fix: _remove_events_with_arg() might probably leak mem on queue overflow

it must free message's mem if failed to requeue the pointer

src/AsyncTCP.cpp

@@ -187,35 +187,42 @@ static inline bool _get_async_event(lwip_event_packet_t** e) {
         log_d("coalescing polls, network congestion or async callbacks might be too slow!");
         continue;
       }
-    } else {
-      /*
-        poor designed apps using asynctcp without proper dataflow control could flood the queue with interleaved pool/ack events.
-        We can try to mitigate it by discarding poll events when queue grows too much.
-        Let's discard poll events using linear probability curve starting from 3/4 of queue length
-        Poll events are periodic and connection could get another chance next time
-      */
-      if (uxQueueMessagesWaiting(_async_queue) > (rand() % CONFIG_ASYNC_TCP_QUEUE_SIZE / 4 + CONFIG_ASYNC_TCP_QUEUE_SIZE * 3 / 4)) {
-        free(next_pkt);
-        next_pkt = NULL;
-        log_d("discarding poll due to queue congestion");
-        // evict next event from a queue
-        return _get_async_event(e);
-      }
     }
-    return true;
+
+    // quit while loop if next event can't be discarded
+    break;
   }
+
+  /*
+    now we have to decide if to proceed with poll callback handler or discard it?
+    poor designed apps using asynctcp without proper dataflow control could flood the queue with interleaved pool/ack events.
+    I.e. on each poll app would try to generate more data to send, which in turn results in additional ack event triggering chain effect
+    for long connections. Or poll callback could take long time starving other connections. Anyway our goal is to keep the queue length
+    grows under control (if possible) and poll events are the safest to discard.
+    Let's discard poll events processing using linear-increasing probability curve when queue size grows over 3/4
+    Poll events are periodic and connection could get another chance next time
+  */
+  if (uxQueueMessagesWaiting(_async_queue) > (rand() % CONFIG_ASYNC_TCP_QUEUE_SIZE / 4 + CONFIG_ASYNC_TCP_QUEUE_SIZE * 3 / 4)) {
+    free(*e);
+    *e = NULL;
+    log_d("discarding poll due to queue congestion");
+    // evict next event from a queue
+    return _get_async_event(e);
+  }
+
   // last resort return
   return true;
 }
 
 static bool _remove_events_with_arg(void* arg) {
-  lwip_event_packet_t* first_packet = NULL;
-  lwip_event_packet_t* packet = NULL;
-
   if (!_async_queue) {
     return false;
   }
-  // figure out which is the first packet so we can keep the order
+
+  lwip_event_packet_t* first_packet = NULL;
+  lwip_event_packet_t* packet = NULL;
+
+  // figure out which is the first non-matching packet so we can keep the order
   while (!first_packet) {
     if (xQueueReceive(_async_queue, &first_packet, 0) != pdPASS) {
       return false;
@@ -224,11 +231,12 @@ static bool _remove_events_with_arg(void* arg) {
     if ((int)first_packet->arg == (int)arg) {
       free(first_packet);
       first_packet = NULL;
-
-      // try to return first packet to the back of the queue
     } else if (xQueueSend(_async_queue, &first_packet, 0) != pdPASS) {
+      // try to return first packet to the back of the queue
       // we can't wait here if queue is full, because this call has been done from the only consumer task of this queue
       // otherwise it would deadlock, we have to discard the event
+      free(first_packet);
+      first_packet = NULL;
       return false;
     }
   }
@@ -238,11 +246,15 @@ static bool _remove_events_with_arg(void* arg) {
       return false;
     }
     if ((int)packet->arg == (int)arg) {
+      // remove matching event
       free(packet);
       packet = NULL;
+      // otherwise try to requeue it
     } else if (xQueueSend(_async_queue, &packet, 0) != pdPASS) {
       // we can't wait here if queue is full, because this call has been done from the only consumer task of this queue
       // otherwise it would deadlock, we have to discard the event
+      free(packet);
+      packet = NULL;
       return false;
     }
   }

src/AsyncTCP.h

@@ -241,7 +241,7 @@ class AsyncClient {
     // will not ack the current packet. Call from onData
     void ackLater() { _ack_pcb = false; }
 
-    const char* errorToString(int8_t error);
+    static const char* errorToString(int8_t error);
     const char* stateToString();
 
     // internal callbacks - Do NOT call any of the functions below in user code!