fix(webpack): forward async load hook errors to the loader callback

[greymoth-jp]

The webpack load loader awaits the user load hook but never catches a rejection. When a load hook throws, the error escapes the loader function instead of being passed to webpack's loader callback. Since the loader calls this.async(), webpack's loader runner doesn't await the returned promise, so the throw surfaces as an unhandled promise rejection rather than a normal build error.

The sibling loaders already forward the error to callback:

• src/webpack/loaders/transform.ts wraps the handler in try/catch and calls callback(error).
• src/rspack/loaders/load.ts got the same handling in fix(rspack): prevents an unhandled promise rejection when throwing in the async loader #536.

This wraps the webpack load loader the same way, and adds a unit test that mirrors the existing transform loader error test. Reverting only the source change makes the new test fail (the rejection propagates out of load instead of reaching the callback); with the change the webpack and rspack loader test files pass.

Summary by CodeRabbit

• Bug Fixes

  • Improved loader error handling so failed async handlers now correctly report errors through the callback.
  • Non-standard thrown values are converted into standard errors before being returned, making failures more consistent.
  • Existing successful result handling remains unchanged for null, string, and object responses.

• Tests

  • Added coverage for rejected or thrown loader handlers to verify errors are passed to the callback.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 38d815ee-5aac-4c97-9580-3e0aa3186ea9

📥 Commits

Reviewing files that changed from the base of the PR and between df47aa2 and 571e620.

📒 Files selected for processing (2)

• src/webpack/loaders/load.ts
• test/unit-tests/webpack/loaders/load.test.ts

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📝 Walkthrough

Walkthrough

The load webpack loader now wraps its async handler.call(...) invocation in a try/catch. On error, it calls callback(error) for Error instances or converts non-Error throws to new Error(String(error)). A unit test verifies that a rejected plugin.load causes the callback to receive the error.

Changes

Load Loader Error Handling

Layer / File(s)	Summary
try/catch error forwarding src/webpack/loaders/load.ts, test/unit-tests/webpack/loaders/load.test.ts	Handler invocation and result dispatch are wrapped in a try block; the catch block forwards Error instances or stringified non-Error throws to the webpack callback. A new test asserts that a rejected plugin.load results in the error being passed to the callback.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5 minutes

Poem

🐇 A try and a catch, now errors won't flee,
The loader reports them with callback(e).
Non-Error? No worries, we'll String it away,
Our webpack won't crash in dismay.
Hop hop, all is well! 🎉

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and accurately summarizes the main change: forwarding async load hook errors to the loader callback.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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