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Bug explanation

There's a bug in Webpack's SideEffectsFlagPlugin that causes nondeterministic build output. Depending on the order of compilation.modules (an unordered Set), the SideEffectsFlagPlugin will sometimes not remove a side-effect-free module from the graph when it's re-exporting a module via a namespace export.

In this example, foo.js re-exports bar.js via a namespace re-export (export * as bar from "./bar.js"). If foo.js is ordered after bar.js in compilation.modules, then it correctly gets removed from the module graph. However, if bar.js is ordered before foo.js, then foo.js does not get removed from the graph.

Because this is a small bundle with small modules, the module order is pretty much guaranteed to be the same between runs. In order to repro the issue, I created two small plugins to ensure module orders that replicate the bug (EnsureOrderForTreeShakingPlugin and WrongOrderForTreeShakingPlugin).

To run webpack and repro the bug, you can run pnpm start.

When foo.js is ordered before bar.js, the optimizeDependencies hook callback in SideEffectsFlagPlugin:

  1. Deactivates the connection between app.js and foo.js and creates a new one between app.js and bar.js (when processing foo.js's incomingConnections)
  2. Deactivates the new connection between app.js and bar.js and creates a new one between app.js and baz.js (when processing bar.js's incomingConnections)

When bar.js is ordered before foo.js, the optimizeDependencies hook callback in SideEffectsFlagPlugin:

  1. Does not create a new connection when processing bar.js (hits this code path: https://github.com/webpack/webpack/blob/64707c9c4fd5307d4997f0e73145ca26c5245bc3/lib/ModuleGraph.js#L229)
  2. Deactivates the connection between app.js and foo.js and creates a new one between app.js and bar.js (when processing foo.js's incomingConnections)

Because it never re-processes bar.js's incomingConnections, it is never able to deactivate the connection between app.js and bar.js + create a new one between app.js and baz.js.