swarm: serve an agent's icon at swarm scope

An agent is not fixed to a hive, so its icon cannot be resolved as
hive -> agent. This adds the swarm-level half: an `agent-icons` KV
bucket keyed by the agent name alone — no hive token, so an agent that
moves hives keeps its icon and one that is stopped still has one — and
`GET /api/agents/<name>/icon` on swarm-controller serving it
same-origin, like every other `/api/*` route swarm-ui calls.

404 is the "this agent has no icon" answer, the same contract the
per-agent harness's own `GET /icon` has for an unconfigured agent.
Until the agent-side publisher lands, that is every agent's answer:
the publisher runs inside the container and an agent's NATS grants are
hive-scoped, which cannot authorise a write to a single-token agent
key. The read side needs no grant change — the controller already
holds `$KV.*.>` and `$JS.API.DIRECT.GET.*.>`.

The response carries `Content-Security-Policy: sandbox` and `nosniff`:
the body is an operator-authored SVG served from this daemon's own
origin, and an SVG can carry script.

Hive-side icon serving is untouched.

Refs #4502

swarm-controller/src/agent_icon.rs

@@ -0,0 +1,57 @@
+//! Reads an agent's icon out of the swarm's agent-icon KV bucket.
+//!
+//! Sibling of [`crate::agent_status`] and built the same way — the bucket
+//! is resolved on first use and cached, a resolution failure is not — but
+//! a much smaller read: one key, no roster to be complete against and no
+//! freshness to derive. An icon is not a report about the agent, it is a
+//! property of it, so there is nothing for a timestamp to mean here.
+//!
+//! **No hive is named on this path**, because the key does not carry one —
+//! see `swarm_queue_client::agent_icon` for why, and for the grant work
+//! the publish side is still waiting on.
+
+use anyhow::{Context, Result};
+
+/// Reads the agent-icon bucket. Holds a NATS client rather than a bucket
+/// handle, so a controller that starts before the bucket exists picks it
+/// up without a restart.
+pub struct AgentIconReader {
+    client: async_nats::Client,
+    store: tokio::sync::OnceCell<async_nats::jetstream::kv::Store>,
+}
+
+impl AgentIconReader {
+    #[must_use]
+    pub fn new(client: async_nats::Client) -> Self {
+        Self {
+            client,
+            store: tokio::sync::OnceCell::new(),
+        }
+    }
+
+    async fn store(
+        &self,
+    ) -> std::result::Result<&async_nats::jetstream::kv::Store, swarm_queue_client::Error> {
+        self.store
+            .get_or_try_init(|| swarm_queue_client::agent_icon::open_or_create(&self.client))
+            .await
+    }
+
+    /// `agent`'s icon, or `None` when it has published none.
+    ///
+    /// The bytes are returned unopened: this daemon does not parse, rewrite
+    /// or validate the SVG, and a consumer that renders it must treat it as
+    /// the untrusted document it is — see `get_agent_icon`'s response
+    /// headers.
+    pub async fn get(&self, agent: &str) -> Result<Option<Vec<u8>>> {
+        // An unconnected client does not fail a JetStream request, it hangs
+        // on it — see `swarm_queue_client::ensure_connected`.
+        swarm_queue_client::ensure_connected(&self.client)?;
+        let store = self.store().await?;
+        let icon = store
+            .get(agent)
+            .await
+            .with_context(|| format!("reading the agent-icon entry for {agent}"))?;
+        Ok(icon.map(Into::into))
+    }
+}

swarm-controller/src/main.rs

@@ -40,6 +40,7 @@ use swarm_authelia_bridge_sock::BridgeResponse;
 use utoipa::{OpenApi, ToSchema};
 use utoipa_axum::{router::OpenApiRouter, routes};
 
+mod agent_icon;
 mod agent_identity;
 mod agent_state_stream;
 mod agent_status;
@@ -607,6 +608,10 @@ struct AppState {
     /// connection, several consumers" rationale as `wanted` above. `None`
     /// in exactly the state `status` is.
     agent_status: Option<Arc<agent_status::AgentStatusReader>>,
+    /// Per-agent icons, sharing `status`'s connection — same "one queue
+    /// connection, several consumers" rationale as `agent_status` above,
+    /// and `None` in exactly the state `status` is.
+    agent_icons: Option<Arc<agent_icon::AgentIconReader>>,
     /// The swarm-level job graph, wrapped in its
     /// [`hive_jobq::scheduler::Scheduler`] now that something drives it
     /// (`spawn_jobq_worker`) — the graph alone was enough for the
@@ -862,7 +867,8 @@ fn error_problem(status: axum::http::StatusCode, detail: &str) -> problem_detail
     problem_details::ProblemDetails::from_status_code(status).with_detail(detail)
 }
 
