op cache: check callback return value

src/op_cache.cpp

@@ -133,8 +133,7 @@ SearchCache::listen(ValueCallback get_cb, Sp<Query> q, Value::Filter filter, std
     auto token = nextToken_++;
     if (nextToken_ == 0)
         nextToken_++;
-    op->second->addListener(token, get_cb, q, filter);
-    return token;
+    return op->second->addListener(token, get_cb, q, filter) ? token : 0;
 }
 
 bool

src/op_cache.h

@@ -87,11 +87,12 @@ public:
     void onValuesAdded(const std::vector<Sp<Value>>& vals);
     void onValuesExpired(const std::vector<Sp<Value>>& vals);
 
-    void addListener(size_t token, ValueCallback cb, Sp<Query> q, Value::Filter filter) {
+    bool addListener(size_t token, ValueCallback cb, Sp<Query> q, Value::Filter filter) {
         listeners.emplace(token, LocalListener{q, filter, cb});
         auto cached = cache.get(filter);
         if (not cached.empty())
-            cb(cached, false);
+            return cb(cached, false);
+        return true;
     }
 
     bool removeListener(size_t token, const time_point& now) {