Attempt to fix #2340

[emilniklas]

↪️ Pull Request

Today, before the bundler creates a child/sibling bundle for a certain asset, it's checked that the asset isn't already an entry point for an existing bundle. However, if it is, the bundle isn't added as an actual child/sibling bundle. That leads to the issue described in #2340.

I think it should be okay to just add the already created bundles as child/sibling to the new bundle, but I'm not so well versed in this codebase, so I cannot way that for sure. I'm also quite unsure how to add a good test case for it (how can I make assertions on the output?).

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

[mischnic]

I can't say much about the code changes themselves (there might be a performance/size reason for the deduplication), but regarding the test:

Take a look at the should insert sibling CSS bundles for JS files in the HEAD test:

parcel/packages/core/integration-tests/test/html.js

Lines 138 to 140 in 08398bd

	it('should insert sibling CSS bundles for JS files in the HEAD', async function() {
	let b = await bundle(
	path.join(__dirname, '/integration/html-css/index.html')

That one already tests the CSS linking with only one HTML asset, you can just duplicate that test and modify it.

[devongovett]

I'm a bit worried about changing the behavior of this at this point. This issue will definitely be fixed in Parcel 2.