hyperhive/swarm-secret-client/README.md

swarm-secret-client

Reading and writing a swarm credential in the secret store, over vaultrs. The HTTP is that crate's job. What this one owns is the agreements both ends of the store have to state identically: where a credential lives, which field its bytes are in, and how this deployment's environment becomes a logged-in client.

The surrounding picture — which secret is minted where, and why delivery is a copy rather than a bind mount — is docs/swarm/secrets.md.

Why a crate and not a module per binary

The store has two Rust ends and they are peers: the swarm controller writes a credential, a hive reads it. Neither is senior to the other, so a path formatted at each call site is an agreement with no owner — it holds right up until one side is edited alone, and then it fails as a missing key rather than as a mismatch.

The third end is what settles it. nix/host-modules/glue-matrix-bao-token.nix reads the store with bao kv get -field=value. That reader is a shell line in a nix module: it cannot be renamed by the same refactor as a Rust struct, and no Rust test reaches it. So the field name is pinned by a test against the serialised literal rather than left to the struct definition.

The identity is a certificate, and the role is the hive's name

Authentication is the store's cert auth method. nix/host-modules/glue-bao-tls.nix mints the client certificate with its CN set to the hive's name, because a cert-auth role matches on the CN. So cert_role is not a free choice for the caller: a hive passes its own name, and the policy attached to that role is what scopes what it may read.

The listener's tls_require_and_verify_client_cert is a different thing and not a substitute. It decides who may open a connection; it says nothing about who the connection belongs to, and a store with the option set and no cert mount configured refuses every login made here. That refusal is what Error::Vault out of connect means.

Configuration

Settings::from_env reads BAO_ADDR, BAO_CLIENT_CERT, BAO_CLIENT_KEY, and the optional BAO_CACERT.

The BAO_ spellings are read explicitly rather than left to vaultrs. Its own defaults look for VAULT_ADDR / VAULT_CLIENT_CERT / VAULT_CLIENT_KEY, which no unit in this tree sets. Falling through to them builds a client with no identity at all, and that surfaces as a TLS handshake failure — a place that names neither the variable nor the reason.

Empty is as absent as unset. systemd renders an unset nix option as Environment=BAO_CACERT=, so empty is the shape a missing value arrives in.

The certificate and key are paths, not values, and are read at connect time — same rule as every other credential in this tree, for the same reason: a value in a nix expression is rendered into the world-readable store.

Reading the environment is separate from connecting (Settings::from_lookup) because every one of those failures is a misconfiguration an operator has to read an error about, and none of them needs a reachable store to happen.

Names that arrive from elsewhere

path::matrix_account is fallible, which for a string formatter needs saying: its segments are an agent name from the topology and an account name from that agent's own config. A / turns one agent's segment into another agent's directory and .. walks out of the prefix entirely, so the charset it accepts is deliberately narrower than what the store would.

What this crate does not do

It has no opinion on what a caller may read. That is the policy attached to the cert role, and it lives in the store.

It holds the token minted at login and renews nothing. A handle is built per credential, so the login is the cheap part of a rare operation — a caller that wanted to keep one alive across a token's lifetime would need more than this.