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niksis02
a4d4519ffe
The S3 gateway and the standalone IAM service exchange authorization decisions over the private endpoints, where a version skew is silently unsafe in both directions: an older service drops a request field it does not know (a `Condition` block, say) and evaluates fail-open, while an older gateway ignores a response field it does not know and misses a deny the service intended. Neither side could previously detect either case. Both peers now declare a protocol version on every exchange via the `X-Vgw-Private-Protocol` header — the gateway on each request, the service on each response, error responses included — and each refuses a peer it cannot serve safely. The service rejects a gateway below `MinClientProtocol` with a `ProtocolMismatch` code; the gateway rejects a service older than the `ProtocolVersion` it speaks, and rejects a response carrying no version at all, since no build of this protocol omits the header and something else answering on that address should not be interpreted as an IAM decision. `ParseProtocolVersion` is shared by both sides and deliberately strict: an unreadable value is a mismatch, never an assumed default. A new root-signed `/private/version` endpoint reports the protocol version, the minimum client the service will serve, and the build tag (`WithPrivateServerVersion`). It is exempt from the service's own client-version check so it can still answer a gateway the service refuses — which is how that gateway learns why. Being authenticated like every other private endpoint, it also lets the gateway's startup probe verify its own credential and its mTLS transport in the same round trip. The gateway probes it once in `NewIAMServiceStandalone` rather than discovering a skew as an opaque per-request 500. An incompatible service is fatal after a 30s window, since a gateway that cannot authorize a single request is more useful refusing to start with the reason in its log; an unreachable one is only a warning, because the two processes legitimately start in parallel and every request checks the version regardless. Only conditions that can resolve on their own are retried — a rejected credential is reported immediately.