agent capabilities live in a swarm KV record, set from swarmctl

An agent's capabilities move from the hive's meta repo
(`meta/capabilities.json`, injected as `HIVE_CAPABILITIES`) into the
`capabilities` field of its key in a new `agent-permissions` KV bucket,
which swarm-controller alone writes.

- swarm-queue-client: `agent_permissions` (bucket, key, `AgentPermissions`,
  direct-get read, the import subject and its wire types).
- swarm-controller: creates the bucket at start; `GET|PUT|DELETE
  /api/agents/{name}/permissions` (deny_unknown_fields and unknown
  capability ⇒ 422, duplicate ⇒ 400, agent must be declared by a hive);
  answers each hive's one-shot import, writing a key only where there is
  none and only for agents that hive's wanted state declares.
- swarmctl: `agent permissions get|set|clear`.
- swarm-nats-auth: a hive reads every key of the bucket and sends its
  import on its own subject; the controller subscribes to the imports and
  may answer them (`responses`).
- hive-c0re: reads each of its agents' keys every 30s into
  /var/lib/hyperhive/swarm-agent-permissions.json and restarts an agent,
  gracefully, when its capabilities change. Unset is the compiled-in
  default (ruth: manage_root_agent; everyone else: none). A failed read
  keeps the stored value. On first boot it seeds that file from
  meta/capabilities.json, sends the grants to the controller, and deletes
  the file once the controller answers.

Deleted: the capabilities.json reader/writer, `HIVE_CAPABILITIES` in the
rendered meta flake, ruth's capability seed, the capabilities half of the
permissions rebuild job (`PermPayload::Capabilities`/`Combined`), the
dashboard's capabilities table and `capabilities_changed` event, and the
now-unused `Capability::description`.

Part of #5170.

