"Error: Module did not self-register" with Node v12 worker threads

[asztal]

I'm finding that node-sass is failing to load when I'm using mocha-parallel-tests. It turns out that mocha-parallel-tests uses worker threads in Node v12, and that seems to be the reason. The first thread to require("node-sass") succeeds, but the second one fails with Error: Module did not self-register.

It seems reasonable that someone might want to use node-sass in worker threads, e.g. something similar to thread-loader. In my case, I'm running integration tests (in parallel) where some SCSS compilation is part of the thing being tested.

Similar error messages have been reported before, but they precede the introduction of worker threads so I presume they are caused to something else.

Minimal reproduction:

const { Worker } = require("worker_threads");

function render(element) {
    return new Promise((resolve, reject) => {
        const worker = new Worker(`
            const sass = require("node-sass");
            const data = "${element} { font-weight: bold }";
            const css = sass.renderSync({ data }).css.toString("utf8");
            console.log(css);
        `, { eval: true });
        worker.on("error", reject);
        worker.on("exit", exitCode => {
            if (exitCode > 0)
                reject(new Error(`Worker exited with code ${exitCode}`));
            resolve();
        })
    });
}

// Spawn the workers in sequence, not in parallel, to make it easier to debug
async function bug() {
    await render("b");
    await render("strong");
}

bug().catch(err => console.error(err));

Output:

lee@lee-desktop:~/Work/restapi$ node sassbug.js 
b {
  font-weight: bold; }

internal/modules/cjs/loader.js:977
  return process.dlopen(module, path.toNamespacedPath(filename));
                 ^

Error: Module did not self-register.
    at Object.Module._extensions..node (internal/modules/cjs/loader.js:977:18)
    at Module.load (internal/modules/cjs/loader.js:790:32)
    at Function.Module._load (internal/modules/cjs/loader.js:703:12)
    at Module.require (internal/modules/cjs/loader.js:830:19)
    at require (internal/modules/cjs/helpers.js:68:18)
    at module.exports (/home/lee/Work/restapi/node_modules/node-sass/lib/binding.js:19:10)
    at Object.<anonymous> (/home/lee/Work/restapi/node_modules/node-sass/lib/index.js:14:35)
    at Module._compile (internal/modules/cjs/loader.js:936:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:947:10)
    at Module.load (internal/modules/cjs/loader.js:790:32)

Versions

• NPM version (npm -v): 6.11.2
• Node version (node -v): v12.10.0
• Node Process (node -p process.versions):

  node: '12.10.0',
  v8: '7.6.303.29-node.16',
  uv: '1.31.0',
  zlib: '1.2.11',
  brotli: '1.0.7',
  ares: '1.15.0',
  modules: '72',
  nghttp2: '1.39.2',
  napi: '4',
  llhttp: '1.1.4',
  http_parser: '2.8.0',
  openssl: '1.1.1c',
  cldr: '35.1',
  icu: '64.2',
  tz: '2019a',
  unicode: '12.1'
}

• Node Platform (node -p process.platform): linux
• Node architecture (node -p process.arch): x64
• node-sass version (node -p "require('node-sass').info"):

node-sass       4.12.0  (Wrapper)       [JavaScript]
libsass         3.5.4   (Sass Compiler) [C/C++]

• npm node-sass versions (npm ls node-sass): └── node-sass@4.12.0

• Binding:
  I installed the binding using standard npm install, but I also tried https://github.com/sass/node-sass/releases/download/v4.12.0/linux-x64-72_binding.node (they have the same SHA256 hash anyway)

[asztal]

I tried switching NODE_MODULE for NAN_MODULE_WORKER_ENABLED - it compiles, works, and only segfaults 10% of the time! OK, maybe 25%. I'm counting that as a success anyway

lee@lee-desktop:~/Work/restapi$ node sassbug.js 
b {
  font-weight: bold; }

Segmentation fault (core dumped)

I sort of expected this to work given that node-sass doesn't have any global state that I can see. Maybe libsass is doing something not thread-safe, but I wouldn't know about that.

[saper]

We currently do not support workers - our add on is not context-aware.

We are using libuv lowlevel routines and it might be difficult to make it worker-aware. Can you try to produce a backtrace from your crash?

[asztal]

Hi, I will give this a try! I did build node-sass with debugging on and didn't get much useful, but I'll try some more when I get a chance.

It didn't seem like node-sass used much global state, so I naïvely assumed it might be okay if I Just changed to NAN_MODULE_WORKER_ENABLED - but it's been a long time since I wrote any node native modules and it's changed a lot :)

[saper]

We have some static initialization that is done by C++ runtime for us.

An example of a problem that was caused by static initialization was this #1283 (comment)

However we run libsass code in a background thread (outside of the main JavaScript loop) so maybe there is something we can investigate.