MiniCssExtractPlugin is incopatible with cross-package configurations

[Zackere]

Bug report

MiniCssExtractPlugin does not work in cross-package compilation setting
Context:
When configuring a monorepo for parallel compilation (https://webpack.js.org/configuration/configuration-types/#parallelism) I noticed that MiniCssExtractPlugin does not work when more than 1 configuration has MiniCssExtractPlugin as a plugin.

I suggest, that src/index.js:206 which now looks like this:
webpack.util.serialization.register(CssModule, "mini-css-extract-plugin/dist/CssModule", null...

to be changed to something like this:
webpack.util.serialization.register(CssModule, "mini-css-extract-plugin/dist/CssModule", __dirname
or
webpack.util.serialization.register(CssModule, "mini-css-extract-plugin/dist/CssModule", Math.random()

1st one would prevent using the loader from the same node_modules twice and would enable using multiple MiniCssExtractPlugin accross multiple configurations (which is what I want)

2nd one would allow using MiniCssExtractPlugin multiple times without any restrictions

Actual Behavior

Webpack throws an error:
Error: ObjectMiddleware.register: serializer for mini-css-extract-plugin/dist/CssModule/null is already registered

Expected Behavior

Webpack should be able to compile multiple configurations with MiniCssExtractPlugin as a plugin

How Do We Reproduce?

Set up two packages, which use MiniCssExtractPlugin. It is important, that webpack configurations have proper loader resolution:
resolveLoader: { modules: [path.resolve(__dirname, 'node_modules'), 'node_modules'], }
Set up a third package, which as a webpack.config.js exports configurations of previous 2 packages.
Run webpack in the third package. It throws the error I described.
If we agree on a solution I'd more than happy to submit a PR

Please paste the results of npx webpack-cli info here, and mention other relevant information

[alexander-akait]

Duplicate #886, bug in webpack, already reported

[Zackere]

Shoot, sorry for bothering you then :) Wasnt aware of that issue, thanks

[Zackere]

@alexander-akait is it a duplicate, though?

[alexander-akait]

hm, I think I was wrong, parallelism on multi compiler level is another thing

[alexander-akait]

So you have multiple mini-css-extract-plugin in multiple packages? Will be great to look at example of structure

[Zackere]

on it

[Zackere]

subpackage sources: https://gist.github.com/Zackere/60549f09db9fd2bd36b4a150ee576dc7
compiler sources: https://gist.github.com/Zackere/08997a266a25b3bdb20bd846bb57385a

the folder structure should look as follows:

• package1 folder with subpackage sources
• package2 folder with subpackage sources
• compiler folder with compiler sources

Each subpackage compiles successfully on their own, but compiling together they crash

[Zackere]

Oh, and I discovered they clash even without parallelism enabled

[alexander-akait]

Yep, I see, you have multiple copied of mini-css-extract-plugin, ideally we should include version of package too, because you can multiple different versions (or we should provide absolute path)... in theory we can try __dirname + "../../mini-css-extract-plugin/dist/CssModule", so path will be different for different packages, I don't test it locally, please try

[Zackere]

Ill try it tomorrow, but it should suffice

[Zackere]

Ok, passing absolute path worked. I submitted a PR

[alexander-akait]

Fixed #894