Fix chunk-merge bug in ReactFlightWebpackPlugin (v19.0.3)

[AbanoubGhadban]

Summary

• Fix recordModule() in ReactFlightWebpackPlugin to merge chunk arrays instead of overwriting when the same module appears in multiple webpack chunk groups
• This fixes TypeError: r[e] is not a function errors during RSC hydration on slow networks (e.g., 3G + CPU throttling)
• Built from React fork rsc-patches/v19.0.3 with the fix applied

Root Cause

When webpack splits modules across multiple chunks (via chunk groups), the plugin's recordModule() was called multiple times for the same module. Each call overwrote the previous entry with only the current chunk group's chunks, losing earlier chunk references. The RSC flight client then failed to preload all necessary chunks, causing __webpack_require__ to fail when a module factory hadn't been registered yet.

The Fix

Changed from:

filePathToModuleMetadata[href] = { id, chunks, name: '*' };

To merge logic that preserves chunks from all chunk groups:

if (filePathToModuleMetadata[href]) {
  // Merge chunks from additional chunk groups
  const existing = filePathToModuleMetadata[href];
  const existingChunkIds = new Set();
  for (let i = 0; i < existing.chunks.length; i += 2) {
    existingChunkIds.add(existing.chunks[i]);
  }
  for (let j = 0; j < chunks.length; j += 2) {
    if (!existingChunkIds.has(chunks[j])) {
      existing.chunks.push(chunks[j], chunks[j + 1]);
    }
  }
} else {
  filePathToModuleMetadata[href] = { id, chunks: chunks.slice(), name: '*' };
}

Test plan

• Built from fixed React fork branch
• Verify with localhub-demo-search under Slow 3G + 6x CPU throttling
• Confirm zero TypeError errors after 3+ hard reloads

Resolves #19

🤖 Generated with Claude Code

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/chunk-merge-bug-v19.0.3

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[greptile-apps]

Greptile Summary

Fixes a critical bug where recordModule() was overwriting chunk arrays instead of merging them when the same module appears in multiple webpack chunk groups. This resolves TypeError: r[e] is not a function errors during RSC hydration on slow networks.

• Implements Set-based deduplication to merge chunks from multiple chunk groups
• Adds defensive chunks.slice() to avoid reference issues when creating new entries
• Uses if-else branching to handle both existing and new module metadata entries

The fix addresses the root cause by preserving all chunk references needed for proper RSC client-side hydration.

Confidence Score: 4/5

• Safe to merge with minor style improvements suggested
• The logic correctly implements chunk merging to fix a critical hydration bug. Variable shadowing exists but doesn't affect correctness. The Set-based deduplication is sound, and the defensive chunks.slice() prevents reference issues.
• No files require special attention - the single file change is straightforward and addresses the stated bug

Important Files Changed

Filename	Overview
src/react-server-dom-webpack/cjs/react-server-dom-webpack-plugin.js	Fixed chunk array overwriting bug by implementing merge logic with Set-based deduplication; variable shadowing exists but doesn't affect correctness

_{Last reviewed commit: ef99ec0}

[greptile-apps]

_{1 file reviewed, 2 comments}

_{Edit Code Review Agent Settings | Greptile}

[greptile-apps]

Variable shadowing: id and module parameters are being reused as local variables. While this works, it reduces readability.

Suggested change

	id = filePathToModuleMetadata[module];
	module = new Set();
	const existing = filePathToModuleMetadata[module];
	const existingChunkIds = new Set();

_{Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!}

[greptile-apps]

Use descriptive variable name in loop instead of reusing i

Suggested change

	for (var i = 0; i < id.chunks.length; i += 2)
	module.add(id.chunks[i]);
	for (let j = 0; j < id.chunks.length; j += 2)
	module.add(id.chunks[j]);

_{Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!}