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refactor(swarm-queue-client): extract the queue connect into a shared crate

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A hive publishing its own status needs the same connect the controller
already has - mint an authelia token, present it at CONNECT for the
callout responder, let async-nats re-run the callback per attempt. Only
the use differs: the controller reads, a hive writes.

Copying it would put credential handling in two places, and a
token-refresh fix would then have to be found twice. That is the same
reasoning that already put hive-sock-client in its own crate rather than
in each daemon that speaks to a unix socket.

`from_env` takes a prefix rather than hardcoding SWARM_CONTROLLER_*: the
variables belong to the consuming unit, since a NixOS module sets them
alongside its other options. What is shared is the RULE - all four
together or none at all - not the spelling. The half-set case gains a
test, because it is the case the rule exists for and it previously had
none.

No jetstream/kv feature on the crate: it ends at a connected client, and
what a consumer does with it should be visible in that consumer's own
Cargo.toml.

Behaviour-preserving, and proven that way rather than by inspection: the
full behavioural gate (real nats-server, credential rotation, mutation)
is 20/0 unchanged, and the controller's own tests still pass.
This commit is contained in:
atlas 2026-08-15 21:43:07 +02:00 • committed by mara
commit a9603214c2
7 changed files with 166 additions and 33 deletions
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21
swarm-queue-client/Cargo.toml Normal file
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[package]
name = "swarm-queue-client"
version.workspace = true
readme = "README.md"
edition.workspace = true
[dependencies]
anyhow.workspace = true
# No `kv`/`jetstream` feature here on purpose: this crate's job ends at a
# connected client. What a consumer does with it - KV for the controller and
# the hive, plain messaging for anything later - is the consumer's business,
# and its Cargo.toml is where that requirement should be visible.
async-nats.workspace = true
reqwest.workspace = true
serde.workspace = true
serde_json.workspace = true
tokio.workspace = true
tracing.workspace = true
[lints]
workspace = true

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swarm-queue-client/README.md Normal file
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# swarm-queue-client
Connecting to the swarm message queue as an authenticated client. Shared by
every process that participates: the swarm controller reads hive status out of
the queue, a hive publishes its own status into it.
## Why a crate and not a module per binary
The *connect* is identical for every participant — mint an authelia token,
present it at CONNECT for the `auth_callout` responder to introspect, let
`async-nats` re-run the callback on each connection attempt. Only the **use**
differs.
Two copies of that would be two copies of credential handling, and a
token-refresh fix would have to be found twice. The same reasoning already put
`hive-sock-client` in its own crate rather than in each daemon that speaks to a
unix socket.
## The two properties that constrain the code
**A token expires.** Authelia issues `client_credentials` access tokens with
`expires_in: 3599`. Authentication happens at CONNECT, so a long-lived
connection is fine — but a *reconnect* an hour later needs a token minted an
hour later.
**The refresh therefore lives in the auth callback, not in a timer.**
`async-nats` invokes it per connection attempt, so there is no window in which
the client holds a token it minted for a previous connection. The alternative —
mint once, own the reconnect loop — fails in the way this subsystem exists to
prevent: the process keeps serving while its data quietly stops moving, and
nothing says so until someone reads a dashboard.
## Configuration
`QueueConfig::from_env(prefix)` reads `<prefix>_NATS_URL`,
`<prefix>_OIDC_TOKEN_ENDPOINT`, `<prefix>_OIDC_CLIENT_ID` and
`<prefix>_OIDC_CLIENT_SECRET_FILE`.
The prefix is a parameter because the variables belong to the consuming unit —
a NixOS module sets them alongside its other options. What is shared is the
rule, not the spelling: **all four together or none at all.** A half-set
environment is a hard error, because the failure it would otherwise produce is
the expensive kind — the process comes up "fine", never connects, and the data
it was supposed to move silently stops.
The client secret is a **path, not a value**: putting it in the environment
would publish it to anything that can read `/proc/<pid>/environ`. It is read
per token request rather than cached, so a rotation the operator believes took
effect actually did.
## What this crate does not do
It ends at a connected client. No `jetstream`/`kv` feature is enabled here —
what a consumer does with the connection is its own business, and its
`Cargo.toml` is where that requirement should be visible.