-/// The status a [`wanted::WantedWriter`] failure should answer with.
+/// The status a [`wanted::WantedWriter`] or [`agent_icon::AgentIconReader`]
+/// failure should answer with.
 ///
 /// `WantedWriter::view`/`set` call `ensure_connected` internally and
 /// propagate through `anyhow`, so the concrete
@@ -936,6 +942,16 @@ fn agent_status_reader(
     })
 }
 
+/// The per-agent icon reader, sharing the status reader's connection —
+/// same rationale as [`wanted_writer`]. No staleness threshold, unlike
+/// [`agent_status_reader`]: an icon is a property of the agent rather than
+/// a report about it, so there is no cadence it could be late against.
+fn agent_icon_reader(
+    status: Option<&Arc<status::StatusReader>>,
+) -> Option<Arc<agent_icon::AgentIconReader>> {
+    status.map(|s| Arc::new(agent_icon::AgentIconReader::new(s.queue_client())))
+}
+
 /// A hive name that is shaped like one and names a hive this swarm has.
 ///
 /// Reports the status and the detail rather than a rendered
@@ -1996,6 +2012,95 @@ where
     Ok(Json(MakeForgeAdminResponse { change }))
 }
 
+/// How long a served icon may be reused without asking again.
+///
+/// An agent roster asks once per row, and an icon changes about as often
+/// as an agent is reconfigured — so a few minutes of browser caching is
+/// what keeps a page render from being one bucket read per agent, and
+/// costs nothing anyone will notice.
+const ICON_MAX_AGE: u32 = 300;
+
+/// `agent`'s icon — the swarm-level answer to "what does this agent look
+/// like".
+///
+/// **Nothing on this path names a hive.** The icon is a property of the
+/// agent, held at swarm scope under the agent's own key, so it is
+/// answerable for an agent that has moved hives or that no hive is
+/// currently running. That is the whole point of it living here rather
+/// than being fetched from wherever the agent happens to run.
+///
+/// **404 means this agent has no icon**, which is the expected
+/// unconfigured state and not an error — the same contract the per-agent
+/// harness's own `GET /icon` already has, so a caller falls back
+/// client-side on a failed load rather than probing first.
+///
+/// ⚠️ Until the agent-side publisher lands (see
+/// `swarm_queue_client::agent_icon`), that 404 is every agent's answer.
+///
+/// 🩸 **The body is an untrusted document served from this daemon's own
+/// origin**, and an SVG can carry script. The response headers say it may
+/// not run any: `Content-Security-Policy: sandbox` with no `allow-scripts`
+/// keeps a direct navigation to this URL from executing it, and `nosniff`
+/// keeps a browser from deciding the bytes are something else. An `<img>`
+/// load — the one consumer — never runs script regardless; the headers are
+/// for every other way a URL gets opened.
+#[utoipa::path(
+    get,
+    path = "/api/agents/{name}/icon",
+    params(("name" = String, Path, description = "agent name")),
+    responses(
+        (status = 200, description = "the agent's icon, an SVG", content_type = "image/svg+xml"),
+        (status = 400, description = "not an identifier (problem+json)", body = String),
+        (status = 404, description = "this agent has no icon (problem+json)", body = String),
+        (status = 500, description = "the icon bucket could not be read (problem+json)", body = String),
+        (status = 503, description = "no swarm queue is wired up, or it is not connected (problem+json)", body = String),
+    ),
+    tag = "agents"
+)]
+async fn get_agent_icon(
+    State(state): State<AppState>,
+    Path(name): Path<String>,
+) -> Result<axum::response::Response, problem_details::ProblemDetails> {
+    use axum::http::{StatusCode, header};
+    use axum::response::IntoResponse as _;
+
+    let Some(reader) = state.agent_icons.as_ref() else {
+        return Err(error_problem(
+            StatusCode::SERVICE_UNAVAILABLE,
+            "no swarm queue is wired up on this host",
+        ));
+    };
+    let agent = hive_types::Ident::parse(&name)
+        .map_err(|reason| error_problem(StatusCode::BAD_REQUEST, reason))?
+        .into_string();
+    let icon = reader.get(&agent).await.map_err(|e| {
+        tracing::warn!(agent = %agent, error = %format!("{e:#}"), "reading the agent icon failed");
+        error_problem(wanted_error_status(&e), &format!("{e:#}"))
+    })?;
+    let Some(icon) = icon else {
+        return Err(error_problem(
+            StatusCode::NOT_FOUND,
+            "this agent has no icon",
+        ));
+    };
+    Ok((
+        [
+            (
+                header::CONTENT_TYPE,
+                swarm_queue_client::agent_icon::MEDIA_TYPE.to_owned(),
+            ),
+            (
+                header::CACHE_CONTROL,
+                format!("public, max-age={ICON_MAX_AGE}"),
+            ),
+            (header::CONTENT_SECURITY_POLICY, "sandbox".to_owned()),
+            (header::X_CONTENT_TYPE_OPTIONS, "nosniff".to_owned()),
+        ],
+        icon,
+    )
+        .into_response())
+}
+
 /// Every agent with an open config PR, in one response — the bulk
 /// counterpart to [`get_agent_config_pr`]. swarm-ui's config-PR table needs
 /// every agent's status to render, and fetching them one at a time doesn't
@@ -2303,6 +2408,7 @@ async fn main() -> Result<()> {
         links: Arc::new(load_links()),
         wanted: wanted_writer(status.as_ref()),
         agent_status: agent_status_reader(status.as_ref()),
+        agent_icons: agent_icon_reader(status.as_ref()),
         status,
         jobq,
         webhook_secret,
@@ -2334,6 +2440,7 @@ fn build_app(state: AppState) -> axum::Router {
         .routes(routes!(get_jobq_graph))
         .routes(routes!(get_jobq_rollup))
         .routes(routes!(get_agent_config_pr))
+        .routes(routes!(get_agent_icon))
         .routes(routes!(get_config_prs))
         .routes(routes!(create_agent))
         .routes(routes!(mint_agent_identity))
@@ -2455,6 +2562,7 @@ mod tests {
             // that would write a declaration.
             wanted: None,
             agent_status: None,
+            agent_icons: None,
             jobq: std::sync::Arc::clone(&sched),
             webhook_secret: None,
             config_prs: None,
@@ -2506,6 +2614,33 @@ mod tests {
         );
     }
 
