Add wasm_bindgen support

[mbalex99]

Glad to see that rand is adding wasm support. However using wasm_bindgen to build towards nodejs we get a build artifact with an unknown require('env')

The make file to run the building is:

build-wasm-dev:
	cargo build --target wasm32-unknown-unknown; \
	mkdir -p dist; \
	wasm-bindgen target/wasm32-unknown-unknown/debug/ditto.wasm --out-dir ./dist --nodejs

And the created file looks like this:

let imports = {};
imports['./ditto'] = require('./ditto');
imports['env'] = require('env'); // HERE, what is this?

            const join = require('path').join;
            const bytes = require('fs').readFileSync(join(__dirname, 'ditto_bg.wasm'));
            const wasmModule = new WebAssembly.Module(bytes);
            const wasmInstance = new WebAssembly.Instance(wasmModule, imports);
            module.exports = wasmInstance.exports;

Therefore running in node throws an unknown env module error.

This seems to only happen when my Cargo.toml includes:

rand = {version = "0.5.0", features = ['stdweb']}

[dhardy]

Probably related to the OsRng implementation, but I don't know much about WASM. Can you investigate further?

[pitdicker]

The implementation lives here: https://github.com/rust-lang-nursery/rand/blob/0.5.0/src/rngs/os.rs#L696.

[mbalex99]

I am unfortunately pretty new to rust (😞) and by my glance I am not sure where the require('env') is coming from in that OsRng implementation

[dhardy]

Since Rand transitively depends on stdweb in this case, is this to do with stdweb? Or is it that require("crypto") implies this? In any case maybe if you ask over there you can find someone who has some idea.

[mbalex99]

Ah I see maybe that's the cause of it.

[celaus]

I have the same issue :(

here's the full error (for google, and if someone knows more than us 😄 )

/workspace/Mine/azure-functions-rust/pibench-rs/wasm_pibench_bg.js:9
            const wasmInstance = new WebAssembly.Instance(wasmModule, imports);
                                 ^

LinkError: WebAssembly Instantiation: Import #0 module="env" function="__js_1" error: function import requires a callable
    at Object.<anonymous> (/workspace/Mine/azure-functions-rust/pibench-rs/wasm_pibench_bg.js:9:34)
    at Module._compile (module.js:660:30)
    at Object.Module._extensions..js (module.js:671:10)
    at Module.load (module.js:573:32)
    at tryModuleLoad (module.js:513:12)
    at Function.Module._load (module.js:505:3)
    at Module.require (module.js:604:17)
    at require (internal/module.js:11:18)
    at Object.<anonymous> (/workspace/Mine/azure-functions-rust/pibench-rs/wasm_pibench.js:168:8)
    at Module._compile (module.js:660:30)

[koute]

The wasm-bindgen doesn't support stdweb's js! macro, which is why this error is being generated.

Basically, the way the js! macro works is something like this: stdweb imports a bunch of functions called env.__js_0, env.__js_1, env.__js_2 and calls them every time the js! macro is used, so, e.g. something like this:

let value = 2;
js! {
    console.log( "1 + 1 = ", @{value} );
}

is effectively equivalent to this under the hood (I'm simplifying a little):

extern "C" {
    pub fn __js_1( a1: i32, code: *const u8 ) -> i32;
}
let value = 2;
__js_1( value, r#"console.log( "1 + 1 = ", $0 );"# );

Now what's supposed to happen here (and cargo-web actually does this) is that the import for the __js_1 (which is emitted as env.__js_1 by the Rust compiler) should be removed and the call to __js_1 replaced by a concrete import of the JS snipped you had in your code.

Since you're not using cargo-web this doesn't happen; the wasm-bindgen sees the env.__js_1 import and does what it does to imports normally - requires the module, in this case a module called env.

You could probably work around this issue (this is super hacky though) by creating a module called env yourself, and implement the __js_1, __js_2, etc. functions through eval. Or you could switch from wasm-bindgen to cargo-web, which may or may not be feasible depending on your usecase. (stdweb in conjunction with cargo-web also supports wasm-bindgen-like exporting of functions through the js_export attribute; see stdweb's README for more details.)

Ideally we'd come up with a cross-tool ABI of specifying JavaScript snippets (which both wasm-bindgen and cargo-web could support), but that is still probably a way off.

[celaus]

Thank you @koute, I had a hunch that the js! macro might be responsible. My project isn't too complex so I might get it to work with cargo-web - thanks for the pointer. 😊

[pitdicker]

Thanks @koute for the good explanation!

So what I think needs to happen here is we need to support wasm_bindgen, similar to stdweb. I.e., add it as an optional dependency in cargo.toml and lib.rs, similar to what we do now with stdweb, and add an implementation in OsRng similar to the one for stdweb (note: I just touched that code a bit in #484).

@dhardy Do you think this is a nice 'help wanted' issue to advertise?

(Changed the title from "rand with stdweb feature and with --nodejs in wasm_bindgen and wasm32-unknown-unknown target generates unknown 'env' module" to "Add wasm_bindgen support").

[dhardy]

Yes, agreed @pitdicker. And thanks @koute for helping us out with ha good explanation of the issue.

I don't know what the best solution for Rand is, but I guess we don't want wasm-bindgen support that conflicts with cargo-web, (at least not unless it's optional).

[celaus]

Here's a quick & dirty fix for the rand crate (as @koute mentioned 👍 )

Nevermind, I was wrong 😭
this does not work
wasm_<project>_bg.js

let imports = {};
imports['./wasm_pibench'] = require('./wasm_pibench');
imports['env'] = {__js_1:  require('crypto') } // that's what stdweb/rand is trying to import on nodejs

            const join = require('path').join;
            const bytes = require('fs').readFileSync(join(__dirname, 'wasm_pibench_bg.wasm'));
            const wasmModule = new WebAssembly.Module(bytes);
            const wasmInstance = new WebAssembly.Instance(wasmModule, imports);
            module.exports = wasmInstance.exports;

[pitdicker]

@quininer continuing from our conversation on reddit https://www.reddit.com/r/rust/comments/8msrfp/help_add_wasmbindgen_support_to_rand/ (I like github a bit more).

wasm-bindgen does not support dynamic feature-detects, I think wbg-rand still uses Math.random for the same reason.

But would it be possible to link to two custom javascript functions, like detect_crypto_random and fill_bytes? And do the detection there?

(and sorry, didn't recognise your name at first as the author of the stdweb implementation 😄)