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//! Implementation of `externref` in Wasmtime.
use crate::prelude::*;
use crate::runtime::vm::VMGcRef;
use crate::{
store::{AutoAssertNoGc, StoreOpaque},
AsContextMut, GcHeapOutOfMemory, GcRefImpl, GcRootIndex, HeapType, ManuallyRooted, RefType,
Result, Rooted, StoreContext, StoreContextMut, ValRaw, ValType, WasmTy,
};
use core::any::Any;
use core::mem;
use core::mem::MaybeUninit;
/// An opaque, GC-managed reference to some host data that can be passed to
/// WebAssembly.
///
/// The `ExternRef` type represents WebAssembly `externref` values. Wasm can't
/// do anything with the `externref`s other than put them in tables, globals,
/// and locals or pass them to other functions (such as imported functions from
/// the host). Unlike `anyref`s, Wasm guests cannot directly allocate new
/// `externref`s; only the host can.
///
/// You can use `ExternRef` to give access to host objects and control the
/// operations that Wasm can perform on them via what functions you allow Wasm
/// to import.
///
/// Like all WebAssembly references, these are opaque and unforgeable to Wasm:
/// they cannot be faked and Wasm cannot, for example, cast the integer
/// `0x12345678` into a reference, pretend it is a valid `externref`, and trick
/// the host into dereferencing it and segfaulting or worse.
///
/// Note that you can also use `Rooted<ExternRef>` and
/// `ManuallyRooted<ExternRef>` as a type parameter with
/// [`Func::typed`][crate::Func::typed]- and
/// [`Func::wrap`][crate::Func::wrap]-style APIs.
///
/// # Example
///
/// ```
/// # use wasmtime::*;
/// # use std::borrow::Cow;
/// # fn _foo() -> Result<()> {
/// let engine = Engine::default();
/// let mut store = Store::new(&engine, ());
///
/// // Define some APIs for working with host strings from Wasm via `externref`.
/// let mut linker = Linker::new(&engine);
/// linker.func_wrap(
/// "host-string",
/// "new",
/// |caller: Caller<'_, ()>| -> Result<Rooted<ExternRef>> {
/// ExternRef::new(caller, Cow::from(""))
/// },
/// )?;
/// linker.func_wrap(
/// "host-string",
/// "concat",
/// |mut caller: Caller<'_, ()>, a: Rooted<ExternRef>, b: Rooted<ExternRef>| -> Result<Rooted<ExternRef>> {
/// let mut s = a
/// .data(&caller)?
/// .downcast_ref::<Cow<str>>()
/// .ok_or_else(|| Error::msg("externref was not a string"))?
/// .clone()
/// .into_owned();
/// let b = b
/// .data(&caller)?
/// .downcast_ref::<Cow<str>>()
/// .ok_or_else(|| Error::msg("externref was not a string"))?;
/// s.push_str(&b);
/// ExternRef::new(&mut caller, s)
/// },
/// )?;
///
/// // Here is a Wasm module that uses those APIs.
/// let module = Module::new(
/// &engine,
/// r#"
/// (module
/// (import "host-string" "concat" (func $concat (param externref externref)
/// (result externref)))
/// (func (export "run") (param externref externref) (result externref)
/// local.get 0
/// local.get 1
/// call $concat
/// )
/// )
/// "#,
/// )?;
///
/// // Create a couple `externref`s wrapping `Cow<str>`s.
/// let hello = ExternRef::new(&mut store, Cow::from("Hello, "))?;
/// let world = ExternRef::new(&mut store, Cow::from("World!"))?;
///
/// // Instantiate the module and pass the `externref`s into it.
/// let instance = linker.instantiate(&mut store, &module)?;
/// let result = instance
/// .get_typed_func::<(Rooted<ExternRef>, Rooted<ExternRef>), Rooted<ExternRef>>(&mut store, "run")?
/// .call(&mut store, (hello, world))?;
///
/// // The module should have concatenated the strings together!