+    /// An icon route with no bucket behind it must say it could not ask,
+    /// not that the agent has no icon.
+    ///
+    /// The two are one status code apart and read identically to an
+    /// `<img>` — both end in the fallback glyph — which is exactly why the
+    /// distinction has to be asserted here: nothing downstream would ever
+    /// notice the 503 silently becoming a 404.
+    #[tokio::test]
+    async fn an_icon_with_no_queue_refuses_rather_than_answering_no_icon() {
+        use axum::response::IntoResponse as _;
+
+        let (state, _sched) = state_with_roster();
+        assert!(
+            state.agent_icons.is_none(),
+            "the fixture must have no queue"
+        );
+
+        let err = super::get_agent_icon(
+            axum::extract::State(state),
+            axum::extract::Path("iris".to_owned()),
+        )
+        .await
+        .expect_err("a queue-less controller cannot answer an icon");
+        let resp = err.into_response();
+        assert_eq!(resp.status(), axum::http::StatusCode::SERVICE_UNAVAILABLE);
+    }
+
     /// The roster check is the half that makes the recorded hive worth
     /// having, so assert it by EFFECT rather than by the message: a hive
     /// that is not in this swarm must be refused **before anything is
@@ -3688,6 +3823,32 @@ mod tests {
             "{problem:?}"
         );
     }
+
+    /// `GET /api/agents/{name}/icon` against a disconnected queue answers
+    /// 503 like its siblings, not the 404 an `<img>` would show identically
+    /// or the 500 of a failed read.
+    #[tokio::test]
+    async fn get_agent_icon_answers_503_on_a_disconnected_queue() {
+        let (state, _sched) = state_with_roster();
+        let state = super::AppState {
+            agent_icons: Some(std::sync::Arc::new(
+                super::agent_icon::AgentIconReader::new(disconnected_client().await),
+            )),
+            ..state
+        };
+
+        let problem = super::get_agent_icon(
+            axum::extract::State(state),
+            axum::extract::Path("iris".to_owned()),
+        )
+        .await
+        .expect_err("a disconnected queue must not read as an icon");
+        assert_eq!(
+            problem.status,
+            Some(axum::http::StatusCode::SERVICE_UNAVAILABLE),
+            "{problem:?}"
+        );
+    }
 }
 
 #[cfg(test)]

swarm-controller/src/matrix_account.rs

@@ -573,6 +573,7 @@ mod tests {
             status: Some(std::sync::Arc::new(status)),
             wanted: None,
             agent_status: None,
+            agent_icons: None,
             jobq: std::sync::Arc::new(std::sync::Mutex::new(hive_jobq::scheduler::Scheduler::new(
                 hive_jobq::Graph::new(),
                 hive_jobq::resources::ResourceTable::new(),