Support S3 conditional writes (If-Match / If-None-Match)

[Robothy]

Summary

local-s3 currently does not support S3 conditional writes. A repo-wide search for If-Match / if-match returns zero hits — neither PutObject, CopyObject nor CompleteMultipartUpload evaluate conditional headers.

Why

AWS GA'd conditional writes in Nov 2024. SDKs may send If-None-Match: * by default in some flows, so clients can silently hit unexpected behaviour against local-s3.

Reference: how competitors handle it

adobe/S3Mock has shipped this and hit multiple follow-up bugs — a signal that the feature is in real demand and that the edge cases are non-trivial:

• S3Mock should support conditional requests in all APIs adobe/S3Mock#829 — S3Mock should support conditional requests in all APIs
• s3 conditional writes adobe/S3Mock#2084 — s3 conditional writes
• Support S3 conditional writes for PutObject API adobe/S3Mock#2371 — Support S3 conditional writes for PutObject API
• PutObject with IfMatch condition on etag succeeds instead of throwing NoSuchKeyException adobe/S3Mock#2502 — PutObject with IfMatch condition on etag succeeds instead of throwing NoSuchKey
• ETag quoting mismatch between AWS S3 and S3Mock when using conditional headers adobe/S3Mock#2665 — ETag quoting mismatch between AWS S3 and S3Mock when using conditional headers

Source: adobe/S3Mock two-year issue backfill (see state/findings.json, entry backfill-s3mock-2y).

Scope

• Operations: PutObject, CopyObject, CompleteMultipartUpload (and consider DeleteObject)
• Expected status codes: 412 PreconditionFailed, 409 ConditionalRequestConflict
• Watch out for ETag quoting semantics — S3 returns quoted ETags, comparisons must account for that (see S3Mock#2665)

Acceptance

• Conditional headers evaluated on the operations above
• Correct error codes returned on failed preconditions
• Integration tests covering match / no-match for each operation