fix(#2851): skip the state PUT when unchanged — it still emits an event

set_room_state PUT unconditionally and its doc called that idempotent.
It is, one level too high: a PUT of identical content is a no-op on the
room's STATE and the homeserver still appends an event to the TIMELINE.
Downstream an event is unread activity, then a todo, then a turn — for
every agent in the room. The provisioning sweep re-wiring the hive
Space's child link therefore woke the whole hive on the sweep's cadence,
forever, for a link that never changed (~1801s between events, measured
across eleven consecutive intervals).

Read the current content first and return early when it matches. The
lookup fails open — an unreadable state means write, because the
re-apply exists to repair a missing link and "don't know" must not be
treated as "fine". Only the steady state goes quiet.

hive-c0re/src/matrix.rs

@@ -1218,8 +1218,47 @@ async fn find_chat_room_by_name(client: &reqwest::Client, admin_token: &str) ->
     None
 }
 
-/// PUT a state event into `room_id` using the admin token. Idempotent —
-/// re-sending identical content is a no-op on the homeserver.
+/// Whether the state event still has to be written.
+///
+/// `current` is the room's existing content for this `(type, state_key)`,
+/// or `None` when it could not be read. **Unreadable means write**: the
+/// re-apply exists so a missing link gets repaired, and one redundant
+/// event is cheaper than a hierarchy that never reconverges.
+///
+/// Comparison is `serde_json::Value` equality, which compares objects as
+/// maps — key order in the response does not matter.
+fn state_needs_write(current: Option<&serde_json::Value>, desired: &serde_json::Value) -> bool {
+    current != Some(desired)
+}
+
+/// Read the room's current content for one state event. `None` on any
+/// failure — absent state, transport error, or an unparseable body are
+/// all "we don't know", and [`state_needs_write`] turns that into a write.
+async fn current_room_state(
+    client: &reqwest::Client,
+    admin_token: &str,
+    url: &str,
+) -> Option<serde_json::Value> {
+    let resp = client.get(url).bearer_auth(admin_token).send().await.ok()?;
+    if !resp.status().is_success() {
+        return None;
+    }
+    resp.json::<serde_json::Value>().await.ok()
+}
+
+/// PUT a state event into `room_id` using the admin token, **skipping the
+/// write when the room already carries identical content**.
+///
+/// The read is not an optimisation. A PUT of identical content is a no-op
+/// on the room's *state*, and the homeserver still appends an event to the
+/// *timeline* — so "idempotent" was true one level too high. Downstream,
+/// an event is unread activity, which is a todo, which is a turn: a caller
+/// re-applying a link on a periodic sweep wakes every agent in the room on
+/// that sweep's cadence, forever. Measured at ~30 minutes per wake per
+/// agent before this guard existed.
+///
+/// The self-healing property the re-apply exists for is unaffected: a
+/// missing or divergent link still gets written.
 async fn set_room_state(
     client: &reqwest::Client,
     admin_token: &str,
@@ -1233,6 +1272,15 @@ async fn set_room_state(
     let encoded_key = encode_room_id_for_url(state_key);
     let url =
         format!("{base}/_matrix/client/v3/rooms/{encoded_room}/state/{event_type}/{encoded_key}");
+    let current = current_room_state(client, admin_token, &url).await;
+    if !state_needs_write(current.as_ref(), content) {
+        tracing::debug!(
+            %room_id,
+            event_type,
+            "matrix: state already current, skipping PUT (no timeline event)"
+        );
+        return Ok(());
+    }
     let resp = client
         .put(&url)
         .bearer_auth(admin_token)
@@ -1261,8 +1309,9 @@ async fn set_room_state(
 /// join instead of an empty Space.
 ///
 /// Dedup mirrors [`ensure_hive_space`]: persisted id wins, else rediscover
-/// by name, else create. The space-child link is re-applied on every call
-/// (idempotent PUT) so a recovered room reconverges its hierarchy link.
+/// by name, else create. The space-child link is re-checked on every call
+/// so a recovered room reconverges its hierarchy link — and written only
+/// when it differs, see [`set_room_state`].
 ///
 /// # Errors
 ///
@@ -1347,9 +1396,9 @@ pub async fn ensure_hive_chat_room(
         id
     };
 
-    // Wire the Space → room child link (idempotent). Without an
-    // `m.space.child` carrying a `via`, the room won't surface in the Space
-    // hierarchy. `suggested` hints clients to surface it prominently.
+    // Wire the Space → room child link. Without an `m.space.child` carrying
+    // a `via`, the room won't surface in the Space hierarchy. `suggested`
+    // hints clients to surface it prominently.
     let child_content = serde_json::json!({
         "via": [server_name],
         "suggested": true,
@@ -1700,6 +1749,43 @@ mod tests {
         assert!(h.chars().all(|c| c.is_ascii_hexdigit()));
     }
 
+    /// The steady state. This is the whole point of the guard: the sweep
+    /// runs forever, and every write it makes is a wake for every agent
+    /// in the room.
+    #[test]
+    fn state_matching_the_room_is_not_rewritten() {
+        let desired = serde_json::json!({ "via": ["example.org"], "suggested": true });
+        let current = desired.clone();
+        assert!(!state_needs_write(Some(&current), &desired));
+    }
+
+    /// Key order in the homeserver's response must not force a write —
+    /// the guard leans on `serde_json::Value` comparing objects as maps,
+    /// and a byte- or order-sensitive comparison would silently degrade
+    /// to writing every time while still looking correct.
+    #[test]
+    fn state_matching_but_reordered_is_not_rewritten() {
+        let desired = serde_json::json!({ "via": ["example.org"], "suggested": true });
+        let current = serde_json::json!({ "suggested": true, "via": ["example.org"] });
+        assert!(!state_needs_write(Some(&current), &desired));
+    }
+
+    #[test]
+    fn diverged_state_is_rewritten() {
+        let desired = serde_json::json!({ "via": ["example.org"], "suggested": true });
+        let current = serde_json::json!({ "via": ["old.example.org"], "suggested": true });
+        assert!(state_needs_write(Some(&current), &desired));
+    }
+
+    /// Fail-open, and deliberately so: an unreadable current state must
+    /// write. The re-apply exists to repair a missing link, so "we don't
+    /// know" has to behave like "it's missing", not like "it's fine".
+    #[test]
+    fn unknown_state_is_written() {
+        let desired = serde_json::json!({ "via": ["example.org"], "suggested": true });
+        assert!(state_needs_write(None, &desired));
+    }
+
     #[test]
     fn random_hex_two_calls_differ() {
         // Sanity check — not a statistical claim, just guards