-model-async")]
        {
            let mut storage = unsafe { Storage::<'_, Params, u32>::new_async::<Return>(storage) };

            // Lift the parameters, either from flat storage or from linear
            // memory.
            let lift = &mut LiftContext::new(store.0.store_opaque_mut(), options, instance);
            lift.enter_call();
            let params = storage.lift_params(lift, param_tys)?;

            // Load the return pointer, if present.
            let retptr = match storage.async_retptr() {
                Some(ptr) => {
                    let mut lower = LowerContext::new(store.as_context_mut(), options, instance);
                    validate_inbounds::<Return>(lower.as_slice_mut(), ptr)?
                }
                // If there's no return pointer then `Return` should have an
                // empty flat representation. In this situation pretend the
                // return pointer was 0 so we have something to shepherd along
                // into the closure below.
                None => {
                    assert_eq!(Return::flatten_count(), 0);
                    0
                }
            };

            let host_result = closure(store.as_context_mut(), params);

            let mut lower_result = {
                move |store: StoreContextMut<T>, ret: Return| {
                    unsafe {
                        flags.set_may_leave(false);
                    }
                    let mut lower = LowerContext::new(store, options, instance);
                    ret.linear_lower_to_memory(&mut lower, result_tys, retptr)?;
                    unsafe {
                        flags.set_may_leave(true);
                    }
                    lower.exit_call()?;
                    Ok(())
                }
            };
            let task = match host_result {
                HostResult::Done(result) => {
                    lower_result(store.as_context_mut(), result?)?;
                    None
                }
                #[cfg(feature = "component-model-async")]
                HostResult::Future(future) => instance.first_poll(
                    store.as_context_mut(),
                    future,
                    caller_instance,
                    lower_result,
                )?,
            };

            let status = if let Some(task) = task {
                Status::Started.pack(Some(task))
            } else {
                Status::Returned.pack(None)
            };

            let mut lower = LowerContext::new(store, options, instance);
            storage.lower_results(&mut lower, InterfaceType::U32, status)?;
        }
        #[cfg(not(feature = "component-model-async"))]
        {
            let _ = caller_instance;
            unreachable!(
                "async-lowered imports should have failed validation \
                 when `component-model-async` feature disabled"
            );
        }
    } else {
        if S::ASYNC {
            // The caller has synchronously lowered an async function, meaning
            // the caller can only call it from an async task (i.e. a task
            // created via a call to an async export).  Otherwise, we'll trap.
            concurrent::check_blocking(store.0)?;
        }

        let mut storage = unsafe { Storage::<'_, Params, Return>::new_sync(storage) };
        let mut lift = LiftContext::new(store.0.store_opaque_mut(), options, instance);
        lift.enter_call();
        let params = storage.lift_params(&mut lift, param_tys)?;

        let ret = match closure(store.as_context_mut(), params) {
            HostResult::Done(result) => result?,
            #[cfg(feature = "component-model-async")]
            HostResult::Future(future) => {
                concurrent::poll_and_block(store.0, future, caller_instance)?
            }
        };

        unsafe {
            flags.set_may_leave(false);
        }
        let mut lower = LowerContext::new(store, options, instance);
        storage.lower_results(&mut lower, result_tys, ret)?;
        unsafe {
            flags.set_may_leave(true);
        }
        lower.exit_call()?;
    }

    return Ok(());

