How to detect if running in a webworker?

[alex-caelus]

Summary

I'm developing a very simple module that need access to either the Window or WorkerGlobalScope object depending on if it is running in a webworker or in the rendering thread.

Currently I haven't been able to figure it out, see below for what I have tried and why it doesn't work.

The reason I'm doing this is mainly because when I run my tests with GECKODRIVER it is running in the main thread but in my application I'm using a webworker.

Additional Details

This is my code, which dosen't work as expected.

use js_sys::global;
use log::trace;
use wasm_bindgen::JsCast;
use web_sys::{Window, WorkerGlobalScope};

impl CSPRNG {
    fn get_u128(&self) -> u128 {
        trace!("CSPRNG::get_u128");
        let mut bytes = [0; 16];
        let g = global();
        if WorkerGlobalScope::instanceof(&g) {
            if let Ok(scope) = g.dyn_into::<WorkerGlobalScope>() {
                scope
                    .crypto()
                    .expect("Crypto API not supported?")
                    .get_random_values_with_u8_array(&mut bytes[..])
                    .expect("Failed to obtain secure randomness");
            } else {
                panic!("failed conversion to WorkerGlobalScope");
            }
        } else if Window::instanceof(&g) {
            if let Ok(scope) = g.dyn_into::<Window>() {
                scope
                    .crypto()
                    .expect("Crypto API not supported?")
                    .get_random_values_with_u8_array(&mut bytes[..])
                    .expect("Failed to obtain secure randomness");
            } else {
                panic!("failed conversion to Window");
            }
        } else {
            panic!("Unsuported global type");
        }

        u128::from_be_bytes(bytes)
    }
}

When running with GECKODRIVER

console.log div contained:
    wasm-bindgen: imported JS function that was not marked as `catch` threw an error:
    WorkerGlobalScope is not defined

    Stack:
    init/imports.wbg.__wbg_instanceof_WorkerGlobalScope_93324c9f5660e5b4<@http://127.0.0.1:57750/wasm-bindgen-test:585:28
    logError/<@http://127.0.0.1:57750/wasm-bindgen-test:214:22
    web_sys::features::gen_WorkerGlobalScope::__wbg_generated_const_WorkerGlobalScope::<impl wasm_bindgen::cast::JsCast for web_sys::features::gen_WorkerGlobalScope::WorkerGlobalScope>::instanceof::h244fb6a2ad7e30cd@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[1060]:0x6fc0b
    <tiny_csprng::wasm32::CSPRNG as tiny_csprng::TinyPRNG>::get_u128::h8b676c7ec7cc1a4e@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[49]:0x126c3
    tiny_csprng::wasm32::test::get_a_random_value::hf04acc041de00098@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[563]:0x5de40
    core::ops::function::FnOnce::call_once::ha728f62e5aef6998@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[1942]:0x7f56f
    wasm_bindgen_test::__rt::Context::execute_sync::{{closure}}::hb7527fde4ab876a3@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[666]:0x62c30
    <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h51512de833fb91a2@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[628]:0x610d0
    <wasm_bindgen_test::__rt::TestFuture<F> as core::future::future::Future>::poll::{{closure}}::{{closure}}::h48ac108cd24bdf66@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[833]:0x6935e
    wasm_bindgen::convert::closures::invoke0_mut::h15117140ce631b6d@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[687]:0x63b0b
    __wbg_adapter_33@http://127.0.0.1:57750/wasm-bindgen-test:332:10
    cb0@http://127.0.0.1:57750/wasm-bindgen-test:488:28
    window.__wbg_test_invoke@http://127.0.0.1:57750/:37:38
    init/imports.wbg.__wbg_wbgtestinvoke_40963ed10b2b93d6<@http://127.0.0.1:57750/wasm-bindgen-test:493:30
    handleError/<@http://127.0.0.1:57750/wasm-bindgen-test:338:22
    wasm_bindgen_test::__rt::__wbg_test_invoke::h895bf8f3be895f05@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[342]:0x4f776
    <wasm_bindgen_test::__rt::TestFuture<F> as core::future::future::Future>::poll::{{closure}}::h64d913946b5019d0@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[723]:0x652b6
    scoped_tls::ScopedKey<T>::set::h8e0b39b0ee232b48@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[455]:0x57ade
    <wasm_bindgen_test::__rt::TestFuture<F> as core::future::future::Future>::poll::h5e6ad99cdbcba28d@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[111]:0x30766
    <wasm_bindgen_test::__rt::ExecuteTests as core::future::future::Future>::poll::h61ef56113a3600cf@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[57]:0x1a79a
    wasm_bindgen_test::__rt::Context::run::{{closure}}::h96c89f6e1b39c00c@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[232]:0x4498c
    <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h2cbe4e48de8d0528@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[482]:0x59571
    wasm_bindgen_futures::future_to_promise::{{closure}}::{{closure}}::hef8d06aea6caac7c@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[117]:0x31e0e
    <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hfeeaadbe23eb316a@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[634]:0x61536
    wasm_bindgen_futures::task::singlethread::Task::run::haf3924a5ccdff81d@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[198]:0x403a9
    wasm_bindgen_futures::queue::QueueState::run_all::h8f257c8f713a39fb@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[250]:0x46b7a
    wasm_bindgen_futures::queue::Queue::new::{{closure}}::he00ae9c50479de2b@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[1115]:0x71250
    <dyn core::ops::function::FnMut<(A,)>+Output = R as wasm_bindgen::closure::WasmClosure>::describe::invoke::h44430f1c3bd26b81@http://127.0.0.1:57750/wasm-bindgen-test_bg.wasm:wasm-function[540]:0x5cb66
    __wbg_adapter_20@http://127.0.0.1:57750/wasm-bindgen-test:236:10
    real@http://127.0.0.1:57750/wasm-bindgen-test:201:20

