swarm-queue-client: the wanted-state bucket, opposite in direction to status

The controller declares each hive's wanted agent set; the hive reads its own
key. Deliberately not a mirror of `status`, which the module documents as a
table: authored by the controller rather than the hive, DECLARED rather than
observed, and — the row that decides how it must be built — unrecoverable if
the store is lost, where status regenerates because every hive republishes
what it is.

Two open functions rather than one shared `open_or_create`. `status` shares its
constructor because either end may legitimately arrive first on a fresh swarm;
here the writer is single and known, so the hive gets a read-only open
returning `Option` and holds no grant to create the bucket. Its absence is the
ordinary pre-publication state, not an error a hive could fix.

Absence is also not a deletion order, which the module says at the place an
implementer will meet it: swarm-side lifecycle does not yet cover agents that
predate it, so a hive finding no key has learned nothing about what it runs —
converging to an empty set would tear those down.

No consumer yet. This is the half that is invariant under the scope semantics
and cadence still being decided.

swarm-queue-client/src/wanted.rs

@@ -0,0 +1,80 @@
+//! The hive-wanted KV bucket: the agent set the controller declares for each
+//! hive, keyed by `hiveName`.
+//!
+//! Sibling of [`crate::status`] and deliberately **not** a mirror of it: that
+//! one is **observed** — each hive republishes what it is, so its store losing
+//! everything "degrades to honesty" (`swarm-nats.nix`). This one is
+//! **declared**, and nothing regenerates it; lose it and a reconcile loop has
+//! nothing to converge to, which is the failure such a loop exists to remove.
+//!
+//! # Absence is not a deletion order
+//!
+//! A hive that finds no key for itself — or no bucket at all — has learned
+//! nothing about the agents it is running, not that it should have none.
+//! Swarm-side lifecycle does not yet cover agents that predate it, so callers
+//! converge the agents a value NAMES and leave the rest alone.
+//!
+//! # Only the controller creates it
+//!
+//! Unlike [`crate::status::open_or_create`], where either end may legitimately
+//! arrive first, the writer here is single and known. A hive opens read-only
+//! and treats absence as the case above, so "not published yet" stays quiet
+//! rather than looking like an error a hive could fix.
+
+#[cfg(feature = "kv")]
+use crate::Error;
+
+/// The KV bucket the controller publishes per-hive wanted state into, one key
+/// per hive keyed by `hiveName`.
+///
+/// A constant and not an option, for the reason [`crate::status::BUCKET`]
+/// gives: writer and reader must name the same bucket, and an option is a way
+/// for two deployments to disagree about which one that is.
+pub const BUCKET: &str = "hive-wanted";
+
+/// Open the wanted-state bucket for writing, creating it if nothing has yet.
+///
+/// **Controller-side only.** `history: 1` because a hive converges to the
+/// current declaration and never asks what the previous one was — that
+/// question is answered by the controller's own records, not by replaying a
+/// bucket.
+#[cfg(feature = "kv")]
+pub async fn open_or_create(
+    client: &async_nats::Client,
+) -> Result<async_nats::jetstream::kv::Store, Error> {
+    let js = async_nats::jetstream::new(client.clone());
+    match js.get_key_value(BUCKET).await {
+        Ok(store) => Ok(store),
+        Err(e) => {
+            tracing::info!(
+                bucket = BUCKET,
+                reason = %e,
+                "wanted-state bucket not available, creating it"
+            );
+            js.create_key_value(async_nats::jetstream::kv::Config {
+                bucket: BUCKET.to_owned(),
+                description: "Agent set the swarm controller declares for each hive".to_owned(),
+                history: 1,
+                ..Default::default()
+            })
+            .await
+            .map_err(|source| Error::CreateBucket {
+                bucket: BUCKET,
+                source,
+            })
+        }
+    }
+}
+
+/// Open the wanted-state bucket for reading, or `None` when it does not exist.
+///
+/// **Hive-side.** `None` is the ordinary pre-publication state, not a failure:
+/// a hive holds no grant to create this bucket and must not treat its absence
+/// as a reason to converge to an empty agent set — see the module docs.
+#[cfg(feature = "kv")]
+pub async fn open_read_only(
+    client: &async_nats::Client,
+) -> Option<async_nats::jetstream::kv::Store> {
+    let js = async_nats::jetstream::new(client.clone());
+    js.get_key_value(BUCKET).await.ok()
+}