    /// Type-level representation of the matrix of possibilities of how
    /// WebAssembly parameters and results are handled in the canonical ABI.
    ///
    /// Wasmtime's ABI here always works with `&mut [MaybeUninit<ValRaw>]` as the
    /// base representation of params/results. Parameters are passed
    /// sequentially and results are returned by overwriting the parameters.
    /// That means both params/results start from index 0.
    ///
    /// The type-level representation here involves working with the typed
    /// `P::Lower` and `R::Lower` values which is a type-level representation of
    /// a lowered value. All lowered values are in essence a sequence of
    /// `ValRaw` values one after the other to fit within this original array
    /// that is the basis of Wasmtime's ABI.
    ///
    /// The various combinations here are cryptic, but only used in this file.
    /// This in theory cuts down on the verbosity below, but an explanation of
    /// the various acronyms here are:
    ///
    /// * Pd - params direct - means that parameters are passed directly in
    ///   their flat representation via `P::Lower`.
    ///
    /// * Pi - params indirect - means that parameters are passed indirectly in
    ///   linear memory and the argument here is `ValRaw` to store the pointer.
    ///
    /// * Rd - results direct - means that results are returned directly in
    ///   their flat representation via `R::Lower`. Note that this is always
    ///   represented as `MaybeUninit<R::Lower>` as well because the return
    ///   values may point to uninitialized memory if there were no parameters
    ///   for example.
    ///
    /// * Ri - results indirect - means that results are returned indirectly in
    ///   linear memory through the pointer specified. Note that this is
    ///   specified as a `ValRaw` to represent the argument that's being given
    ///   to the host from WebAssembly.
    ///
    /// * Ar - async results - means that the parameters to this call
    ///   additionally include an async result pointer. Async results are always
    ///   transmitted via a pointer so this is always a `ValRaw`.
    ///
    /// Internally this type makes liberal use of `Union` and `Pair` helpers
    /// below which are simple `#[repr(C)]` wrappers around a pair of types that
    /// are a union or a pair.
    ///
    /// Note that for any combination of `P` and `R` this `enum` is actually
    /// pointless as a single variant will be used. In theory we should be able
    /// to monomorphize based on `P` and `R` to a specific type. This
    /// monomorphization depends on conditionals like `flatten_count() <= N`,
    /// however, and I don't know how to encode that in Rust easily. In lieu of
    /// that we assume LLVM will figure things out and boil away the actual enum
    /// and runtime dispatch.
    enum Storage<'a, P: ComponentType, R: ComponentType> {
        /// Params: direct, Results: direct
        ///
        /// The lowered representation of params/results are overlaid on top of
        /// each other.
        PdRd(&'a mut Union<P::Lower, MaybeUninit<R::Lower>>),

        /// Params: direct, Results: indirect
        ///
        /// The return pointer comes after the params so this is sequentially
        /// laid out with one after the other.
        PdRi(&'a Pair<P::Lower, ValRaw>),

        /// Params: indirect, Results: direct
        ///
        /// Here the return values are overlaid on top of the pointer parameter.
        PiRd(&'a mut Union<ValRaw, MaybeUninit<R::Lower>>),

        /// Params: indirect, Results: indirect
        ///
        /// Here the two parameters are laid out sequentially one after the
        /// other.
        PiRi(&'a Pair<ValRaw, ValRaw>),

        /// Params: direct + async result, Results: direct
        ///
        /// This is like `PdRd` except that the parameters additionally include
        /// a pointer for where to store the result.
        #[cfg(feature = "component-model-async")]
        PdArRd(&'a mut Union<Pair<P::Lower, ValRaw>, MaybeUninit<R::Lower>>),

        /// Params: indirect + async result, Results: direct
        ///
        /// This is like `PiRd` except that the parameters additionally include
        /// a pointer for where to store the result.
        #[cfg(feature = "component-model-async")]
        PiArRd(&'a mut Union<Pair<ValRaw, ValRaw>, MaybeUninit<R::Lower>>),
    }

    // Helper structure used above in `Storage` to represent two consecutive
    // values.
    #[repr(C)]
    #[derive(Copy, Clone)]
    struct Pair<T, U> {
        a: T,
        b: U,
    }

    // Helper structure used above in `Storage` to represent two values overlaid
    // on each other.
    #[repr(C)]
    union Union<T: Copy, U: Copy> {
        a: T,
        b: U,
    }

    /// Representation of where parameters are lifted from.
    enum Src<'a, T> {
        /// Parameters are directly lifted from `T`, which is under the hood a
        /// sequence of `ValRaw`. This is `P::Lower` for example.
        Direct(&'a T),

        /// Parameters are loaded from linear memory, and this is the wasm
        /// parameter representing the pointer into linear memory to load from.
        Indirect(&'a ValRaw),
    }

    /// Dual of [`Src`], where to store results.
    enum Dst<'a, T> {
        /// Results are stored directly in this pointer.
        ///
        /// Note that this is a mutable pointer but it's specifically
        /// `MaybeUninit` as trampolines do not initialize it. The `T` here will
        /// be `R::Lower` for example.
        Direct(&'a mut MaybeUninit<T>),

