[dergachoff]

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• Have you followed our Contributing guidelines?
• Have you checked for other open Pull Requests for the same change?
• Have you explained what your changes do? Performance claims (e.g. "this is faster") must include brew benchmark results.
• Have you explained why you'd like these changes included, not just what they do?
• For bug fixes, have you given step-by-step brew commands to reproduce the bug?
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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

I used Claude Code (Claude Opus 5.5) to find the bug, write the patch and tests, and draft this text. I used Codex CLI (GPT-6 Astra) to review the patch. I reviewed the diff and the test results.

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Problem

Since #23034, brew extracts each cask to var/homebrew/tmp/.caskroom/<token>/<version> before it installs it.

If the install or upgrade fails before the move into the Caskroom, this copy stays.

brew cleanup does not remove it. It only cleans tmp/.cellar.

On my Mac, tmp/.caskroom/chatgpt had 22 old versions (31.4GB).

Steps to reproduce

1. Make a local cask, version 1.0, with an uninstall_preflight block that raises.
2. Install it.
3. Change the version to 2.0.
4. Run brew upgrade --cask <cask>. It fails.
5. Run brew cleanup.
6. ls $(brew --prefix)/var/homebrew/tmp/.caskroom/<cask> still shows 2.0 and 2.0.staged.

Fix

• A failed install or upgrade now removes its extracted copy.
• It does this after the old version is restored.
• If the removal fails, brew prints a warning and shows the original error.
• brew cleanup now also empties tmp/.caskroom. This covers interrupted runs.
• If cleanup cannot remove a folder, it warns and continues.
• New constant HOMEBREW_TEMP_CASKROOM replaces the path string.

Validation

• brew lgtm passes.
• The full brew tests suite passes.
• Each new test fails without its fix.
• On my Mac, the patched brew cleanup removed the 31.4GB.

[Copilot]

Copilot review overview

🟡 Changes recommended

Moderate cleanup and error-preservation issues remain unresolved.

Review effort: Lite
Findings: 2

Open (2)

• Ensure staged copy is purged when restore_backup fails · New
• Ensure staged copy is purged when rollback fails · New

What changed in this PR

This pull request cleans up pre-staged cask downloads after failed installs/upgrades and extends brew cleanup to remove interrupted staging data.

Changes:

• Adds HOMEBREW_TEMP_CASKROOM.
• Purges staged downloads during failure handling.
• Adds cleanup behavior and regression tests.

Outstanding moderate findings concern cleanup being skipped when restore/rollback fails, and staging cleanup errors masking the original failure.

File	Reviewed changes
Library/​Homebrew/​test/​support/​lib/​startup/​config.rb	Adds the test caskroom path constant.
Library/​Homebrew/​test/​spec_helper.rb	Preserves the new constant during tests.
Library/​Homebrew/​test/​cleanup_spec.rb	Tests temporary cask staging cleanup.
Library/​Homebrew/​test/​cask/​upgrade_spec.rb	Tests upgrade-failure cleanup.
Library/​Homebrew/​test/​cask/​installer_spec.rb	Tests install-failure cleanup.
Library/​Homebrew/​startup/​config.rb	Defines the temporary caskroom path.
Library/​Homebrew/​cleanup.rb	Cleans temporary cellar and caskroom contents.
Library/​Homebrew/​cask/​upgrade.rb	Purges staged downloads during upgrade rollback.
Library/​Homebrew/​cask/​installer.rb	Removes staged downloads after install failures.
Library/​Homebrew/​cask/​download.rb	Uses the new caskroom constant.

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[MikeMcQuaid]

Thanks!