Share the yarn berry project gates between hosted and vendored modes

[mikolalysenko]

[agent] Filed by the scheduled architecture audit routine (ecosystems and formats). Register: discussion #560 register.

Kind: refactor. Source: review Part 4.4 ("Yarn berry project gates are written twice") and 4.7 D; register E09.

Problem

The berry project-level refusals (mixed line endings, cacheKey, compressionLevel) are implemented once per mode. Verified on 045d7ec:

Piece	Vendored	Hosted
Supported cacheKey	SUPPORTED_CACHE_KEY	YARN_BERRY_SUPPORTED_CACHE_KEY ("matches the vendored backend's")
cacheKey extraction	berry_metadata + berry_field over scan_blocks	berry_cache_key,`` a split("\n\n") scan ("mirroring the vendored backend's `berry_field`")
Gate driver	refuse_mixed_line_endings / refuse_unsupported_cache / refuse_unsupported_compression + yarn_berry_vendor_preflight	preflight_yarn_berry_hosted
Refusal text	three copies, worded differently	three copies

Only yarnrc_compression_level is already shared (since #508 fixed #370 once for both modes). The rest has drifted:

• Hosted mode doesn't gate the root package.json at all (Hosted yarn berry rewrites a mixed-line-ending package.json that vendored mode refuses #628).
• Vendored mode refuses an unreadable .yarnrc.yml ("cannot read … to verify the cache configuration"). The hosted takeover preflight treats an unreadable one as absent (.ok() in scan/hosted.rs#L1671-L1675).``

Hosted mode also imports vendor::yarn_berry_lock::yarnrc_compression_level (redirect/mod.rs#L30), which is a redirect → vendor layering edge.

Symptoms and impact

#628. Any future berry gate, such as a new cache-format version, would again have to be added twice. The risk is low.

Proposed change

Add formats/yarn/berry_gates.rs, a pure module with no I/O:

• SUPPORTED_CACHE_KEY, plus cache_key(lock_text) -> Option<&str> built on scan_blocks/berry_field;
• yarnrc_compression_level, moved here;
• fn check(lock: &str, manifest: Option<&str>, yarnrc: YarnrcState) -> Result<(), BerryGate>, where BerryGate is { MixedLineEndings { file }, CacheKey { found }, Compression { level }, YarnrcUnreadable } and owns the detail text.

Each mode maps BerryGate to its own code prefix (vendor_yarn_berry_* or redirect_yarn_berry_*), so the codes don't change.

Delete: YARN_BERRY_SUPPORTED_CACHE_KEY, berry_cache_key, the three refuse_* bodies' text, and the hosted refusal text. Keep this PR behavior-neutral apart from what #628 decides: either land #628's policy here, or keep hosted mode's manifest argument None until #628.

Size and scope

About 150 production lines moved and about 90 deleted, across vendor/yarn_berry_lock.rs, patch/redirect/mod.rs, a new formats/yarn/berry_gates.rs and the takeover caller in commands/scan/hosted.rs. Out of scope: the berry entry writers and restorers (E08), and the classic grammar.

Acceptance criteria

• One cacheKey constant, one extractor and one gate function, used by vendored vendor/preflight, the hosted rewriter and the vendored→hosted takeover.
• The codes are unchanged: vendor_yarn_berry_cache_unsupported, vendor_yarn_berry_mixed_line_endings, redirect_yarn_berry_cache_unsupported, redirect_yarn_berry_mixed_line_endings.
• yarn_berry_warning_branches, mixed_line_endings_refuse_before_any_write, the yarnrc_compression_level_* tests and the takeover gate tests from Fix berry mode takeover reverting before gates (#468, #369) #470 stay green.
• A table test feeds the same inputs (cacheKey 10, missing __metadata, compressionLevel: mixed, a mixed lock, a BOM lock) to both modes and asserts the same BerryGate.
• patch/redirect no longer imports from vendor::yarn_berry_lock for gates.

Dependencies

Blocks nothing, and pairs with #628. It is part of review fix D (hosted yarn writers on LockBlock, E08).

[mikolalysenko]

[agent] Shares root cause with #628: the yarn berry project gates are implemented once per mode, and the hosted copy (preflight_yarn_berry_hosted, redirect/mod.rs:3357) checks only yarn.lock for mixed line endings, never the root package.json. Will be fixed together. Triaged as priority:p1 (Yarn Berry).

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[mikolalysenko]

[agent] Claiming this issue (with #628; shared root cause: the yarn berry project gates are written once per mode, and the hosted copy never checks the root package.json). Branch: agent/fix-yarn-berry-shared-gates. Claim-ID: 2026-10-03T05:20:28Z-5423ae

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[mikolalysenko]

[agent] Draft PR: #657

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