I3951 — The mesh store is spoken natively: signed S3 requests, bounded in time, with the credential read from a private file and never answered

✕BLOCKED wave 1 · p0 · implementation profile · runs alone

Part of builds-reuse-the-mesh-cache — Builds reuse the mesh cache: compiled Rust, the cargo build directory, mix deps and npm packages, keyed by platform, toolchain and inputs.

Objective

An S3 client under mesh/build_cache/: AWS Signature V4 over a bounded HTTP/1.1 connection, HEAD, GET into a file and PUT from a file, path-style; the node's credential read from the AWS-format file of mode 0600 that sccache also reads (profile mesh-build-cache), never from the environment.

Why

The owner, 2026-10-22, after lundra (the build and deploy host) reached zero free bytes: the shared cache must be part of Majordomus — used when MinIO is available, enforced, dynamic, derived — for compiled Rust, the shared cargo build directory, mix deps.get and npm install, keyed by a triplet for the most reuse. Decided the same day after critique: the store is trusted only through a signed offer (I2671), the build directory archive is measured before it is built (I3955), and the dependency cache is generic and wired first into prismatic-platform. See ADR 0157 and intent a-build-reuses-what-the-mesh-already-built.

Current state

Measured on master 2034d4a3f0b on 2026-10-22: no key derivation, no S3 client and no dependency cache exist in the crate; I2670 (#1118) holds the offer model and the pure selection only.

Desired state

The crate can probe, read and write a store with no dependency added, and no answer, log line, error or child environment carries the secret.

Scope

• apps/majordomus-cli/src/mesh/build_cache.rs
• apps/majordomus-cli/src/mesh/build_cache/
• .ai/repo/project/intents/a-build-reuses-what-the-mesh-already-built.yaml
• test/cases/3951_the_mesh_store_is_spoken_natively.sh

Out of scope

• Multipart upload: an object larger than one PUT is not saved, and says so
• Choosing a store (I2671, I2672) and provisioning one (I2674)

Dependencies

• I2670○READY A build-cache offer is a typed model and its selection is a pure, explained function

What waits on this

• I3952✕BLOCKED A dependency install is restored before and saved after, verified by its digest, local level first and a trusted store second

Acceptance criteria

• The signature and HMAC reproduce the AWS SigV4 GET Object example and RFC 4231 byte for byte
• A credential file readable by its group or others is refused, with the cause named and no value
• Every network call has a bound; a store that does not answer within it is unknown, never down
• A refused credential, a full store and a body cut short are distinct errors, and a cut body leaves no file

Validation

• bin/majordomus plan validate
• MJ_TEST_JOBS=2 bash test/run.sh 3951_the_mesh_store_is_spoken_natively

Evidence required

• sigv4_vectors
• store_failure_paths_case

Evidence

None recorded. Every token above needs a command or an artifact behind it before this issue can be completed; narrative is refused.

Timeline

started

—

verified

—

completed

—

Those three fields, the evidence above and the state of the dependencies are all the status is made of. There is no status field to disagree with them.

Canonical record: .ai/repo/project/issues/I3951.yaml. Read it back with majordomus plan show I3951.