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wasmtime-c-api: Add support for externref values

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This required that `wasm_val_t` have a `Drop` implementation, an explicit
`Clone` implementation, and no longer be `Copy`, which rippled out through the
crate a bit.

Additionally, `wasm_func_call` and friends were creating references to
uninitialized data for its out pointers and assigning to them. As soon as
`wasm_val_t` gained a `Drop` impl and tried to drop the old value of the
assignment (which is uninitialized data), then things blew up. The fix is to
properly represent the out pointers with `MaybeUninit`, and use `ptr::write` to
initialize them without dropping the old data.

Part of #929
This commit is contained in:
Nick Fitzgerald
2020-07-08 10:20:56 -07:00
parent 2a4f72aeb7
commit 4cdf8b7cfd
3 changed files with 71 additions and 42 deletions
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17
crates/c-api/src/func.rs
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@@ -2,6 +2,7 @@ use crate::{wasm_extern_t, wasm_functype_t, wasm_store_t, wasm_val_t};
use crate::{wasm_name_t, wasm_trap_t, wasmtime_error_t};
use anyhow::anyhow;
use std::ffi::c_void;
use std::mem::MaybeUninit;
use std::panic::{self, AssertUnwindSafe};
use std::ptr;
use std::str;
@@ -89,6 +90,7 @@ fn create_function(
let func = Func::new(store, ty, move |caller, params, results| {
let params = params
.iter()
.cloned()
.map(|p| wasm_val_t::from_val(p))
.collect::<Vec<_>>();
let mut out_results = vec![wasm_val_t::default(); results.len()];
@@ -163,7 +165,7 @@ pub extern "C" fn wasmtime_func_new_with_env(
pub unsafe extern "C" fn wasm_func_call(
wasm_func: &wasm_func_t,
args: *const wasm_val_t,
results: *mut wasm_val_t,
results: *mut MaybeUninit<wasm_val_t>,
) -> *mut wasm_trap_t {
let func = wasm_func.func();
let mut trap = ptr::null_mut();
@@ -186,7 +188,7 @@ pub unsafe extern "C" fn wasmtime_func_call(
func: &wasm_func_t,
args: *const wasm_val_t,
num_args: usize,
results: *mut wasm_val_t,
results: *mut MaybeUninit<wasm_val_t>,
num_results: usize,
trap_ptr: &mut *mut wasm_trap_t,
) -> Option<Box<wasmtime_error_t>> {
@@ -201,7 +203,7 @@ pub unsafe extern "C" fn wasmtime_func_call(
fn _wasmtime_func_call(
func: &wasm_func_t,
args: &[wasm_val_t],
results: &mut [wasm_val_t],
results: &mut [MaybeUninit<wasm_val_t>],
trap_ptr: &mut *mut wasm_trap_t,
) -> Option<Box<wasmtime_error_t>> {
let func = func.func();
@@ -217,8 +219,13 @@ fn _wasmtime_func_call(
let result = panic::catch_unwind(AssertUnwindSafe(|| func.call(&params)));
match result {
Ok(Ok(out)) => {
for (slot, val) in results.iter_mut().zip(out.iter()) {
*slot = wasm_val_t::from_val(val);
for (slot, val) in results.iter_mut().zip(out.into_vec().into_iter()) {
unsafe {
// NB: The results array is likely uninitialized memory, so
// use `ptr::write` rather than assignment (which tries to
// run destructors).
ptr::write(slot.as_mut_ptr(), wasm_val_t::from_val(val));
}
}
None
}