Wasm-bindgen: Methods that take a WebIDL `sequence` argument are missing

[RSSchermer]

Some methods seem to be missing from the WebGl2RenderingContext:

• invalidateFramebuffer
• invalidateSubFramebuffer
• drawBuffers
• transformFeedbackVaryings
• getUniformIndices
• getActiveUniforms

What they all seem to have in common is that they all take a WebIDL sequence as an argument. I've not checked other APIs, so I'm not sure whether or not this affects other APIs besides the WebGl2 API.

[jpmartin2]

I just noticed this when looking at the SubtleCrypto api, which has the same problem:

• generateKey
• deriveKey
• importKey
• unwrapKey

are all missing, and all have sequence parameters.

[alexcrichton]

Er sorry for taking awhile to get back to this! I believe this is handled here where we don't handle it right now. I suspect that we probably want to return either an Array or some sort of Rust-defined iterator here perhaps. It'd definitely be nice to bind these!

This'll need some work to figure out what the best interface is, an implementation, and then a PR!

[derekdreery]

Relevant webidl spec: https://heycam.github.io/webidl/#es-sequence

It seems like the best thing to do for now is require a js_sys::Array, and then maybe expand to supporting iterators later.

[derekdreery]

I've added a patch to use Array, because I think all sequences are converted to arrays in ECMAScript. PR: https://github.com/rustwasm/wasm-bindgen/pull/1152. I'm going to see if I get any errors with it like this - not 100% sure it's right.

[derekdreery]

Update: I've re-read the spec and it seems ambiguous. It says:

IDL sequence values are represented by ECMAScript Array values.

but then says some other things about iterators. Does this mean "values of type sequence<T>", or "values of the sequence (T)". I think Array is a safe option since passing arrays has to work, and I'm pretty sure you get arrays back.

[Pauan]

Rather than guessing, we should test and see what the browsers actually do.

[derekdreery]

I've experimented with DocumentOrShadowRoot elementsFromPoint in the console window (Firefox):

$ temp0 = document.elementsFromPoint(1.0, 1.0);
Array [ html ]
$ temp0 instanceof Array
true

EDIT Chrome is the same

$ temp1 = document.elementsFromPoint(1.0, 1.0)
(3) [div#TZA4S, body.default-theme.des-mat, html]
$ temp1 instanceof Array
true

EDIT 2 In argument position it's the same again (from the [SubtleCrypto] interface, available at window.crypto.subtle, emphasis added)

var result = crypto.subtle.generateKey(algorithm, extractable, keyUsages);

...

• keyUsages is an Array indicating what can be done with the newly generated key. Possible values for array elements are:

...

[alexcrichton]

@derekdreery it may be worthwhile testing a few sequences in argument position too, I think it makes sense that coming out they're an array, but going in it may be anything iterable

[derekdreery]

@alexcrichton that's what I thought too, they might accept stuff that's iterable. I'll do some tests.

[derekdreery]

Here's a test case showing that for at least indexedDB, iterables are supported in argument position:

const keys = ['first', 'second'];
var iterable = {
    [Symbol.iterator]() {
        let idx = 0;

        return {
            next: function() {
                if (idx >= keys.length) {
                    return { done: true };
                }
                idx += 1;
                return {value: keys[idx-1], done: false};
            }
        };
    }
}

var request = indexedDB.open("test2", 1);
request.onupgradeneeded = function(evt) {
    var db = request.result;
    var store = db.createObjectStore("test", {keyPath: iterable});
    console.log(store.keyPath);
}

request.onerror = function(evt) {
    console.warn("Error");
    console.warn(evt);
}

request.onsuccess = function(event) {
    console.log("js Success");
}

How should we handle this in web-sys? Should we emit different types depending on whether the sequence is in argument position, or return position?

[alexcrichton]

Ok thanks for the investigation! In that case I think we'd ideally use &JsValue in argument position and js_sys::Array in return position, but it may also be possible to just use JsValue everywhere.

[alexcrichton]

Fixed in https://github.com/rustwasm/wasm-bindgen/pull/1152!