feat(cache): stamp S3 content ETag onto disk copy at write time#326
feat(cache): stamp S3 content ETag onto disk copy at write time#326worstell wants to merge 1 commit into
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A tiered Create writes the same byte stream to disk and S3, but only an S3-backed Open backfill persisted the content ETag, so freshly generated disk entries weren't pinnable and pinned ranges proxied through S3 until a backfill ran. Capture S3's ETag from PutObject and stamp it onto the disk copy on Close, so the disk tier serves pinned ranges immediately. Best effort: a stamp failure only falls pinned reads back to S3.
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| w.headers = http.Header{} | ||
| } | ||
| w.headers.Set(ContentETagHeader, etag) | ||
| return errors.WithStack(w.disk.db.set(w.key, w.namespace, w.expiresAt, w.headers)) |
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When two tiered Creates for the same key close concurrently, the first disk writer can commit its file, wait for its S3 upload, and then run this stamp after a later writer has already renamed a different file over w.path. Because this only checks that some file exists before rewriting metadata with this writer's S3 ETag, it can attach an old pin to newer disk bytes; a client holding the old pin would then be served the wrong range from disk instead of falling through to S3 and getting ErrPinStale. The stamp needs a generation/same-file check, or it must happen while the disk commit is still known to be the current entry.
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Stacks on #324.
A tiered
Createfeeds the same byte stream to disk and S3, but only an S3-backedOpenbackfill persisted the content ETag onto the disk copy. So freshly generated disk entries weren't pinnable, and pinned ranges proxied through S3 until a backfill happened to run — defeating disk-first range serving for hot repos.This captures S3's ETag from
PutObjectand stamps it onto the disk copy when the tiered writer closes. Both tiers were written from the same bytes, so the ETag is valid for the disk copy and it serves pinned ranges immediately. Stamping is best effort: a failure only falls pinned reads back to S3, and a disk ETag that doesn't match the authoritative S3 pin simply misses to S3.