AssetGraph: assert fails .type is 'asset' instead of 'asset_group'

🐛 bug report

When building/watching, I often get this crash

🚨 Build failed.
Error: The expression evaluated to a falsy value:

  (0, assert().default)(n.type === 'asset_group')

AssertionError [ERR_ASSERTION]: The expression evaluated to a falsy value:

  (0, _assert().default)(n.type === 'asset_group')

    at AssetGraph.removeNode (node_modules/@parcel/core/lib/AssetGraph.js:214:31)
    at AssetGraph.removeEdge (node_modules/@parcel/core/lib/Graph.js:248:12)
    at AssetGraph.removeNode (node_modules/@parcel/core/lib/Graph.js:212:14)
    at AssetGraph.removeNode (node_modules/@parcel/core/lib/AssetGraph.js:219:18)
    at AssetGraph.removeEdge (node_modules/@parcel/core/lib/Graph.js:248:12)
    at AssetGraph.removeNode (node_modules/@parcel/core/lib/Graph.js:212:14)
    at AssetGraph.removeNode (node_modules/@parcel/core/lib/AssetGraph.js:219:18)
    at AssetGraph.removeEdge (node_modules/@parcel/core/lib/Graph.js:248:12)

(when inspecting the .type, we see it’s a asset instead of asset_group).

It probably has something to do with my custom transformer: it makes 1x/2x/3x assets for require('asset:/myFile.png?size=100x100') by exposing these as extra parts in the transform step:

async transform({asset, config, options}) {
    if (!asset.query.size) {
        // probably the variant!
        asset.isIsolated = true;
        return [asset];
    }
    
    let code = 'module.exports = {\n';
    const parts = [];
    // ...
    for (variant of ["3x","2x","1x"]) {
        const result = await imagePipeline.clone().toBuffer();
        const uniqueKey = asset.id+'@'+variant;
        asset.addDependency({
            moduleSpecifier: uniqueKey,
        });
        parts.push({
             type: 'png',
             pipeline: 'asset', // ??
             uniqueKey,
             content: result,
             isIsolated: true,
         });
        code += JSON.stringify(variant) + ": require(" + JSON.stringify(uniqueKey) + "),\n";
    });
    code += '}';
    asset.type = 'js';
    asset.setCode(code);
    return [asset, ...parts];
}

Note: re-inserting the parts into the same asset: “pipeline” feels rather clunky (might be the source if the issue?), but if I don’t do it I get:

No transformers found for assets/myImage.png.

🎛 Configuration (.babelrc, package.json, cli command)

const parcel = new Parcel({
    entries: "index.html",
    distDir: 'built/',
    config: require.resolve("./.parcelrc"),
    mode: "development",
    contentHash: true,
});

await parcel.watch((e) => {
    console.log("built! ", e);
})

{
    "extends": "@parcel/config-default",
    "transformers": {
        "asset:*": ["parcel-transformer-my-asset"]
    }
}

Also, using PARCEL_WORKER_BACKEND=process env.

💻 Code Sample

I can try to extract the crash case on request, but it’s quite some work!

🌍 Your Environment

Software	Version(s)
Parcel	2.0.0-nightly.462
Node	node-v14.15.1-linux-x64
npm/Yarn	yarn 1.22.4
Operating System	linux

[Banou26]

I edited the comment last night saying that upgrading to 2.0.0-nightly.540 fixed the problem. So I think everything is all good?

[mischnic]

This should have been fixed by https://github.com/parcel-bundler/parcel/pull/5522