fix(presence-sync): tick mutex (#8)

The presence-sync tick iterates accounts serially with await on each
agent-login + PUT round-trip — a single tick can easily run 20+s when
there are several accounts. setInterval(intervalMs) does NOT wait for
the previous tick to finish, so on a busy gateway the next tick fires
on top of a still-running one and two parallel iterations each PUT
the same agentId within ~10 ms. That tipped the guild backend's
first-time-insert race (separate fix in nav/Fabric.Backend.Guild) into
500s on prod (caught in t2 gateway 2026-05-25 23:23:35Z; 6 of 6 agents
showed paired log lines 4-10 ms apart for the same agent → idle).

Fix: a simple `inflight` boolean. tick() returns immediately if
already running; the next interval beat catches up. lastStatus !==
bridge.get gating already means status changes catch the next tick
anyway, so skipping a beat costs nothing the next beat won't fix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dist/fabric/src/presence-sync.js

@@ -8,6 +8,15 @@ export class PresenceSync {
     lastStatus = new Map(); // by agentId
     accounts = new Map();
     tokenCache = new Map(); // by agentId
+    // Mutex flag: a tick iterates accounts serially with `await` on each
+    // agent-login + PUT round-trip, so a single tick can easily run 20+s
+    // when there are many accounts. setInterval(intervalMs) does NOT wait
+    // for the previous tick to finish — without this guard the next tick
+    // fires on top of a still-running one and two parallel iterations
+    // PUT the same agentId within milliseconds. That tipped the backend's
+    // first-time-insert race (separate fix in Fabric.Backend.Guild) into
+    // 500s on prod. Guarded ticks just skip a beat instead.
+    inflight = false;
     constructor(logger, client) {
         this.logger = logger;
         this.client = client;
@@ -60,6 +69,21 @@ export class PresenceSync {
         return entry.token;
     }
     async tick() {
+        // Mutex: see the `inflight` field declaration for the why. Drop
+        // overlapping ticks rather than letting them run concurrently —
+        // status is gated by `lastStatus !== bridge.get`, so skipping a
+        // beat costs nothing the next beat won't catch.
+        if (this.inflight)
+            return;
+        this.inflight = true;
+        try {
+            await this.tickInner();
+        }
+        finally {
+            this.inflight = false;
+        }
+    }
+    async tickInner() {
         const bridge = globalThis['__hfAgentStatus'];
         if (!bridge || typeof bridge.get !== 'function')
             return; // HF plugin not loaded — skip

src/presence-sync.ts

@@ -50,6 +50,16 @@ export class PresenceSync {
   private readonly accounts = new Map<string, PresenceSyncAccount>();
   private readonly tokenCache = new Map<string, CachedToken>(); // by agentId
 
+  // Mutex flag: a tick iterates accounts serially with `await` on each
+  // agent-login + PUT round-trip, so a single tick can easily run 20+s
+  // when there are many accounts. setInterval(intervalMs) does NOT wait
+  // for the previous tick to finish — without this guard the next tick
+  // fires on top of a still-running one and two parallel iterations
+  // PUT the same agentId within milliseconds. That tipped the backend's
+  // first-time-insert race (separate fix in Fabric.Backend.Guild) into
+  // 500s on prod. Guarded ticks just skip a beat instead.
+  private inflight = false;
+
   constructor(private readonly logger: Logger, private readonly client: FabricClient) {}
 
   setAccounts(accounts: PresenceSyncAccount[]): void {
@@ -103,6 +113,20 @@ export class PresenceSync {
   }
 
   private async tick(): Promise<void> {
+    // Mutex: see the `inflight` field declaration for the why. Drop
+    // overlapping ticks rather than letting them run concurrently —
+    // status is gated by `lastStatus !== bridge.get`, so skipping a
+    // beat costs nothing the next beat won't catch.
+    if (this.inflight) return;
+    this.inflight = true;
+    try {
+      await this.tickInner();
+    } finally {
+      this.inflight = false;
+    }
+  }
+
+  private async tickInner(): Promise<void> {
     const bridge = (globalThis as Record<string, unknown>)['__hfAgentStatus'] as Bridge | undefined;
     if (!bridge || typeof bridge.get !== 'function') return; // HF plugin not loaded — skip