Crate externref

Expand description

Low-cost reference type shims for WASM modules.

Reference type (aka externref or anyref) is an opaque reference made available to a WASM module by the host environment. Such references cannot be forged in the WASM code and can be associated with arbitrary host data, thus making them a good alternative to ad-hoc handles (e.g., numeric ones). References cannot be stored in WASM linear memory; they are confined to the stack and tables with externref elements.

Rust does not support reference types natively; there is no way to produce an import / export that has externref as an argument or a return type. wasm-bindgen patches WASM if externrefs are enabled. This library strives to accomplish the same goal for generic low-level WASM ABIs (wasm-bindgen is specialized for browser hosts).

`externref` use cases

Since externrefs are completely opaque from the module perspective, the only way to use them is to send an externref back to the host as an argument of an imported function. (Depending on the function semantics, the call may or may not consume the externref and may or may not modify the underlying data; this is not reflected by the WASM function signature.) An externref cannot be dereferenced by the module, thus, the module cannot directly access or modify the data behind the reference. Indeed, the module cannot even be sure which kind of data is being referenced.

It may seem that this limits externref utility significantly, but externrefs can still be useful, e.g. to model capability-based security tokens or resource handles in the host environment. Another potential use case is encapsulating complex data that would be impractical to transfer across the WASM API boundary (especially if the data shape may evolve over time), and/or if interactions with data must be restricted from the module side.

Usage

Use Resources as arguments / return results for imported and/or exported functions in a WASM module in place of externrefs . Reference args (including mutable references) and the Option<_> wrapper are supported as well.
Add the #[externref] proc macro on the imported / exported functions.
Post-process the generated WASM module with the processor.

How it works

The externref macro detects Resource args / return types for imported and exported functions. All Resource args or return types are replaced with usizes and a wrapper function is added that performs the necessary transform from / to usize. Additionally, a function signature describing where Resource args are located is recorded in a WASM custom section.

To handle usize (~i32 in WASM) <-> externref conversions, managing resources is performed using 3 function imports from a surrogate module:

Creating a Resource (“real” signature fn(externref) -> usize) stores a reference into an externref table and returns the table index. The index is what is actually stored within the Resource, meaning that Resources can be easily placed on heap.
Getting a reference from a Resource (“real” signature fn(usize) -> externref) is an indexing operation for the externref table.
Resource::drop() (“real” signature fn(usize)) removes the reference from the table.

Real externrefs are patched back to the imported / exported functions by the WASM module post-processor:

Imports from a surrogate module referenced by Resource methods are replaced with local WASM functions. Functions for getting an externref from the table and dropping an externref are more or less trivial. Storing an externref is less so; we don’t want to excessively grow the externrefs table, thus we search for null refs among its existing elements first, and only grow the table if all existing table elements are occupied.
Patching changes function types, and as a result types of some locals. This is OK because the post-processor also changes the signatures of affected imported / exported functions. The success relies on the fact that a reference is only stored immediately after receiving it from the host; likewise, a reference is only obtained immediately before passing it to the host. Resources can be dropped anywhere, but the corresponding externref removal function does not need its type changed.

use externref::{externref, Resource};

// Two marker types for different resources.
pub struct Sender(());
pub struct Bytes(());

#[externref]
#[link(wasm_import_module = "test")]
extern "C" {
    // This import will have signature `(externref, i32, i32) -> externref`
    // on host.
    fn send_message(
        sender: &Resource<Sender>,
        message_ptr: *const u8,
        message_len: usize,
    ) -> Resource<Bytes>;

    // `Option`s are used to deal with null references. This function will have
    // `(externref) -> i32` signature.
    fn message_len(bytes: Option<&Resource<Bytes>>) -> usize;
    // This one has `() -> externref` signature.
    fn last_sender() -> Option<Resource<Sender>>;
}

// This export will have signature `(externref)` on host.
#[externref]
#[export_name = "test_export"]
pub extern "C" fn test_export(sender: Resource<Sender>) {
    let messages: Vec<_> = ["test", "42", "some other string"]
        .into_iter()
        .map(|msg| {
            unsafe { send_message(&sender, msg.as_ptr(), msg.len()) }
        })
        .collect();
    // ...
    // All 4 resources are dropped when exiting the function.
}