Fix brew bundle topological sort crash on missing dependency node

[bendrucker]

Fixes a brew bundle crash on Linux where the dependency topological sort dies with key not found: "libice" (or another library name).

Topo#tsort_each_child fetched a node's children with an unguarded fetch(node.downcase). That raises KeyError whenever a dependency edge points at a formula that is not in the node set. Nodes come from installed formulae, where kegs that fail to load are silently dropped. Edges come from formulae_by_name loaded from the tap or API. The two sources can disagree, so an edge target can have no matching key and the sort blows up instead of completing.

The fix defaults a missing node to a leaf (no children), matching the existing defensive pattern at formula_dep_names (find_formula(name)&.fetch(:dependencies, []) || []). I also dropped the .downcase: the Topo keys are stored verbatim from each formula's name/full_name, and the node passed in is always one of those exact strings, so .downcase could only ever break a lookup, never enable one. It was incidental dead code.

Tests

Added a unit spec for Homebrew::Bundle::Brew::Topo: build a graph whose edge references a node with no key of its own, then assert .tsort returns the present nodes in dependency order without raising. Verified red/green with --only=bundle/brew (the example raises KeyError before the change, passes after).

Reproduction

This triggers when an installed keg fails to load and is dropped from the node set while still appearing as a dependency edge of another formula. On an affected Linux machine:

brew bundle dump

crashes in the sort with key not found: "<library>". After this change the sort completes.

I reproduced this in GitHub Actions CI and am hitting it deterministically:

https://github.com/bendrucker/dotfiles/actions/runs/27803390575/job/82279728443

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• AI was used to generate or assist with generating this PR.

AI was used to aid in diagnosing the crash and drafting the fix and test. I reviewed all of the code in detail before opening this PR. Tool: Claude Code (Opus 4.8).

[Copilot]

Pull request overview

Fixes a brew bundle crash during dependency topological sorting when a dependency edge references a formula that is not present in the Topo node set (previously raising KeyError from fetch).

Changes:

• Make Homebrew::Bundle::Brew::Topo#tsort_each_child treat missing nodes as leaves by defaulting to an empty child list.
• Remove the .downcase lookup transformation (keys/nodes are already stored and passed in the correct canonical form).
• Add a unit spec asserting Topo#tsort does not raise when an edge targets a missing node key.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/bundle/brew.rb	Prevents KeyError in Topo#tsort_each_child by defaulting missing nodes to [] (leaf behavior).
Library/Homebrew/test/bundle/brew_spec.rb	Adds a regression test covering an edge that points to a node without a corresponding key.

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[MikeMcQuaid]

Thanks!