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swarm-queue-client/src/lib.rs Normal file
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//! Connecting to the swarm message queue as an authenticated client.
//!
//! Shared by every process that needs the queue — the swarm controller reads
//! hive status out of it, a hive publishes its own status into it — because
//! the *connect* is identical for all of them and only the use differs.
//! Duplicating it per binary would put credential handling in two places, and
//! a token-refresh fix would then have to be found twice.
//!
//! The queue admits every non-responder client through `auth_callout`: a
//! client presents a token at CONNECT, the callout responder introspects it
//! against authelia and mints a user JWT if it is good. So each participant is
//! an ordinary client that needs an identity of its own — the controller is
//! not a hive, and the per-hive clients issued from the roster are not its to
//! use.
//!
//! Two things about that shape drive everything here:
//!
//! - **A token expires.** Authelia issues `client_credentials` access tokens
//! with `expires_in: 3599`. Authentication happens at CONNECT, so a
//! long-lived connection is fine — but a *reconnect* an hour later needs a
//! token that was minted an hour later.
//! - **`async-nats` re-runs an auth callback per connection attempt** (it is
//! handed that attempt's nonce). So the refresh belongs in the callback and
//! not in a timer: there is no window in which the client holds a token it
//! minted for a previous connection.
//!
//! The alternative — mint once, pass a static `auth_token`, own the reconnect
//! loop — fails in the way this subsystem exists to prevent: the controller
//! keeps serving, its status data quietly stops updating, and nothing says so
//! until someone reads a dashboard.
use std::path::PathBuf;
use anyhow::{Context, Result, bail};
/// Only the one field this needs; authelia returns several.
#[derive(serde::Deserialize)]
struct TokenResponse {
access_token: String,
}
/// Where the controller finds the queue and what it authenticates with.
///
/// Every field comes from an environment variable the NixOS module sets, the
/// same way `load_hives` takes the roster — a config change is a redeploy, and
/// this process reads no file it was not pointed at.
#[derive(Debug, Clone)]
pub struct QueueConfig {
/// `nats://host:port` for the swarm queue.
pub url: String,
/// Authelia's token endpoint, e.g. `https://auth.<swarm>/api/oidc/token`.
pub token_endpoint: String,
/// The controller's own `OAuth2` client id.
pub client_id: String,
/// File holding the client secret's PLAINTEXT.
///
/// A path and not a value: the secret is minted on the authelia host and
/// read here, and putting it in the environment would publish it to
/// anything that can read `/proc/<pid>/environ`.
pub client_secret_file: PathBuf,
}
impl QueueConfig {
/// Read the config from `<prefix>_NATS_URL`, `<prefix>_OIDC_TOKEN_ENDPOINT`,
/// `<prefix>_OIDC_CLIENT_ID` and `<prefix>_OIDC_CLIENT_SECRET_FILE`, or
/// `None` when the queue was not wired up for this deployment.
///
/// The prefix is a parameter rather than a constant because the variables
/// belong to the *consuming unit* — a NixOS module sets them alongside its
/// other options, and two daemons sharing one name would be a worse
/// coupling than passing four characters. What is shared is the RULE
/// below, not the spelling.
///
/// `None` rather than an error on purpose: a daemon serves its other
/// surfaces on hosts where the queue is not enabled, and refusing to start
/// there would trade a missing feature for a dead process. What must NOT
/// happen is a *half* configuration silently behaving like an absent one —
/// hence the explicit partial check below.
pub fn from_env(prefix: &str) -> Result<Option<Self>> {
let url = std::env::var(format!("{prefix}_NATS_URL")).ok();
let token_endpoint = std::env::var(format!("{prefix}_OIDC_TOKEN_ENDPOINT")).ok();
let client_id = std::env::var(format!("{prefix}_OIDC_CLIENT_ID")).ok();
let secret = std::env::var(format!("{prefix}_OIDC_CLIENT_SECRET_FILE")).ok();
match (url, token_endpoint, client_id, secret) {
(None, None, None, None) => Ok(None),
(Some(url), Some(token_endpoint), Some(client_id), Some(secret)) => Ok(Some(Self {
url,
token_endpoint,
client_id,
client_secret_file: PathBuf::from(secret),
})),
// A partially-set environment is a deployment bug, and the failure
// it would otherwise produce is the expensive kind: the process
// comes up "fine", never connects, and the data it was supposed to
// move silently stops moving. Naming the variables costs one line.
_ => bail!(
"swarm queue is half-configured: {prefix}_NATS_URL, \
{prefix}_OIDC_TOKEN_ENDPOINT, {prefix}_OIDC_CLIENT_ID and \
