fix: invalidate the pack when the generator changes

[alexander-akait]

Summary

generate rewrites a module's own build result, and the filesystem cache restores that result without rebuilding the module — so changing generate or generatorOptions changed nothing. Reproduced: a second run configured for avif emitted image.webp with the first run's bytes and never called the new generator. Silent, and only deleting the cache directory recovered.

Nothing per-module keys on a plugin, so the generator's identity goes into cache.version. Identity is the implementation's source rather than getMinimizerVersion, since a generator already travels to a worker as source and a custom one reports no version. This reaches the cache only from a configured plugin — plugins or optimization.minimizer — because webpack builds the cache strategy while it applies those; the README says so.

Based on #721 rather than main: the README paragraph this extends is added there. Please merge that one first.

What kind of change does this PR introduce?

fix

Did you add tests for your changes?

Yes — test/generate-filesystem-cache.test.js, four cases across two compiler runs sharing a cache directory. The rename surviving a restore (with the module asserted cached, not built) and changed bytes re-encoding both pass without the fix; a changed generator and changed generatorOptions both fail without it.

Does this PR introduce a breaking change?

No. A pack written before this is invalidated once, since the version it was written under no longer matches.

If relevant, what needs to be documented once your changes are merged or what have you already documented?

Documented here, in the generate section: that the pack is invalidated by a changed generator, and that the plugin has to be in the config for it to be — a plugin applied by hand after webpack() returns is too late.

Use of AI

Claude Code found and fixed this. It went looking after the tests in #721 closed the watch-mode gap, first ruling out the canonical defect by running three builds under infrastructureLogging.debug and confirming zero Pack got invalid because of write to: lines with the probe shown to be capturing output. The bug then surfaced on the next hypothesis. Its first probe missed it — two generators built by the same factory had identical source and differed only by closure, which is also what settled on source as the identity — and the fix was confirmed by A/B on one probe with nothing else changed. Reading webpack's own ordering is what established the plugins requirement, after the first version of the test applied the plugin post-construction and failed.

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🤖 Generated with Claude Code

https://claude.ai/code/session_016TQeNpahUSDUjD2Crugy5H

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Generated by Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 8aa3bfa

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

This PR includes changesets to release 1 package

Name	Type
minimizer-webpack-plugin	Patch

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.88%. Comparing base (cc1f712) to head (8aa3bfa).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #722      +/-   ##
==========================================
+ Coverage   94.73%   94.88%   +0.14%     
==========================================
  Files           4        4              
  Lines        1102     1114      +12     
  Branches      386      391       +5     
==========================================
+ Hits         1044     1057      +13     
+ Misses         51       50       -1     
  Partials        7        7              

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