[iBug]

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I am using my custom setup that allows multiple Linux users to install Homebrew independently without conflict, by having a FUSE program present /home/linuxbrew/.linuxbrew as a symbolic link that points to the visiting user's home directory. This causes brew install and brew upgrade to intermittently attempt to operate on the root directory:

Error: An unexpected error occurred during the `brew link` step
The formula built, but is not symlinked into /home/linuxbrew/.linuxbrew
Permission denied @ dir_s_mkdir - /linuxbrew
Error: <irrelevant>: Permission denied @ dir_s_mkdir - /linuxbrew

This is reproduceable by having /home/linuxbrew/.linuxbrew as a symbolic link that points to somewhere else and trying to install certain formulae.

This PR fixes the issue described above.

AI Use Disclosure: Codex (GPT-5.6 Sol) identified the problem from the code and made the changes. I personally reviewed the changes before submitting this PR.

[coderabbitai]

No actionable comments were generated in the recent review.

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📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Fixed an issue where linking kegs with a symlinked installation prefix could make headers from another keg unavailable. Headers in nested directories now remain accessible when linking multiple kegs, including when those kegs contain different files in the same nested directory. This keeps the expected headers available under the installation prefix after linking.

Keg.for now constructs the keg path relative to the resolved Cellar path. A regression test checks that linking two kegs with a symlinked prefix leaves a nested header accessible.

Changes: Keg path resolution

Layer / File(s)	Summary
Resolve Cellar paths and test symlinked-prefix linking Library/Homebrew/keg.rb, Library/Homebrew/test/keg_spec.rb	Keg.for constructs the keg path from the Cellar constant and the input path relative to the resolved Cellar. The test checks that a nested header remains accessible after linking two kegs with a symlinked prefix.

Priority: Normal

Merge Risk: Minimal · up to e47bf

The reported symlinked-prefix linking failure is addressed, and the regression test checks that the header remains accessible after the merge. No actionable merge-blocking risk remains after normal checks.

Pre-merge checks: 4

Check name	Status	Explanation
Description check		The description clearly explains the symlinked-prefix linking failure, the intended fix, testing, and AI use disclosure.
Title check		The title clearly and concisely describes the main change: handling symlinked prefixes when linking kegs.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

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[coderabbitai]

Actionable comments posted: 1

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[coderabbitai]

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[MikeMcQuaid]

Is this the only issue you see with this configuration and everything else works perfectly? If so, surprising but cool. My concern is there's likely a bunch of failure states and we drip-drip PRs adding complexity for an unsupported configuration.

[iBug]

Is this the only issue you see with this configuration and everything else works perfectly?

Surprisingly, yes. This is the only issue that I've run into in the past few months with this symlink setup.

It would be nice if we can have this case supported as, you know, Homebrew is not known for allowing each user to install their own copy other than having to recompile everything from scratch.

[MikeMcQuaid]

Ok, let's try this, thanks!