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Fix a panic calling host functions with refs in async mode (#8434) (#…
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…8439)

This commit fixes a panic when a host function defined with `Func::new`
returned GC references and was called in async mode. The logic to
auto-gc before the return values go to wasm asserted that a synchronous
GC was possible but the context this function is called in could be
either async or sync. The fix applied in this commit is to remove the
auto-gc. This means that hosts will need to explicitly GC in these
situations until auto-gc is re-added back to Wasmtime.

cc #8433 as this will make the behavior consistent, but we'll want to
re-add the gc behavior.
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alexcrichton authored Apr 22, 2024
1 parent 0c83363 commit 0c92290
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Showing 2 changed files with 62 additions and 18 deletions.
18 changes: 0 additions & 18 deletions crates/wasmtime/src/runtime/func.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1345,24 +1345,6 @@ impl Func {
let (params, results) = val_vec.split_at_mut(nparams);
func(caller.sub_caller(), params, results)?;

#[cfg(feature = "gc")]
{
// See the comment in `Func::call_impl_check_args`.
let num_gc_refs = ty.as_wasm_func_type().non_i31_gc_ref_returns_count();
if let Some(num_gc_refs) = NonZeroUsize::new(num_gc_refs) {
if caller
.as_context()
.0
.gc_store()?
.gc_heap
.need_gc_before_entering_wasm(num_gc_refs)
{
assert!(!caller.as_context().0.async_support());
caller.as_context_mut().gc();
}
}
}

// Unlike our arguments we need to dynamically check that the return
// values produced are correct. There could be a bug in `func` that
// produces the wrong number, wrong types, or wrong stores of
Expand Down
62 changes: 62 additions & 0 deletions tests/all/async_functions.rs
Original file line number Diff line number Diff line change
Expand Up @@ -983,3 +983,65 @@ async fn gc_preserves_externref_on_historical_async_stacks() -> Result<()> {

Ok(())
}

#[tokio::test]
async fn async_gc_with_func_new_and_func_wrap() -> Result<()> {
let _ = env_logger::try_init();

let mut config = Config::new();
config.async_support(true);
let engine = Engine::new(&config)?;

let module = Module::new(
&engine,
r#"
(module $m1
(import "" "a" (func $a (result externref)))
(import "" "b" (func $b (result externref)))
(table 2 funcref)
(elem (i32.const 0) func $a $b)
(func (export "a")
(call $call (i32.const 0))
)
(func (export "b")
(call $call (i32.const 1))
)
(func $call (param i32)
(local $cnt i32)
(loop $l
(drop (call_indirect (result externref) (local.get 0)))
(local.set $cnt (i32.add (local.get $cnt) (i32.const 1)))
(if (i32.lt_u (local.get $cnt) (i32.const 1000))
(then (br $l)))
)
)
)
"#,
)?;

let mut linker = Linker::new(&engine);
linker.func_wrap("", "a", |mut cx: Caller<'_, _>| {
Ok(Some(ExternRef::new(&mut cx, 100)?))
})?;
let ty = FuncType::new(&engine, [], [ValType::EXTERNREF]);
linker.func_new("", "b", ty, |mut cx, _, results| {
results[0] = ExternRef::new(&mut cx, 100)?.into();
Ok(())
})?;

let mut store = Store::new(&engine, ());
let instance = linker.instantiate_async(&mut store, &module).await?;
let a = instance.get_typed_func::<(), ()>(&mut store, "a")?;
a.call_async(&mut store, ()).await?;

let mut store = Store::new(&engine, ());
let instance = linker.instantiate_async(&mut store, &module).await?;
let b = instance.get_typed_func::<(), ()>(&mut store, "b")?;
b.call_async(&mut store, ()).await?;

Ok(())
}

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