Git object-store A3 conformance probe fails on GCS S3-interop (`if_match_race`) — consider a native GCS backend

[vsima]

Environment

• Relay: ghcr.io/block/buzz:sha-2d26db6 (2d26db6d8a7381ff0dc9b50194fd28d42957de74); reproduced identically on sha-acfbb1b (2026-07-22 main)
• Object storage: Google Cloud Storage via the S3-interop XML API (BUZZ_S3_ENDPOINT=https://storage.googleapis.com, HMAC service-account credentials)
• Deployment: GKE, chart buzz 0.1.6, external Postgres/Redis

What happens

Relay startup is fatal at the A3 git conformance gate:

INFO  running git object-store conformance probe (A3 gate)
Error: git conformance probe failed: conformance probe failed in phase 'if_match_race' (round 0, key probe/pointer-0d592977-…)

Media paths on the same bucket/credentials work (Media storage connected;
put/get verified end-to-end through the app).

Why

The probe is right to fail: GCS's S3-interoperability layer does not honor
S3-style conditional writes (If-Match/If-None-Match preconditions on PUT),
so the linearizable pointer CAS that run_conformance_probe verifies (and
that the manifest-pointer protocol needs) cannot be satisfied. This makes
git-on-object-storage unusable on GCS for any deployer using the documented
S3 configuration — the probe converts that into a hard startup failure.

What we did (and why it's not a fix)

Booted the pilot with BUZZ_GIT_CONFORMANCE_PROBE=false and quarantined git
features operationally. That's an availability workaround, not a solution —
the probe's verdict about pointer CAS still stands.

Proposal

GCS does support real CAS — via its native preconditions
(x-goog-if-generation-match: 0 for create-if-absent, generation-match for
compare-and-swap). Options in rough order of preference:

1. Native GCS storage backend for the git object store (JSON/XML API with
   generation preconditions) behind the existing storage trait.
2. Conditional-write shim for GCS endpoints: detect
   storage.googleapis.com and translate the pointer-CAS operations to
   x-goog-* preconditions while leaving plain put/get on the S3 path.
3. At minimum, document that git-on-object-storage requires a backend with
   S3 conditional-write semantics (AWS S3 ≥ 2024 If-Match support, MinIO, R2?)
   and name GCS S3-interop as unsupported — so deployers hit a doc instead of
   a fatal probe.

We (Apiary — hosted Buzz on GCP) are happy to test any of these against real
GCS, and would rather contribute the backend than run a MinIO shim in front
of GCS.

[Chessing234]

Fixed in #2511 — clearer A3 failure text and Helm git.conformanceProbe for skipping on non-CAS backends (e.g. GCS).