RequireJS resolver sometimes fails to detect blocked resources

[ishakhsuvarov]

Preconditions (*)

1. Magento 2.3.5-p1. Most likely any recent release would be affected.

Steps to reproduce (*)

1. Create a custom RequireJS config which defines a component complete with its url and fallback path. For example:

var config = {
    paths: {
        a: ['https://example.com/advertisement.js', 'Custom_Module/js/noop']
    }
};

2. Include this component on Checkout (may be reproducible elsewhere)

<!-- Something simple like this would work: -->
<script type="text/x-magento-init">
    {
        "*": {
            "a": {}
        }
    }
</script>

3. Use a browser with any aggressive Ad Blocker extension which would block the required url
4. Open Checkout page

Expected result (*)

1. ✅ Ad Blocker extension blocks the script download from the url stated in RequireJS config (https://example.com/advertisement.js)
2. ✅ Checkout page loads and renders just fine

Actual result (*)

1. ✅ Ad Blocker Extension blocks script download from the original url
2. ℹ️ RequireJS uses path fallback mechanism to load a resource from second url, therefore not triggering an error.
3. ℹ️ Magento's RequireJS resolver cannot keep track of such events and continues to wait for original resource to load (as no error was produced), therefore not calling any of the callbacks.
4. ❌ Checkout page is displaying 2 spinning loaders and a fully rendered page under it as resolver does not track components which were loaded via fallbacks.

[m2-assistant]

Hi @ishakhsuvarov. Thank you for your report.
To help us process this issue please make sure that you provided the following information:

• Summary of the issue
• Information on your environment
• Steps to reproduce
• Expected and actual results

Please make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, please, add a comment to the issue:

@magento give me 2.4-develop instance - upcoming 2.4.x release

For more details, please, review the Magento Contributor Assistant documentation.

@ishakhsuvarov do you confirm that you were able to reproduce the issue on vanilla Magento instance following steps to reproduce?

• yes
• no

---

• Join Magento Community Engineering Slack and ask your questions in #github channel.

[magento-engcom-team]

@VladimirZaets Thank you for verifying the issue.

Unfortunately, not enough information was provided to acknowledge ticket. Please consider adding the following:

• Add "Reproduced on " label(s) to this ticket based on verification result

Once all required information is added, please add label "Issue: Confirmed" again.
Thanks!