{prefix}_OIDC_CLIENT_SECRET_FILE must be set together or not \
at all"
),
}
}
}
/// Mint a fresh access token for the controller's own client.
///
/// `client_credentials`, because there is no user here: the controller
/// authenticates as itself. Authelia refuses the `openid` scope for this grant
/// (a machine client receives an access token and never an id-token), so no
/// scope is requested.
async fn mint_token(http: &reqwest::Client, cfg: &QueueConfig) -> Result<String> {
// Read per call rather than caching: the file is small, and a cached
// secret would survive a rotation that the operator believes took effect.
let secret = tokio::fs::read_to_string(&cfg.client_secret_file)
.await
.with_context(|| {
format!(
"reading the queue client secret from {}",
cfg.client_secret_file.display()
)
})?;
let response = http
.post(&cfg.token_endpoint)
.form(&[
("grant_type", "client_credentials"),
("client_id", cfg.client_id.as_str()),
("client_secret", secret.trim()),
])
.send()
.await
.context("requesting an access token from authelia")?;
// The body carries authelia's own error description, and it is far more
// useful than the status alone: a wrong grant says `unauthorized_client`,
// a wrong secret says `invalid_client`, and those point at different
// config.
let status = response.status();
let body = response.text().await.unwrap_or_default();
if !status.is_success() {
bail!("authelia refused the controller's token request ({status}): {body}");
}
let parsed: TokenResponse =
serde_json::from_str(&body).context("parsing authelia's token response")?;
Ok(parsed.access_token)
}
/// Connect to the swarm queue, minting a token for each connection attempt.
pub async fn connect(cfg: QueueConfig) -> Result<async_nats::Client> {
// A timeout, because this client runs INSIDE the auth callback: a token
// endpoint that accepts the connection and then never answers would hang
// the callback, and with it the connection attempt that invoked it, with
// no retry and nothing in the log to say why. Failing fast lets
// `async-nats` do what it already does well — back off and try again.
// 10s is generous for a form POST to a local IdP.
let http = reqwest::Client::builder()
.timeout(std::time::Duration::from_secs(10))
.build()
.context("building the token-endpoint HTTP client")?;
let url = cfg.url.clone();
let client = async_nats::ConnectOptions::with_auth_callback(move |_nonce| {
let http = http.clone();
let cfg = cfg.clone();
async move {
let token = mint_token(&http, &cfg)
.await
// The callback's error type carries a string, so the context
// chain would be lost; flatten it rather than dropping it.
.map_err(|e| async_nats::AuthError::new(format!("{e:#}")))?;
let mut auth = async_nats::Auth::new();
auth.token = Some(token);
Ok(auth)
}
})
// The controller and the queue are separate units on (possibly)
// separate hosts, and nothing orders them. Without this, a queue that
// comes up one second later leaves the controller permanently
// queue-less until someone restarts it — a boot-order race that
// presents as "status has been unavailable since Tuesday".
//
// It also composes with the callback above rather than fighting it:
// each background attempt is a connection attempt, so each one mints
// its own token instead of retrying a stale one.
.retry_on_initial_connect()
.connect(&url)
.await
.with_context(|| format!("connecting to the swarm queue at {url}"))?;
Ok(client)
}
#[cfg(test)]
mod tests {
use super::*;
/// The all-unset case is the common one — most hosts do not run the queue.
///
/// Uses a prefix no deployment sets, so it cannot pass vacuously by
/// running inside a configured environment (the guard below covers the
/// same ground, and both are cheap).
#[test]
fn an_absent_environment_is_not_an_error() {
for k in [
"SWARM_QUEUE_TEST_NATS_URL",
"SWARM_QUEUE_TEST_OIDC_TOKEN_ENDPOINT",
"SWARM_QUEUE_TEST_OIDC_CLIENT_ID",
"SWARM_QUEUE_TEST_OIDC_CLIENT_SECRET_FILE",
] {
assert!(std::env::var(k).is_err(), "{k} must be unset for this test");
}
assert!(
QueueConfig::from_env("SWARM_QUEUE_TEST")
.expect("absent is not an error")
.is_none()
);
}
/// The half-set case is the one the rule exists for: a deployment bug that
/// would otherwise look exactly like "no queue configured".
///
/// SAFETY: single-threaded mutation of a process env var under a prefix no
/// other test or deployment uses; removed before returning.
#[test]
fn a_half_set_environment_is_a_hard_error() {
unsafe {
std::env::set_var("SWARM_QUEUE_HALF_NATS_URL", "nats://127.0.0.1:4222");
}
let err = QueueConfig::from_env("SWARM_QUEUE_HALF")
.expect_err("a partial set must not read as absent");
let msg = format!("{err}");
assert!(
msg.contains("SWARM_QUEUE_HALF_OIDC_CLIENT_ID"),
"the error must name the missing variables, got: {msg}"
);
unsafe {
std::env::remove_var("SWARM_QUEUE_HALF_NATS_URL");
}
}
}