swarm-queue-client/src/agent_permissions.rs

@@ -0,0 +1,255 @@
+//! The per-agent permissions KV bucket: one key per agent, holding the
+//! capabilities the swarm grants it.
+//!
+//! **swarm-controller is the only writer**, and creates the bucket when it
+//! starts, so a bucket that does not answer means the queue is not serving it,
+//! never "nothing set". A field left unset leaves the agent on its hive's
+//! compiled-in default; an empty list grants nothing.
+//!
+//! **The hive running the agent reads the key**, not the agent: capabilities
+//! are bind mounts the host sets up at container start. A hive reads with one
+//! `DIRECT.GET` per agent
+//! ([`crate::agent_permissions::direct_get_subject`]), and its grant covers
+//! every key of this bucket, read-only, for the reason [`crate::agent_limits`]
+//! gives.
+//!
+//! Keyed by the agent alone, like [`crate::agent_limits`]: an agent can move
+//! between hives, and its permissions go with it.
+//!
+//! Also carries the one-shot import a hive sends of the grants its meta repo
+//! held ([`crate::agent_permissions::import_subject`]), answered by the
+//! controller.
+
+/// The KV bucket permissions are published into, keyed by agent name.
+pub const BUCKET: &str = "agent-permissions";
+
+/// The stream backing [`BUCKET`]. `create_key_value` prefixes `KV_`.
+pub const STREAM: &str = "KV_agent-permissions";
+
+/// The subject `agent`'s entry is stored under.
+#[must_use]
+pub fn key_subject(agent: &str) -> String {
+    format!("$KV.{BUCKET}.{agent}")
+}
+
+/// The request subject that reads `agent`'s current entry.
+#[must_use]
+pub fn direct_get_subject(agent: &str) -> String {
+    format!("$JS.API.DIRECT.GET.{STREAM}.{}", key_subject(agent))
+}
+
+const IMPORT_SUBJECT_PREFIX: &str = "$SWARM.permissions-import";
+
+/// The subject `hive` sends its import on. One per hive, so the hive's grant
+/// names only its own, and the controller takes the hive from the subject
+/// rather than from the payload.
+#[must_use]
+pub fn import_subject(hive: &str) -> String {
+    format!("{IMPORT_SUBJECT_PREFIX}.{hive}")
+}
+
+/// Every [`import_subject`] at once, for the controller's subscription.
+#[must_use]
+pub fn import_subject_wildcard() -> String {
+    import_subject("*")
+}
+
+/// The hive an [`import_subject`] names, or `None` for any other subject.
+#[must_use]
+pub fn hive_from_import_subject(subject: &str) -> Option<&str> {
+    let hive = subject
+        .strip_prefix(IMPORT_SUBJECT_PREFIX)?
+        .strip_prefix('.')?;
+    (!hive.is_empty() && !hive.contains('.')).then_some(hive)
+}
+
+/// How long a hive waits for the controller's answer to an import, and the
+/// queue lets the controller answer.
+pub const IMPORT_REPLY_DEADLINE: std::time::Duration = std::time::Duration::from_secs(30);
+
+/// One agent's permissions: the value under `BUCKET`/`<agent>`. `None` is
+/// "not set by the swarm", which a hive reads as its default.
+///
+/// No `deny_unknown_fields`: a field a newer controller adds must not cost an
+/// older hive the fields it knows.
+#[cfg(feature = "agent-permissions")]
+#[derive(Debug, Clone, Default, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub struct AgentPermissions {
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub capabilities: Option<Vec<hive_sh4re::permissions::Capability>>,
+}
+
+/// What a hive sends once: the capabilities its meta repo held, by agent.
+#[cfg(feature = "agent-permissions")]
+#[derive(Debug, Clone, Default, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub struct Import {
+    pub capabilities: std::collections::BTreeMap<String, Vec<hive_sh4re::permissions::Capability>>,
+}
+
+/// The controller's answer to an [`Import`].
+#[cfg(feature = "agent-permissions")]
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+#[serde(tag = "state", rename_all = "kebab-case")]
+pub enum ImportReply {
+    /// Every agent was handled. `written` had no key and got the hive's value;
+    /// `kept` already had a key, which was left alone; `skipped` is not an
+    /// agent the sending hive's wanted state declares.
+    Imported {
+        written: Vec<String>,
+        kept: Vec<String>,
+        skipped: Vec<String>,
+    },
+    /// Nothing is known to be written; the hive sends again later.
+    Failed { error: String },
+}
+
+/// Open the permissions bucket, creating it if nothing has yet.
+///
+/// **Controller-side only.** `history: 1`: a hive wants the current value.
+#[cfg(feature = "kv")]
+pub async fn open_or_create(
+    client: &async_nats::Client,
+) -> Result<async_nats::jetstream::kv::Store, crate::Error> {
+    let js = async_nats::jetstream::new(client.clone());
+    crate::kv::open_or_create(
+        &js,
+        BUCKET,
+        "agent-permissions",
+        "Permissions the swarm controller sets for each agent".to_owned(),
+    )
+    .await
+}
+