/// assert_eq!(
/// result.data(&store)?.downcast_ref::<Cow<str>>().unwrap(),
/// "Hello, World!"
/// );
/// # Ok(())
/// # }
/// ```
#[derive(Debug, Clone)]
#[repr(transparent)]
pub struct ExternRef {
pub(crate) inner: GcRootIndex,
}
unsafe impl GcRefImpl for ExternRef {
#[allow(private_interfaces)]
fn transmute_ref(index: &GcRootIndex) -> &Self {
// Safety: `ExternRef` is a newtype of a `GcRootIndex`.
let me: &Self = unsafe { mem::transmute(index) };
// Assert we really are just a newtype of a `GcRootIndex`.
assert!(matches!(
me,
Self {
inner: GcRootIndex { .. },
}
));
me
}
}
impl ExternRef {
/// Creates a new instance of `ExternRef` wrapping the given value.
///
/// The resulting value is automatically unrooted when the given `context`'s
/// scope is exited. See [`Rooted<T>`][crate::Rooted]'s documentation for
/// more details.
///
/// This method will *not* automatically trigger a GC to free up space in
/// the GC heap; instead it will return an error. This gives you more
/// precise control over when collections happen and allows you to choose
/// between performing synchronous and asynchronous collections.
///
/// # Errors
///
/// If the allocation cannot be satisfied because the GC heap is currently
/// out of memory, but performing a garbage collection might free up space
/// such that retrying the allocation afterwards might succeed, then a
/// `GcHeapOutOfMemory<T>` error is returned.
///
/// The `GcHeapOutOfMemory<T>` error contains the host value that the
/// `externref` would have wrapped. You can extract that value from this
/// error and reuse it when attempting to allocate an `externref` again
/// after GC or otherwise do with it whatever you see fit.
///
/// # Example
///
/// ```
/// # use wasmtime::*;
/// # fn _foo() -> Result<()> {
/// let mut store = Store::<()>::default();
///
/// {
/// let mut scope = RootScope::new(&mut store);
///
/// // Create an `externref` wrapping a `str`.
/// let externref = match ExternRef::new(&mut scope, "hello!") {
/// Ok(x) => x,
/// // If the heap is out of memory, then do a GC and try again.
/// Err(e) if e.is::<GcHeapOutOfMemory<&'static str>>() => {
/// // Do a GC! Note: in an async context, you'd want to do
/// // `scope.as_context_mut().gc_async().await`.
/// scope.as_context_mut().gc();
///
/// // Extract the original host value from the error.
/// let host_value = e
/// .downcast::<GcHeapOutOfMemory<&'static str>>()
/// .unwrap()
/// .into_inner();
///
/// // Try to allocate the `externref` again, now that the GC
/// // has hopefully freed up some space.
/// ExternRef::new(&mut scope, host_value)?
/// }
/// Err(e) => return Err(e),
/// };
///
/// // Use the `externref`, pass it to Wasm, etc...
/// }
///
/// // The `externref` is automatically unrooted when we exit the scope.
/// # Ok(())
/// # }
/// ```
pub fn new<T>(mut context: impl AsContextMut, value: T) -> Result<Rooted<ExternRef>>
where
T: 'static + Any + Send + Sync,
{
let ctx = context.as_context_mut().0;
let value: Box<dyn Any + Send + Sync> = Box::new(value);
let gc_ref = ctx
.gc_store_mut()?
.alloc_externref(value)
.err2anyhow()
.context("unrecoverable error when allocating new `externref`")?
.map_err(|x| GcHeapOutOfMemory::<T>::new(*x.downcast().unwrap()))
.err2anyhow()
.context("failed to allocate `externref`")?;
let mut ctx = AutoAssertNoGc::new(ctx);
Ok(Self::from_cloned_gc_ref(&mut ctx, gc_ref.into()))
}
/// Creates a new, manually-rooted instance of `ExternRef` wrapping the
/// given value.
///
/// The resulting value must be manually unrooted, or else it will leak for
/// the entire duration of the store's lifetime. See
/// [`ManuallyRooted<T>`][crate::ManuallyRooted]'s documentation for more
/// details.