        /// Results are stored in linear memory, and this value is the wasm
        /// parameter given which represents the pointer into linear memory.
        ///
        /// Note that this is not mutable as the parameter is not mutated, but
        /// memory will be mutated.
        Indirect(&'a ValRaw),
    }

    impl<P, R> Storage<'_, P, R>
    where
        P: ComponentType + Lift,
        R: ComponentType + Lower,
    {
        /// Classifies a new `Storage` suitable for use with sync functions.
        ///
        /// There's a 2x2 matrix of whether parameters and results are stored on the
        /// stack or on the heap. Each of the 4 branches here have a different
        /// representation of the storage of arguments/returns.
        ///
        /// Also note that while four branches are listed here only one is taken for
        /// any particular `Params` and `Return` combination. This should be
        /// trivially DCE'd by LLVM. Perhaps one day with enough const programming in
        /// Rust we can make monomorphizations of this function codegen only one
        /// branch, but today is not that day.
        ///
        /// # Safety
        ///
        /// Requires that the `storage` provided does indeed match an wasm
        /// function with the signature of `P` and `R` as params/results.
        unsafe fn new_sync(storage: &mut [MaybeUninit<ValRaw>]) -> Storage<'_, P, R> {
            // SAFETY: this `unsafe` is due to the `slice_to_storage_*` helpers
            // used which view the slice provided as a different type. This
            // safety should be upheld by the contract of the `ComponentType`
            // trait and its `Lower` type parameter meaning they're valid to
            // view as a sequence of `ValRaw` types. Additionally the
            // `ComponentType` trait ensures that the matching of the runtime
            // length of `storage` should match the actual size of `P::Lower`
            // and `R::Lower` or such as needed.
            unsafe {
                if P::flatten_count() <= MAX_FLAT_PARAMS {
                    if R::flatten_count() <= MAX_FLAT_RESULTS {
                        Storage::PdRd(slice_to_storage_mut(storage).assume_init_mut())
                    } else {
                        Storage::PdRi(slice_to_storage_mut(storage).assume_init_ref())
                    }
                } else {
                    if R::flatten_count() <= MAX_FLAT_RESULTS {
                        Storage::PiRd(slice_to_storage_mut(storage).assume_init_mut())
                    } else {
                        Storage::PiRi(slice_to_storage_mut(storage).assume_init_ref())
                    }
                }
            }
        }

        fn lift_params(&self, cx: &mut LiftContext<'_>, ty: InterfaceType) -> Result<P> {
            match self.lift_src() {
                Src::Direct(storage) => P::linear_lift_from_flat(cx, ty, storage),
                Src::Indirect(ptr) => {
                    let ptr = validate_inbounds::<P>(cx.memory(), ptr)?;
                    P::linear_lift_from_memory(cx, ty, &cx.memory()[ptr..][..P::SIZE32])
                }
            }
        }

        fn lift_src(&self) -> Src<'_, P::Lower> {
            match self {
                // SAFETY: these `unsafe` blocks are due to accessing union
                // fields. The safety here relies on the contract of the
                // `ComponentType` trait which should ensure that the types
                // projected onto a list of wasm parameters are indeed correct.
                // That means that the projections here, if the types are
                // correct, all line up to initialized memory that's well-typed
                // to access.
                Storage::PdRd(storage) => unsafe { Src::Direct(&storage.a) },
                Storage::PdRi(storage) => Src::Direct(&storage.a),
                #[cfg(feature = "component-model-async")]
                Storage::PdArRd(storage) => unsafe { Src::Direct(&storage.a.a) },
                Storage::PiRd(storage) => unsafe { Src::Indirect(&storage.a) },
                Storage::PiRi(storage) => Src::Indirect(&storage.a),
                #[cfg(feature = "component-model-async")]
                Storage::PiArRd(storage) => unsafe { Src::Indirect(&storage.a.a) },
            }
        }

        fn lower_results<T>(
            &mut self,
            cx: &mut LowerContext<'_, T>,
            ty: InterfaceType,
            ret: R,
        ) -> Result<()> {
            match self.lower_dst() {
                Dst::Direct(storage) => ret.linear_lower_to_flat(cx, ty, storage),
                Dst::Indirect(ptr) => {
                    let ptr = validate_inbounds::<R>(cx.as_slice_mut(), ptr)?;
                    ret.linear_lower_to_memory(cx, ty, ptr)
                }
            }
        }

