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atlas 67ba28448f swarm-matrix-ctl: one control binary for the matrix container, not one per job

Renames `swarm-matrix-minter` and reshapes it around subcommands. Minting
is now `swarm-matrix-ctl mint`.

Running rust inside `containers.hive-matrix` is not free: it needs its own
store identity, its own cert role and its own bind mounts, and every one of
those is per-*container*, not per-task. A second single-purpose crate would
have had to duplicate that plumbing to add one action, so the next thing
that has to run in there should be a verb here rather than a new crate.
The old name guaranteed the opposite.

`main.rs` is clap dispatch; the minting logic moves to `mint.rs` unchanged.
A bare invocation is refused: `mint` writes a credential, so "no verb"
defaulting to it would make a typo in the unit mint rather than fail.

The environment prefix moves with it, `MATRIX_MINTER_*` → `MATRIX_MINT_*`.
Scoped to the verb and not to the binary, because a binary-scoped prefix is
one the next verb has to share or widen, and a widened one never narrows
again. A test asserts every variable carries the verb's prefix.

The principal renames too. The cert role, bao policy, granting unit, leaf
filename and `certAuthCns` entry all have to spell one string the same way,
so leaving them as `swarm-matrix-minter` would have rebuilt the naming
split this branch exists to remove. Renaming the nix options alongside is
free here: every one of them is introduced by this PR and has never been
released, so no operator config names them yet.

`ExecStart` now names the verb, which is a contract between a nix string
and a clap enum that fails at deploy time with no local signal. Both ends
assert it: `mint_is_spelled_the_way_the_unit_invokes_it` in the crate, and
a new module-eval arm reading the rendered `ExecStart`.

docs/getting-started/setup.md drops the sender token from its "live on the
host" list: setup does not touch this credential, so a setup guide has no
reason to name it.

2026-09-20 22:07:16 +02:00

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swarm-matrix-ctl

The rust that runs inside containers.hive-matrix, beside the homeserver.

One binary with subcommands rather than one binary per job. Running code in that container is not free: it needs its own store identity, its own cert role and its own bind mounts, and every one of those is per-container, not per-task. A second single-purpose crate would have had to duplicate that plumbing to add one action, so the next thing that has to run in here is a verb, not a new crate.

Verbs

`mint`

Puts the appservice sender account's access token into the swarm's secret store, under an identity of its own. A boot-time oneshot.

Configured entirely by the MATRIX_MINT_* environment the unit sets — no flags. A systemd Environment= block is what a nix module can render; a command line full of paths is not. The prefix is scoped to the verb rather than to the binary so the next verb brings its own, instead of widening a shared one nobody can then narrow.

Why this lives in the matrix container

The credential mint writes is authorised by the appservice as_token, and the container already holds that: nix/host-modules/hive-matrix.nix bind-mounts the rendered appservice registration into it read-only, because that is how tuwunel itself is handed the registration. Minting anywhere else would mean copying the as_token to a second holder — and the point of this component is that the hive stops being one.

It is not the swarm controller for the same reason, plus a structural one: a homeserver has exactly one appservice registration and so one sender account, and a swarm runs one homeserver, so "mint it once" needs no lock, no lease and no trigger surface — it is a property of the thing being minted.

Idempotency

The store is the key, not the homeserver. A mint run reads swarm/services/matrix/sender-token first and returns without touching the homeserver when something is already there. Only an empty path reaches the mint ladder:

POST /_matrix/client/v3/register with "type": "m.login.application_service" — one round trip, no UIAA.
M_USER_IN_USE (the expected arm on a homeserver that has already loaded the registration, since the account is the appservice's own sender_localpart) → POST /_matrix/client/v3/login as the appservice, same pinned device_id, so the old device is replaced rather than duplicated.
Write the result to the store.

A crash between the homeserver call and the store write is recoverable: the next run takes arm 2.

🩸 A secret is a path, never a value

Nothing here logs, prints or interpolates a token. The mint ladder's errors are built from the homeserver's status and its errcode, never its body, because a /login response body is an access token. The one identifier this binary logs is the store path it wrote.

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