///
/// # Errors
///
/// This function returns the same errors in the same scenarios as
/// [`ExternRef::new`][crate::ExternRef::new].
///
/// # Example
///
/// ```
/// # use wasmtime::*;
/// # fn _foo() -> Result<()> {
/// let mut store = Store::<()>::default();
///
/// // Create a manually-rooted `externref` wrapping a `str`.
/// let externref = ExternRef::new_manually_rooted(&mut store, "hello!")?;
///
/// // Use `externref` a bunch, pass it to Wasm, etc...
///
/// // Don't forget to explicitly unroot the `externref` when you're done
/// // using it!
/// externref.unroot(&mut store);
/// # Ok(())
/// # }
/// ```
pub fn new_manually_rooted<T>(
mut store: impl AsContextMut,
value: T,
) -> Result<ManuallyRooted<ExternRef>>
where
T: 'static + Any + Send + Sync,
{
let ctx = store.as_context_mut().0;
let value: Box<dyn Any + Send + Sync> = Box::new(value);
let gc_ref = ctx
.gc_store_mut()?
.alloc_externref(value)
.err2anyhow()
.context("unrecoverable error when allocating new `externref`")?
.map_err(|x| GcHeapOutOfMemory::<T>::new(*x.downcast().unwrap()))
.err2anyhow()
.context("failed to allocate `externref`")?;
let mut ctx = AutoAssertNoGc::new(ctx);
Ok(ManuallyRooted::new(&mut ctx, gc_ref.into()))
}
/// Create a new `Rooted<ExternRef>` from the given GC reference.
///
/// Does not invoke the `GcRuntime`'s clone hook; callers should ensure it
/// has been called.
///
/// `gc_ref` should be a GC reference pointing to an instance of `externref`
/// that is in this store's GC heap. Failure to uphold this invariant is
/// memory safe but will result in general incorrectness such as panics and
/// wrong results.
pub(crate) fn from_cloned_gc_ref(
store: &mut AutoAssertNoGc<'_>,
gc_ref: VMGcRef,
) -> Rooted<Self> {
assert!(
gc_ref.is_extern_ref(&*store.unwrap_gc_store().gc_heap),
"GC reference {gc_ref:#p} is not an externref"
);
Rooted::new(store, gc_ref)
}
/// Get a shared borrow of the underlying data for this `ExternRef`.
///
/// Returns an error if this `externref` GC reference has been unrooted (eg
/// if you attempt to use a `Rooted<ExternRef>` after exiting the scope it
/// was rooted within). See the documentation for
/// [`Rooted<T>`][crate::Rooted] for more details.
///
/// # Example
///
/// ```
/// # use wasmtime::*;
/// # fn _foo() -> Result<()> {
/// let mut store = Store::<()>::default();
///
/// let externref = ExternRef::new(&mut store, "hello")?;
///
/// // Access the `externref`'s host data.
/// let data = externref.data(&store)?;
/// // Dowcast it to a `&str`.
/// let data = data.downcast_ref::<&str>().ok_or_else(|| Error::msg("not a str"))?;
/// // We should have got the data we created the `externref` with!
/// assert_eq!(*data, "hello");
/// # Ok(())
/// # }
/// ```
pub fn data<'a, T>(
&self,
store: impl Into<StoreContext<'a, T>>,
) -> Result<&'a (dyn Any + Send + Sync)>
where
T: 'a,
{
let store = store.into().0;
let gc_ref = self.inner.try_gc_ref(&store)?;
let externref = gc_ref.as_externref_unchecked();
Ok(store.gc_store()?.externref_host_data(externref))
}
/// Get an exclusive borrow of the underlying data for this `ExternRef`.
///
/// Returns an error if this `externref` GC reference has been unrooted (eg
/// if you attempt to use a `Rooted<ExternRef>` after exiting the scope it
/// was rooted within). See the documentation for
/// [`Rooted<T>`][crate::Rooted] for more details.