        fn lower_dst(&mut self) -> Dst<'_, R::Lower> {
            match self {
                // SAFETY: these unsafe blocks are due to accessing fields of a
                // `union` which is not safe in Rust. The returned value is
                // `MaybeUninit<R::Lower>` in all cases, however, which should
                // safely model how `union` memory is possibly uninitialized.
                // Additionally `R::Lower` has the `unsafe` contract that all
                // its bit patterns must be sound, which additionally should
                // help make this safe.
                Storage::PdRd(storage) => unsafe { Dst::Direct(&mut storage.b) },
                Storage::PiRd(storage) => unsafe { Dst::Direct(&mut storage.b) },
                #[cfg(feature = "component-model-async")]
                Storage::PdArRd(storage) => unsafe { Dst::Direct(&mut storage.b) },
                #[cfg(feature = "component-model-async")]
                Storage::PiArRd(storage) => unsafe { Dst::Direct(&mut storage.b) },
                Storage::PdRi(storage) => Dst::Indirect(&storage.b),
                Storage::PiRi(storage) => Dst::Indirect(&storage.b),
            }
        }

        #[cfg(feature = "component-model-async")]
        fn async_retptr(&self) -> Option<&ValRaw> {
            match self {
                // SAFETY: like above these are `unsafe` due to accessing a
                // `union` field. This should be safe via the construction of
                // `Storage` which should correctly determine whether or not an
                // async return pointer is provided and classify the args/rets
                // appropriately.
                Storage::PdArRd(storage) => unsafe { Some(&storage.a.b) },
                Storage::PiArRd(storage) => unsafe { Some(&storage.a.b) },
                Storage::PdRd(_) | Storage::PiRd(_) | Storage::PdRi(_) | Storage::PiRi(_) => None,
            }
        }
    }

    #[cfg(feature = "component-model-async")]
    impl<P> Storage<'_, P, u32>
    where
        P: ComponentType + Lift,
    {
        /// Classifies a new `Storage` suitable for use with async functions.
        ///
        /// # Safety
        ///
        /// Requires that the `storage` provided does indeed match an `async`
        /// wasm function with the signature of `P` and `R` as params/results.
        unsafe fn new_async<R>(storage: &mut [MaybeUninit<ValRaw>]) -> Storage<'_, P, u32>
        where
            R: ComponentType + Lower,
        {
            // SAFETY: see `Storage::new` for discussion on why this should be
            // safe given the unsafe contract of the `ComponentType` trait.
            unsafe {
                if P::flatten_count() <= wasmtime_environ::component::MAX_FLAT_ASYNC_PARAMS {
                    if R::flatten_count() == 0 {
                        Storage::PdRd(slice_to_storage_mut(storage).assume_init_mut())
                    } else {
                        Storage::PdArRd(slice_to_storage_mut(storage).assume_init_mut())
                    }
                } else {
                    if R::flatten_count() == 0 {
                        Storage::PiRd(slice_to_storage_mut(storage).assume_init_mut())
                    } else {
                        Storage::PiArRd(slice_to_storage_mut(storage).assume_init_mut())
                    }
                }
            }
        }
    }
}

pub(crate) fn validate_inbounds<T: ComponentType>(memory: &[u8], ptr: &ValRaw) -> Result<usize> {
    // FIXME(#4311): needs memory64 support
    let ptr = usize::try_from(ptr.get_u32())?;
    if ptr % usize::try_from(T::ALIGN32)? != 0 {
        bail!("pointer not aligned");
    }
    let end = match ptr.checked_add(T::SIZE32) {
        Some(n) => n,
        None => bail!("pointer size overflow"),
    };
    if end > memory.len() {
        bail!("pointer out of bounds")
    }
    Ok(ptr)
}

unsafe fn call_host_and_handle_result<T>(
    cx: NonNull<VMOpaqueContext>,
    func: impl FnOnce(StoreContextMut<'_, T>, Instance) -> Result<()>,
) -> bool
where
    T: 'static,
{
    let cx = unsafe { VMComponentContext::from_opaque(cx) };
    unsafe {
        ComponentInstance::enter_host_from_wasm(cx, |store, instance| {
            let mut store = store.unchecked_context_mut();
            store.0.call_hook(CallHook::CallingHost)?;
            let res = func(store.as_context_mut(), instance);
            store.0.call_hook(CallHook::ReturningFromHost)?;
            res
        })
    }
}

