feat(cache): support conditional requests on Cache.Open and Stat

[alecthomas]

Summary

Unifies conditional-request handling into a single option set shared by the client wire protocol, the cache backends, and the server handlers. Previously If-Match/If-None-Match were only evaluated at the HTTP layer (httputil.ServeCacheHit), and Cache.Open/Stat had no way to express preconditions — so the Remote tier couldn't forward them upstream and backends always read the body before a 304/412 could be decided.

This pushes precondition evaluation down into the cache, establishing the option plumbing for forthcoming Range / If-Range support.

Changes

• client: replace the http.Request-based RequestOption with an inspectable RequestOptions struct (IfMatch/IfNoneMatch) plus a Check(etag) evaluator, so non-HTTP backends can evaluate preconditions locally. The matching logic (etagListMatches) moves here from httputil.
• internal/cache: Cache.Open/Stat accept variadic Option args (an idiomatic alias of client.RequestOption). Disk/Memory/S3 short-circuit with ErrNotModified/ErrPreconditionFailed before reading the body; Remote forwards opts to upstream (zero translation — same type); Tiered treats the sentinels as definitive (no backfill, headers preserved for 304).
• internal/httputil: ServeCacheHit/ServeCacheStat now serve the Open/Stat outcome, mapping sentinels to 304/412; CheckConditionals delegates to the unified client logic (still used by the git snapshot path).
• handlers: apiv1 and the generic caching handler parse request conditionals and push them into Open/Stat.

Note on S3

S3's native conditional options (SetMatchETag etc.) match against S3's object ETag, but cachew advertises a SHA256 content-hash ETag stored in the companion .meta object — a different value. So preconditions are evaluated locally against the stored ETag rather than delegated to S3.

Tests

• Reworked httputil/conditional_test.go for the outcome-based serve helpers.
• Added a Conditional case to the shared cache suite, exercising If-Match/If-None-Match on Open and Stat across all backends (including Remote end-to-end).

🤖 Generated with Claude Code