Normalize HOMEBREW_CELLAR for immutable Fedora

[coffeejunk]

Fedora immutable systems use /home as a symlink to /var/home. Homebrew has code in brew.sh (lines 76-94) to normalize HOMEBREW_PREFIX and HOMEBREW_REPOSITORY for this situation, but it didn't normalize HOMEBREW_CELLAR. This caused a path mismatch when creating symlinks; for example:

• Destination: /home/linuxbrew/.linuxbrew/opt/gum (using symlink)
• Source: /var/home/linuxbrew/.linuxbrew/Cellar/gum/0.17.0 (using real path)

Ruby's relative_path_from then generated broken symlinks like ../../../../var/home/linuxbrew/.linuxbrew/Cellar/gum/0.17.0.

The commit adds normalization for HOMEBREW_CELLAR in Library/Homebrew/brew.sh to ensure it uses /home consistently with HOMEBREW_PREFIX.

Without the patch

$ brew install jq
✔︎ JSON API cask.jws.json
✔︎ JSON API formula.jws.json
==> Fetching downloads for: jq
✔︎ Bottle Manifest jq (1.8.1)
✔︎ Bottle Manifest oniguruma (6.9.10)
✔︎ Bottle oniguruma (6.9.10)
✔︎ Bottle jq (1.8.1)
==> Installing jq dependency: oniguruma
==> Pouring oniguruma--6.9.10.x86_64_linux.bottle.tar.gz
Error: No such file or directory @ rb_sysopen - /home/linuxbrew/.linuxbrew/opt/oniguruma/.sbom.spdx.json20251105-58550-9j303o

$ ls -la /home/linuxbrew/.linuxbrew/var/homebrew/linked/
total 36
drwxr-xr-x. 1 max  max       100 Nov  5 14:22 .
drwxrwxr-x. 1 root linuxbrew  22 Nov  5 13:02 ..
lrwxrwxrwx. 1 max  max        71 Nov  5 14:22 oniguruma -> ../../../../../../var/home/linuxbrew/.linuxbrew/Cellar/oniguruma/6.9.10

With the patch

$ brew install jq
==> Fetching downloads for: jq
✔︎ Bottle Manifest jq (1.8.1)
✔︎ Bottle Manifest oniguruma (6.9.10)
✔︎ Bottle oniguruma (6.9.10)
✔︎ Bottle jq (1.8.1)
==> Installing jq dependency: oniguruma
==> Pouring oniguruma--6.9.10.x86_64_linux.bottle.tar.gz
🍺  /home/linuxbrew/.linuxbrew/Cellar/oniguruma/6.9.10: 16 files, 1.7MB
==> Pouring jq--1.8.1.x86_64_linux.bottle.tar.gz
🍺  /home/linuxbrew/.linuxbrew/Cellar/jq/1.8.1: 21 files, 1.4MB
==> Running `brew cleanup jq`...
Disable this behaviour by setting `HOMEBREW_NO_INSTALL_CLEANUP=1`.
Hide these hints with `HOMEBREW_NO_ENV_HINTS=1` (see `man brew`).
$ jq
jq - commandline JSON processor [version 1.8.1]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

[Copilot]

Pull Request Overview

This PR adds support for systems where HOMEBREW_CELLAR's parent directory is a symlink (e.g., Fedora Silverblue) and fixes a typo in a related comment. The changes ensure that when the resolved path of the default cellar location matches the current HOMEBREW_CELLAR, the default prefix path is used instead for consistency.

• Added symlink resolution logic for HOMEBREW_CELLAR similar to existing logic for HOMEBREW_PREFIX and HOMEBREW_REPOSITORY
• Fixed typo in comment at line 82 (added missing / in /var/home)

Comments suppressed due to low confidence (1)

Library/Homebrew/brew.sh:90

• Corrected spelling of 'var/home' to '/var/home'.

# Example: Fedora Silverblue symlinks /home -> var/home

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[MikeMcQuaid]

Thanks, makes sense to me and don't think it should cause any regressions on other configurations. Great work @coffeejunk, you rock!

Maintainers: if this does cause ANY reported regressions, please favour reverting rather than waiting for someone else to fix forward ❤️