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Add a safe method for accessing memory and T (#2971)

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This is currently a very common operation in host bindings where if wasm
gives a host function a relative pointer you'll want to simulataneously
work with the host state and the wasm memory. These two regions are
distinct and safe to borrow mutably simulataneously but it's not obvious
in the Rust type system that this is so, so add a helper method here to
assist in doing so.
This commit is contained in:
Alex Crichton
2021-06-08 09:37:31 -05:00
committed by GitHub
parent 8b4bdf92e2
commit 884a6500e9
2 changed files with 36 additions and 19 deletions
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22
crates/wiggle/generate/src/wasmtime.rs
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@@ -118,25 +118,9 @@ fn generate_func(
return Err(#rt::wasmtime_crate::Trap::new("missing required memory export"));
}
};
// Note the unsafety here. Our goal is to simultaneously borrow the
// memory and custom data from `caller`, and the store it's connected
// to. Rust will not let us do that, however, because we must call two
// separate methods (both of which borrow the whole `caller`) and one of
// our borrows is mutable (the custom data).
//
// This operation, however, is safe because these borrows do not overlap
// and in the process of borrowing them mutability doesn't actually
// touch anything. This is akin to mutably borrowing two indices in an
// array, which is safe so long as the indices are separate.
//
// TODO: depending on how common this is for other users to run into we
// may wish to consider adding a dedicated method for this. For now the
// future of `GuestPtr` may be a bit hazy, so let's just get this
// working from the previous iteration for now.
let (ctx, mem) = unsafe {
let mem = &mut *(mem.data_mut(&mut caller) as *mut [u8]);
(get_cx(caller.data_mut()), #rt::wasmtime::WasmtimeGuestMemory::new(mem))
};
let (mem , ctx) = mem.data_and_store_mut(&mut caller);
let ctx = get_cx(ctx);
let mem = #rt::wasmtime::WasmtimeGuestMemory::new(mem);
match #abi_func(ctx, &mem #(, #arg_names)*) #await_ {
Ok(r) => Ok(<#ret_ty>::from(r)),
Err(#rt::Trap::String(err)) => Err(#rt::wasmtime_crate::Trap::new(err)),