unsafe fn call_host_dynamic<T, F, S>(
    store: StoreContextMut<'_, T>,
    instance: Instance,
    ty: TypeFuncIndex,
    options: OptionsIndex,
    storage: &mut [MaybeUninit<ValRaw>],
    closure: F,
) -> Result<()>
where
    F: Fn(
            StoreContextMut<'_, T>,
            ComponentFunc,
            Vec<Val>,
            usize,
        ) -> Pin<Box<dyn Future<Output = Result<Vec<Val>>> + Send + 'static>>
        + Send
        + Sync
        + 'static,
    T: 'static,
    S: FunctionStyle,
{
    let (component, store) = instance.component_and_store_mut(store.0);
    let mut store = StoreContextMut(store);
    let vminstance = instance.id().get(store.0);
    let opts = &component.env_component().options[options];
    let async_lower = opts.async_;
    let caller_instance = opts.instance;
    let mut flags = vminstance.instance_flags(caller_instance);

    // Perform a dynamic check that this instance can indeed be left. Exiting
    // the component is disallowed, for example, when the `realloc` function
    // calls a canonical import.
    if unsafe { !flags.may_leave() } {
        return Err(anyhow!(crate::Trap::CannotLeaveComponent));
    }

    let types = component.types();
    let func_ty = &types[ty];
    let param_tys = &types[func_ty.params];
    let result_tys = &types[func_ty.results];

    let mut params_and_results = Vec::new();
    let mut lift = &mut LiftContext::new(store.0.store_opaque_mut(), options, instance);
    lift.enter_call();
    let max_flat = if async_lower {
        MAX_FLAT_ASYNC_PARAMS
    } else {
        MAX_FLAT_PARAMS
    };
    let ty = ComponentFunc::from(ty, &lift.instance_type());

    let ret_index = unsafe {
        dynamic_params_load(
            &mut lift,
            types,
            storage,
            param_tys,
            &mut params_and_results,
            max_flat,
        )?
    };
    let result_start = params_and_results.len();
    for _ in 0..result_tys.types.len() {
        params_and_results.push(Val::Bool(false));
    }

    if async_lower {
        #[cfg(feature = "component-model-async")]
        {
            let retptr = if result_tys.types.len() == 0 {
                0
            } else {
                let retptr = unsafe { storage[ret_index].assume_init() };
                let mut lower = LowerContext::new(store.as_context_mut(), options, instance);
                validate_inbounds_dynamic(&result_tys.abi, lower.as_slice_mut(), &retptr)?
            };

            let future = closure(store.as_context_mut(), ty, params_and_results, result_start);

            let task = instance.first_poll(store, future, caller_instance, {
                let result_tys = func_ty.results;
                move |store: StoreContextMut<T>, result_vals: Vec<Val>| {
                    unsafe {
                        flags.set_may_leave(false);
                    }

                    let mut lower = LowerContext::new(store, options, instance);
                    let result_tys = &lower.types[result_tys];
                    let result_vals = &result_vals[result_start..];
                    assert_eq!(result_vals.len(), result_tys.types.len());
                    let mut ptr = retptr;
                    for (val, ty) in result_vals.iter().zip(result_tys.types.iter()) {
                        let offset = lower.types.canonical_abi(ty).next_field32_size(&mut ptr);
                        val.store(&mut lower, *ty, offset)?;
                    }

                    unsafe {
                        flags.set_may_leave(true);
                    }

                    lower.exit_call()?;

                    Ok(())
                }
            })?;

            let status = if let Some(task) = task {
                Status::Started.pack(Some(task))
            } else {
                Status::Returned.pack(None)
            };

            storage[0] = MaybeUninit::new(ValRaw::i32(status as i32));
        }
        #[cfg(not(feature = "component-model-async"))]
        {
            unreachable!(
                "async-lowered imports should have failed validation \
                 when `component-model-async` feature disabled"
            );
        }
    } else {
        if S::ASYNC {
            // The caller has synchronously lowered an async function, meaning
            // the caller can only call it from an async task (i.e. a task
            // created via a call to an async export).  Otherwise, we'll trap.
            concurrent::check_blocking(store.0)?;
        }

        let future = closure(store.as_context_mut(), ty, params_and_results, result_start);
        let result_vals = concurrent::poll_and_block(store.0, future, caller_instance)?;
        let result_vals = &result_vals[result_start..];