+/// What reading one agent's key found.
+#[cfg(feature = "agent-permissions")]
+#[derive(Debug)]
+pub enum Read {
+    /// The bucket answered; an absent or deleted key has every field unset.
+    Current(AgentPermissions),
+    /// Nothing answers for the bucket's stream. Not an unset value: it says
+    /// nothing about the agent's permissions.
+    NoBucket,
+}
+
+/// Read `agent`'s current permissions with one direct get.
+///
+/// # Errors
+///
+/// A failed request or an undecodable value. A request the queue refused for
+/// permissions reaches the client as a timeout, not as a refusal.
+#[cfg(feature = "agent-permissions")]
+pub async fn read(
+    client: &async_nats::Client,
+    agent: &str,
+) -> Result<Read, Box<dyn std::error::Error + Send + Sync>> {
+    use async_nats::jetstream::stream::DirectGetErrorKind;
+
+    let js = async_nats::jetstream::new(client.clone());
+    let stream = js.get_stream_no_info(STREAM).await?;
+    let message = match stream.direct_get_last_for_subject(key_subject(agent)).await {
+        Ok(message) => message,
+        Err(e) if matches!(e.kind(), DirectGetErrorKind::NotFound) => {
+            return Ok(Read::Current(AgentPermissions::default()));
+        }
+        Err(e)
+            if matches!(
+                e.kind(),
+                DirectGetErrorKind::ErrorResponse(status, _)
+                    if status == async_nats::StatusCode::NO_RESPONDERS
+            ) =>
+        {
+            return Ok(Read::NoBucket);
+        }
+        Err(e) => return Err(e.into()),
+    };
+    Ok(Read::Current(decode(
+        message
+            .headers
+            .get("KV-Operation")
+            .map(async_nats::HeaderValue::as_str),
+        &message.payload,
+    )?))
+}
+
+/// One stored message as a value: a KV delete or purge is stored as an empty
+/// marker carrying the `KV-Operation` header, and reads as every field unset.
+#[cfg(feature = "agent-permissions")]
+fn decode(operation: Option<&str>, payload: &[u8]) -> serde_json::Result<AgentPermissions> {
+    if matches!(operation, Some("DEL" | "PURGE")) {
+        return Ok(AgentPermissions::default());
+    }
+    serde_json::from_slice(payload)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// The names the grants in `swarm-nats-auth` are built from.
+    #[test]
+    fn names_match_the_queue_grant() {
+        assert_eq!(STREAM, format!("KV_{BUCKET}"));
+        assert_eq!(key_subject("iris"), "$KV.agent-permissions.iris");
+        assert_eq!(
+            direct_get_subject("iris"),
+            "$JS.API.DIRECT.GET.KV_agent-permissions.$KV.agent-permissions.iris"
+        );
+        assert_eq!(import_subject("pr1ma"), "$SWARM.permissions-import.pr1ma");
+        assert_eq!(import_subject_wildcard(), "$SWARM.permissions-import.*");
+    }
+
+    #[test]
+    fn the_hive_is_read_back_from_its_import_subject_only() {
+        assert_eq!(
+            hive_from_import_subject(&import_subject("pr1ma")),
+            Some("pr1ma")
+        );
+        for other in [
+            "$SWARM.permissions-import",
+            "$SWARM.permissions-import.",
+            "$SWARM.permissions-import.a.b",
+            "$SWARM.permissions-importx.pr1ma",
+            "$SWARM.deploy.pr1ma",
+        ] {
+            assert_eq!(hive_from_import_subject(other), None, "{other}");
+        }
+    }
+
+    /// Unset is left out on the wire, `{}` is unset, and an empty list stays
+    /// an empty list: "grants nothing" is not "not set".
+    #[cfg(feature = "agent-permissions")]
+    #[test]
+    fn unset_and_empty_are_different_values() {
+        use hive_sh4re::permissions::Capability;
+        assert_eq!(
+            serde_json::to_string(&AgentPermissions::default()).unwrap(),
+            "{}"
+        );
+        let none: AgentPermissions = serde_json::from_str(r#"{"capabilities":[]}"#).unwrap();
+        assert_eq!(none.capabilities, Some(Vec::new()));
+        let ruth: AgentPermissions =
+            serde_json::from_str(r#"{"capabilities":["manage_root_agent"]}"#).unwrap();
+        assert_eq!(ruth.capabilities, Some(vec![Capability::ManageRootAgent]));
+    }
+
+    /// Fields a newer controller adds must not cost an older hive the ones it
+    /// knows; a deleted key is unset.
+    #[cfg(feature = "agent-permissions")]
+    #[test]
+    fn decoding_tolerates_unknown_fields_and_reads_a_delete_as_unset() {
+        let decoded = decode(
+            None,
+            br#"{"capabilities":["manage_root_agent"],"tool_groups":["meta"]}"#,
+        )
+        .unwrap();
+        assert_eq!(decoded.capabilities.map(|c| c.len()), Some(1));
+        assert_eq!(
+            decode(Some("DEL"), b"").unwrap(),
+            AgentPermissions::default()
+        );
+        assert_eq!(
+            decode(Some("PURGE"), b"").unwrap(),
+            AgentPermissions::default()
+        );
+    }
+}