Fix entirely broken TopicTree fuzz target

fuzzing/TopicTree.cpp

@@ -10,8 +10,16 @@
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     /* Create topic tree */
     uWS::TopicTree topicTree([](uWS::Subscriber *s, std::pair<std::string_view, std::string_view> message) {
-        /* We assume sane output */
-        if (!s || !message.first.length()) {
+
+        /* Subscriber must not be null, and at this point we have to have subscriptions.
+         * This assumption seems to hold true. */
+        if (!s->subscriptions.size()) {
+            free((void *) -1);
+        }
+
+        /* Depending on what publishing we do below (with or without empty strings),
+         * this assumption can hold true or not. For now it should hold true */
+        if (!message.first.length()) {
             free((void *) -1);
         }
 
@@ -35,10 +43,12 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
             /* Then one byte action */
             if (data[4] == 'S') {
                 /* Subscribe */
-                if (subscribers.find(id) != subscribers.end()) {
+                if (subscribers.find(id) == subscribers.end()) {
                     uWS::Subscriber *subscriber = new uWS::Subscriber(nullptr);
                     subscribers[id] = std::unique_ptr<uWS::Subscriber>(subscriber);
                     topicTree.subscribe(lastString, subscriber);
+                } else {
+                    topicTree.subscribe(lastString, subscribers[id].get());
                 }
             } else if (data[4] == 'U') {
                 /* Unsubscribe */
@@ -53,8 +63,13 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
                     topicTree.unsubscribeAll(it->second.get());
                 }
             } else if (data[4] == 'P') {
-                /* Publish */
-                topicTree.publish(lastString, {lastString, lastString});
+                /* Publish only if we actually have data */
+                if (lastString.length()) {
+                    topicTree.publish(lastString, {lastString, lastString});
+                } else {
+                    /* We could use having more strings */
+                    topicTree.publish("", {"anything", "something else"});
+                }
             } else if (data[4] == 'D') {
                 /* Drain */
                 topicTree.drain();
@@ -62,6 +77,11 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         }
     });
 
+    /* Remove any subscriber from the tree */
+    for (auto &p : subscribers) {
+        topicTree.unsubscribeAll(p.second.get());
+    }
+
     return 0;
 }