///
/// # Example
///
/// ```
/// # use wasmtime::*;
/// # fn _foo() -> Result<()> {
/// let mut store = Store::<()>::default();
///
/// let externref = ExternRef::new::<usize>(&mut store, 0)?;
///
/// // Access the `externref`'s host data.
/// let data = externref.data_mut(&mut store)?;
/// // Dowcast it to a `usize`.
/// let data = data.downcast_mut::<usize>().ok_or_else(|| Error::msg("not a usize"))?;
/// // We initialized to zero.
/// assert_eq!(*data, 0);
/// // And we can mutate the value!
/// *data += 10;
/// # Ok(())
/// # }
/// ```
pub fn data_mut<'a, T>(
&self,
store: impl Into<StoreContextMut<'a, T>>,
) -> Result<&'a mut (dyn Any + Send + Sync)>
where
T: 'a,
{
let store = store.into().0;
// NB: need to do an unchecked copy to release the borrow on the store
// so that we can get the store's GC store. But importantly we cannot
// trigger a GC while we are working with `gc_ref` here.
let gc_ref = self.inner.try_gc_ref(store)?.unchecked_copy();
let externref = gc_ref.as_externref_unchecked();
Ok(store.gc_store_mut()?.externref_host_data_mut(externref))
}
/// Creates a new strongly-owned [`ExternRef`] from the raw value provided.
///
/// This is intended to be used in conjunction with [`Func::new_unchecked`],
/// [`Func::call_unchecked`], and [`ValRaw`] with its `externref` field.
///
/// This function assumes that `raw` is an externref value which is
/// currently rooted within the [`Store`].
///
/// # Unsafety
///
/// This function is particularly `unsafe` because `raw` not only must be a
/// valid externref value produced prior by `to_raw` but it must also be
/// correctly rooted within the store. When arguments are provided to a
/// callback with [`Func::new_unchecked`], for example, or returned via
/// [`Func::call_unchecked`], if a GC is performed within the store then
/// floating externref values are not rooted and will be GC'd, meaning that
/// this function will no longer be safe to call with the values cleaned up.
/// This function must be invoked *before* possible GC operations can happen
/// (such as calling wasm).
///
/// When in doubt try to not use this. Instead use the safe Rust APIs of
/// [`TypedFunc`] and friends.
///
/// [`Func::call_unchecked`]: crate::Func::call_unchecked
/// [`Func::new_unchecked`]: crate::Func::new_unchecked
/// [`Store`]: crate::Store
/// [`TypedFunc`]: crate::TypedFunc
/// [`ValRaw`]: crate::ValRaw
pub unsafe fn from_raw(mut store: impl AsContextMut, raw: u32) -> Option<Rooted<ExternRef>> {
let mut store = AutoAssertNoGc::new(store.as_context_mut().0);
Self::_from_raw(&mut store, raw)
}
// (Not actually memory unsafe since we have indexed GC heaps.)
pub(crate) fn _from_raw(store: &mut AutoAssertNoGc, raw: u32) -> Option<Rooted<ExternRef>> {
let gc_ref = VMGcRef::from_raw_u32(raw)?;
let gc_ref = store.unwrap_gc_store_mut().clone_gc_ref(&gc_ref);
Some(Self::from_cloned_gc_ref(store, gc_ref))
}
/// Converts this [`ExternRef`] to a raw value suitable to store within a
/// [`ValRaw`].
///
/// Returns an error if this `externref` has been unrooted.
///
/// # Unsafety
///
/// Produces a raw value which is only safe to pass into a store if a GC
/// doesn't happen between when the value is produce and when it's passed
/// into the store.
