[emanueleperuffo]

Summary

HermesBytecodePlugin runs hermesc in the assetEmitted hooks and checks whether the chunk's .map exists. A chunk's .map may not have been written yet at that moment, so hermesc runs without source maps:

• debug info is then embedded in the bytecode (.bundle), making the chunk bigger
• composeSourceMaps is skipped, so stack traces for that chunk can't be symbolicated back to the original source

This change calls the hermesc from the afterEmit hook instead, once all assets are guaranteed to be written.

Our app has many lazy chunks, and most of them hit the race. With this fix, the debug info is moved from the chunks into the source maps:

Platform	Before	After	Diff
iOS	115.5 MB	111.0 MB	−4.45 MB (−3.85%)
Android	133.8 MB	128.9 MB	−4.88 MB (−3.65%)

Lazy chunks now ship with a composed source map, and frames resolve to the correct source file and line. The main bundle is not affected, since React Native compiles it with its own hermesc step.

Test plan

• I bundled our apps and tester-app for Android with the plugin enabled. All chunks were compiled and had their source maps composed.
• We shipped the fix in our app (webpack) and measured the sizes above, and confirmed that production errors thrown inside lazy chunks are symbolicated to the right source lines.
• The tester app is too small to reproduce the race with both the old and new code, since it depends on timing and chunk count, so it needs to be tested with bigger apps.

[vercel]

Someone is attempting to deploy a commit to the Callstack Team on Vercel.

A member of the Team first needs to authorize it.

[changeset-bot]

🦋 Changeset detected

Latest commit: 6eb39e7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@callstack/repack	Patch
@callstack/repack-plugin-expo-modules	Patch
@callstack/repack-plugin-nativewind	Patch
@callstack/repack-plugin-reanimated	Patch
@callstack/repack-dev-server	Patch
@callstack/repack-init	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[dannyhw]

Thanks for this! I can confirm the race, on Rspack too. On the super-app-showcase trading app, the current plugin composed source maps for only 36/66 chunks, and this PR composes all 66 (4.53 MB → 3.98 MB).

Nit (non-blocking): compilation.getAssets() also returns assets that weren't written in this compilation. When rebuilding in the same process (e.g. bundle --watch with caching enabled), unchanged chunks are skipped and are still bytecode on disk, so hermesc fails on them. Normal release builds aren't affected. Recording files in assetEmitted and transforming them in afterEmit keeps the old "only files written this build" behaviour:

let emittedAssets = new Map<string, string>();

compiler.hooks.assetEmitted.tap('RepackHermesBytecodePlugin', (file, { outputPath }) => {
  emittedAssets.set(file, outputPath);
});

compiler.hooks.afterEmit.tapPromise(
  { name: 'RepackHermesBytecodePlugin', stage: -100 },
  async () => {
    const assets = emittedAssets;
    emittedAssets = new Map();
    await Promise.all(
      Array.from(assets, async ([file, outputPath]) => {
        // …same body as now
      })
    );
  }
);

(Rspack doesn't expose webpack's compilation.emittedAssets, hence the Map.) Happy to push this if you'd like.

[emanueleperuffo]

Thanks for this! I can confirm the race, on Rspack too. On the super-app-showcase trading app, the current plugin composed source maps for only 36/66 chunks, and this PR composes all 66 (4.53 MB → 3.98 MB).

Nit (non-blocking): compilation.getAssets() also returns assets that weren't written in this compilation. When rebuilding in the same process (e.g. bundle --watch with caching enabled), unchanged chunks are skipped and are still bytecode on disk, so hermesc fails on them. Normal release builds aren't affected. Recording files in assetEmitted and transforming them in afterEmit keeps the old "only files written this build" behaviour:

let emittedAssets = new Map<string, string>();

compiler.hooks.assetEmitted.tap('RepackHermesBytecodePlugin', (file, { outputPath }) => {
  emittedAssets.set(file, outputPath);
});

compiler.hooks.afterEmit.tapPromise(
  { name: 'RepackHermesBytecodePlugin', stage: -100 },
  async () => {
    const assets = emittedAssets;
    emittedAssets = new Map();
    await Promise.all(
      Array.from(assets, async ([file, outputPath]) => {
        // …same body as now
      })
    );
  }
);

(Rspack doesn't expose webpack's compilation.emittedAssets, hence the Map.) Happy to push this if you'd like.

Sure! I'll take a look at it next Monday if it's ok.

[emanueleperuffo]

Hi @dannyhw I have pushed a new commit with your suggestions. I used a emittedAssets.clear() and a const instead of a let just out of my preference for immutable variables. Let me know if it's ok.

I have tried changing our patch with these changes and it works correctly, tests pass and also rebuilds work.