When running in webworker (in application)

The code above actually works, but let's say for instance that I reverse the order of the if-else statements above and try the conversion to Window first then I would expect the same error as above to be shown in the developer console. But that is not what happens, instead my application hangs silently.

Solution

If I could detect, at runtime, if I'm running in a webworker or not (perhaps by checking if WorkerGlobalScope is defined, somehow) then I could use that and avoid to trigger the faults above.

[Pauan]

You need to use code like this. We plan to eventually make the above code public, so you can easily just import gloo and use it. But for now you can copy-paste it.

[alex-caelus]

Thank you, I will try this approach.

[alex-caelus]

For posterity, (in case it would help someone else with the same or similar problem) I ended up constructing the following macro:

use wasm_bindgen::prelude::*;
use wasm_bindgen::{JsCast, JsValue};
use web_sys::{Window, WorkerGlobalScope};

macro_rules! window_or_worker {
    ($w:ident => $block:stmt) => {
        #[wasm_bindgen]
        extern "C" {
            type Global;

            #[wasm_bindgen(method, getter, js_name = Window)]
            fn window(this: &Global) -> JsValue;

            #[wasm_bindgen(method, getter, js_name = WorkerGlobalScope)]
            fn worker(this: &Global) -> JsValue;
        }

        let global: Global = js_sys::global().unchecked_into();

        if !global.window().is_undefined() {
            let $w: Window = global.unchecked_into();
            $block
        } else if !global.worker().is_undefined() {
            let $w: WorkerGlobalScope = global.unchecked_into();
            $block
        } else {
            panic!("Only supported in a browser or web worker");
        }
    };
}

I used it like this:

fn get_u128(&self) -> u128 {
    let mut bytes = [0; 16];

    window_or_worker!(scope => {
        scope
            .crypto()
            .expect("Crypto API not supported?")
            .get_random_values_with_u8_array(&mut bytes[..])
            .expect("Failed to obtain secure randomness");
    });

    u128::from_be_bytes(bytes)
}

I would just like to add, as an aside, that it would have been nice to have the common functionalities of Window and WorkerGlobalScope extracted into a common trait. But I'm sure it has been discussed before.

[Pauan]

@alex-caelus That code is inefficient. It is better to use the code I posted, since it is faster.

It also has a macro for generating methods:

impl_window_or_worker! {
    fn crypto() -> Result<Crypto, JsValue>;
}

fn get_u128(&self) -> u128 {
    let mut bytes = [0; 16];

    GLOBAL.with(|global| {
        global
            .crypto()
            .expect("Crypto API not supported?")
            .get_random_values_with_u8_array(&mut bytes[..])
            .expect("Failed to obtain secure randomness");
    });

    u128::from_be_bytes(bytes)
}

I would just like to add, as an aside, that it would have been nice to have the common functionalities of Window and WorkerGlobalScope extracted into a common trait. But I'm sure it has been discussed before.

Yes, it has been discussed a few times, and our plan is to add it into gloo.

[alex-caelus]

Thank you for pointing it out! Yes, you are correct, I did not really consider it to be costly to run the code once per invocation compared to running it only once with thread_local!. Your linked code is ofc better.