fix(bootstrap): AIPCC-29282 remove dead cache-server fallback

[jenlevy]

Pull Request Description

What

Remove the unreachable cache-server fallback from multi-version bootstrap resolution, including its resolver API, bootstrapper argument, unused import, and obsolete tests. Retain the AgeFallback coverage.

Why

AgeFallback.NEWEST now guarantees a candidate whenever matches exist, making the cache-server fallback unreachable. Removing it reflects the actual resolution flow and avoids misleading fallback behavior, while leaving the Bootstrapper’s separate cache-server usage unchanged.

Fixes AIPCC-29282
https://redhat.atlassian.net/browse/AIPCC-29282

• PR follows CONTRIBUTING.md guidelines

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📚 Code guidelines (1)

AGENTS.md — auto-discovered

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ℹ️ Recent review info

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• docs/concepts/resolver-architecture.rst
• src/fromager/bootstrap_requirement_resolver.py

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📝 Walkthrough

Walkthrough

BootstrapRequirementResolver no longer accepts a cache wheel server URL or queries a remote cache when source resolution returns no results. Bootstrapper no longer passes the URL. Tests remove cache-server fallback checks. The resolver documentation describes the cache server as a lookup for previously built wheels, separate from source-version resolution.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 7912b

This change removes a cache-server fallback from source-version resolution that is not reached in practice, and it updates the tests and docs to match. Built-wheel cache lookups are unchanged, so merge risk is minimal.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 7912b

The change removes a cache fallback rather than expanding access or authority. Built-wheel reuse remains separate and restricted to the selected version. No introduced security issue was established, but external constructor callers and the broader deployment context were not verified.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — For the reviewed source-resolution path, removal of the fallback eliminates one way remote cache contents could become source candidates. The separate wheel-cache download path remains reachable; this PR does not eliminate that existing artifact trust boundary.

Trust Boundaries and Controls

• observed — Source-mode resolution and built-wheel reuse remain distinct producer and consumer paths. The wheel-cache lookup consumes a pinned resolved version and does not populate the resolver’s known-version state.

Resilience and Maintainability Implications

• observed — A resolver lock serializes known-version and rule-state updates. Exceptions during resolution occur before the rule is marked resolved, allowing retry. Non-exception outcomes are memoized, while Bootstrapper explicitly contains empty-result failures rather than treating them as resolved dependencies.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the removal of the dead cache-server fallback from bootstrap resolution. The issue identifier does not obscure the main change.
Description check	✅ Passed	The description accurately explains the removed fallback, related API and test changes, and the reason for the change.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@src/fromager/bootstrap_requirement_resolver.py`:
- Line 212: Update the source-resolution branch so that when
`sources.get_source_provider()` returns no candidates, resolution falls back to
the configured `PyPICacheProvider` and can select a matching cached wheel;
preserve the existing source-candidate behavior. Add a regression test covering
an empty source-provider result with a matching cached wheel and verify
multi-version resolution returns it.

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ℹ️ Review info

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Reviewing files that changed from the base of the PR and between e724cb4 and a04d316.

📒 Files selected for processing (3)

• src/fromager/bootstrap_requirement_resolver.py
• src/fromager/bootstrapper/_bootstrapper.py
• tests/test_bootstrap_requirement_resolver.py

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• src/fromager/bootstrapper/_bootstrapper.py

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[rd4398]

@jenlevy can you please remove downstream references like "AIPCC" from title and commit messages?

[jenlevy]

@jenlevy can you please remove downstream references like "AIPCC" from title and commit messages?

Removed all references!

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