///
/// [`ValRaw`]: crate::ValRaw
pub unsafe fn to_raw(&self, mut store: impl AsContextMut) -> Result<u32> {
let mut store = AutoAssertNoGc::new(store.as_context_mut().0);
self._to_raw(&mut store)
}
pub(crate) fn _to_raw(&self, store: &mut AutoAssertNoGc) -> Result<u32> {
let gc_ref = self.inner.try_clone_gc_ref(store)?;
let raw = gc_ref.as_raw_u32();
store.unwrap_gc_store_mut().expose_gc_ref_to_wasm(gc_ref);
Ok(raw)
}
}
unsafe impl WasmTy for Rooted<ExternRef> {
#[inline]
fn valtype() -> ValType {
ValType::Ref(RefType::new(false, HeapType::Extern))
}
#[inline]
fn compatible_with_store(&self, store: &StoreOpaque) -> bool {
self.comes_from_same_store(store)
}
#[inline]
fn dynamic_concrete_type_check(&self, _: &StoreOpaque, _: bool, _: &HeapType) -> Result<()> {
unreachable!()
}
fn store(self, store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
self.wasm_ty_store(store, ptr, ValRaw::externref)
}
unsafe fn load(store: &mut AutoAssertNoGc<'_>, ptr: &ValRaw) -> Self {
Self::wasm_ty_load(store, ptr.get_externref(), ExternRef::from_cloned_gc_ref)
}
}
unsafe impl WasmTy for Option<Rooted<ExternRef>> {
#[inline]
fn valtype() -> ValType {
ValType::EXTERNREF
}
#[inline]
fn compatible_with_store(&self, store: &StoreOpaque) -> bool {
self.map_or(true, |x| x.comes_from_same_store(store))
}
#[inline]
fn dynamic_concrete_type_check(&self, _: &StoreOpaque, _: bool, _: &HeapType) -> Result<()> {
unreachable!()
}
#[inline]
fn is_vmgcref_and_points_to_object(&self) -> bool {
self.is_some()
}
fn store(self, store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
<Rooted<ExternRef>>::wasm_ty_option_store(self, store, ptr, ValRaw::externref)
}
unsafe fn load(store: &mut AutoAssertNoGc<'_>, ptr: &ValRaw) -> Self {
<Rooted<ExternRef>>::wasm_ty_option_load(
store,
ptr.get_externref(),
ExternRef::from_cloned_gc_ref,
)
}
}
unsafe impl WasmTy for ManuallyRooted<ExternRef> {
#[inline]
fn valtype() -> ValType {
ValType::Ref(RefType::new(false, HeapType::Extern))
}
#[inline]
fn compatible_with_store(&self, store: &StoreOpaque) -> bool {
self.comes_from_same_store(store)
}
#[inline]
fn dynamic_concrete_type_check(&self, _: &StoreOpaque, _: bool, _: &HeapType) -> Result<()> {
unreachable!()
}
#[inline]
fn is_vmgcref_and_points_to_object(&self) -> bool {
true
}
fn store(self, store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
self.wasm_ty_store(store, ptr, ValRaw::externref)
}
unsafe fn load(store: &mut AutoAssertNoGc<'_>, ptr: &ValRaw) -> Self {
Self::wasm_ty_load(store, ptr.get_externref(), ExternRef::from_cloned_gc_ref)
}
}
unsafe impl WasmTy for Option<ManuallyRooted<ExternRef>> {
#[inline]
fn valtype() -> ValType {
ValType::EXTERNREF
}
#[inline]
fn compatible_with_store(&self, store: &StoreOpaque) -> bool {
self.as_ref()
.map_or(true, |x| x.comes_from_same_store(store))
}
#[inline]
fn dynamic_concrete_type_check(&self, _: &StoreOpaque, _: bool, _: &HeapType) -> Result<()> {
unreachable!()
}
#[inline]
fn is_vmgcref_and_points_to_object(&self) -> bool {
self.is_some()
}
fn store(self, store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
<ManuallyRooted<ExternRef>>::wasm_ty_option_store(self, store, ptr, ValRaw::externref)
}
unsafe fn load(store: &mut AutoAssertNoGc<'_>, ptr: &ValRaw) -> Self {
<ManuallyRooted<ExternRef>>::wasm_ty_option_load(
store,
ptr.get_externref(),
ExternRef::from_cloned_gc_ref,
)
}
}