fix: Renew presence on sub/pub

server/internal/bus/bus.go

@@ -204,6 +204,21 @@ func SetPresence(ctx context.Context, serverID string, ttl time.Duration) error
 	return rdb.Set(ctx, PresenceKey+serverID, nodeID, ttl).Err()
 }
 
+// RenewPresence extends serverID's claim, but only while this node still holds
+// it, and reports whether it did.
+//
+// A blind SET here is wrong, not merely untidy. When an agent reconnects, its
+// previous stream can stay half-open on another pod for the length of a
+// keepalive cycle, and that pod goes on renewing. Two processes then overwrite
+// each other's claim every renewal interval and the key names whichever wrote
+// last rather than whichever holds the live stream. A superseded pod must lose
+// quietly instead.
+func RenewPresence(ctx context.Context, serverID string, ttl time.Duration) bool {
+	n, err := renewPresenceIfOwner.Run(ctx, rdb,
+		[]string{PresenceKey + serverID}, nodeID, int64(ttl/time.Millisecond)).Int64()
+	return err == nil && n == 1
+}
+
 // ClearPresence releases serverID, but only if this node still holds it. A
 // blind DEL would let a pod whose stream had already been re-established
 // elsewhere delete the new owner's claim on its way out.
@@ -274,6 +289,13 @@ end
 return ""
 `)
 
+var renewPresenceIfOwner = redis.NewScript(`
+if redis.call("GET", KEYS[1]) == ARGV[1] then
+	return redis.call("PEXPIRE", KEYS[1], ARGV[2])
+end
+return 0
+`)
+
 var releaseIfOwner = redis.NewScript(`
 if redis.call("GET", KEYS[1]) == ARGV[1] then
 	return redis.call("DEL", KEYS[1])