        unsafe {
            flags.set_may_leave(false);
        }

        let mut cx = LowerContext::new(store, options, instance);
        if let Some(cnt) = result_tys.abi.flat_count(MAX_FLAT_RESULTS) {
            let mut dst = storage[..cnt].iter_mut();
            for (val, ty) in result_vals.iter().zip(result_tys.types.iter()) {
                val.lower(&mut cx, *ty, &mut dst)?;
            }
            assert!(dst.next().is_none());
        } else {
            let ret_ptr = unsafe { storage[ret_index].assume_init_ref() };
            let mut ptr = validate_inbounds_dynamic(&result_tys.abi, cx.as_slice_mut(), ret_ptr)?;
            for (val, ty) in result_vals.iter().zip(result_tys.types.iter()) {
                let offset = types.canonical_abi(ty).next_field32_size(&mut ptr);
                val.store(&mut cx, *ty, offset)?;
            }
        }

        unsafe {
            flags.set_may_leave(true);
        }

        cx.exit_call()?;
    }

    Ok(())
}

/// Loads the parameters for a dynamic host function call into `params`
///
/// Returns the number of flat `storage` values consumed.
///
/// # Safety
///
/// Requires that `param_tys` matches the type signature of the `storage` that
/// was passed in.
unsafe fn dynamic_params_load(
    cx: &mut LiftContext<'_>,
    types: &ComponentTypes,
    storage: &[MaybeUninit<ValRaw>],
    param_tys: &TypeTuple,
    params: &mut Vec<Val>,
    max_flat_params: usize,
) -> Result<usize> {
    if let Some(param_count) = param_tys.abi.flat_count(max_flat_params) {
        // NB: can use `MaybeUninit::slice_assume_init_ref` when that's stable
        let storage =
            unsafe { mem::transmute::<&[MaybeUninit<ValRaw>], &[ValRaw]>(&storage[..param_count]) };
        let mut iter = storage.iter();
        for ty in param_tys.types.iter() {
            params.push(Val::lift(cx, *ty, &mut iter)?);
        }
        assert!(iter.next().is_none());
        Ok(param_count)
    } else {
        let mut offset = validate_inbounds_dynamic(&param_tys.abi, cx.memory(), unsafe {
            storage[0].assume_init_ref()
        })?;
        for ty in param_tys.types.iter() {
            let abi = types.canonical_abi(ty);
            let size = usize::try_from(abi.size32).unwrap();
            let memory = &cx.memory()[abi.next_field32_size(&mut offset)..][..size];
            params.push(Val::load(cx, *ty, memory)?);
        }
        Ok(1)
    }
}

pub(crate) fn validate_inbounds_dynamic(
    abi: &CanonicalAbiInfo,
    memory: &[u8],
    ptr: &ValRaw,
) -> Result<usize> {
    // FIXME(#4311): needs memory64 support
    let ptr = usize::try_from(ptr.get_u32())?;
    if ptr % usize::try_from(abi.align32)? != 0 {
        bail!("pointer not aligned");
    }
    let end = match ptr.checked_add(usize::try_from(abi.size32).unwrap()) {
        Some(n) => n,
        None => bail!("pointer size overflow"),
    };
    if end > memory.len() {
        bail!("pointer out of bounds")
    }
    Ok(ptr)
}

extern "C" fn dynamic_entrypoint<T, F, S>(
    cx: NonNull<VMOpaqueContext>,
    data: NonNull<u8>,
    ty: u32,
    options: u32,
    storage: NonNull<MaybeUninit<ValRaw>>,
    storage_len: usize,
) -> bool
where
    F: Fn(
            StoreContextMut<'_, T>,
            ComponentFunc,
            Vec<Val>,
            usize,
        ) -> Pin<Box<dyn Future<Output = Result<Vec<Val>>> + Send + 'static>>
        + Send
        + Sync
        + 'static,
    T: 'static,
    S: FunctionStyle,
{
    let data = SendSyncPtr::new(NonNull::new(data.as_ptr() as *mut F).unwrap());
    unsafe {
        call_host_and_handle_result(cx, |store, instance| {
            call_host_dynamic::<T, _, S>(
                store,
                instance,
                TypeFuncIndex::from_u32(ty),
                OptionsIndex::from_u32(options),
                NonNull::slice_from_raw_parts(storage, storage_len).as_mut(),
                &*data.as_ptr(),
            )
        })
    }
}