Coalesce poll events on queue eviction

Refer: mathieucarbou/ESPAsyncWebServer#165

Let's try to coalesce two (or more) consecutive poll events into one
this usually happens with poor implemented user-callbacks that are runs too long and makes poll events to stack in the queue
if consecutive user callback for a same connection runs longer that poll time then it will fill the queue with events until it deadlocks.
This is a workaround to mitigate such poor designs and won't let other events/connections to starve the task time.
It won't be effective if user would run multiple simultaneous long running callbacks due to message interleaving.
todo: implement some kind of fair dequeing or (better) simply punish user for a bad designed callbacks by resetting hog connections

src/AsyncTCP.cpp

@@ -61,6 +61,12 @@ extern "C" {
 
 #define INVALID_CLOSED_SLOT -1
 
+/*
+  TCP poll interval is specified in terms of the TCP coarse timer interval, which is called twice a second
+  https://github.com/espressif/esp-lwip/blob/2acf959a2bb559313cd2bf9306c24612ba3d0e19/src/core/tcp.c#L1895
+*/
+#define CONFIG_ASYNC_TCP_POLL_TIMER   1
+
 /*
  * TCP/IP Event Task
  * */
@@ -148,7 +154,34 @@ static inline bool _prepend_async_event(lwip_event_packet_t** e) {
 }
 
 static inline bool _get_async_event(lwip_event_packet_t** e) {
-  return _async_queue && xQueueReceive(_async_queue, e, portMAX_DELAY) == pdPASS;
+  if (!_async_queue) {
+    return false;
+  }
+
+  if (xQueueReceive(_async_queue, e, portMAX_DELAY) != pdPASS)
+    return false;
+
+  /*
+    Let's try to coalesce two (or more) consecutive poll events into one
+    this usually happens with poor implemented user-callbacks that are runs too long and makes poll events to stack in the queue
+    if consecutive user callback for a same connection runs longer that poll time then it will fill the queue with events until it deadlocks.
+    This is a workaround to mitigate such poor designs and won't let other events/connections to starve the task time.
+    It won't be effective if user would run multiple simultaneous long running callbacks due to message interleaving.
+    todo: implement some kind of fair dequeing or (better) simply punish user for a bad designed callbacks by resetting hog connections
+  */
+  lwip_event_packet_t* next_pkt = NULL;
+  while (xQueuePeek(_async_queue, &next_pkt, 0) == pdPASS){
+    if (next_pkt->arg == (*e)->arg && next_pkt->event == LWIP_TCP_POLL && (*e)->event == LWIP_TCP_POLL){
+      if (xQueueReceive(_async_queue, &next_pkt, 0) == pdPASS){
+        free(next_pkt);
+        next_pkt = NULL;
+        log_w("coalescing polls, async callback might be too slow!");
+      } else
+        return true;
+    } else
+      return true;
+  }
+  return true;
 }
 
 static bool _remove_events_with_arg(void* arg) {
@@ -311,6 +344,7 @@ static int8_t _tcp_connected(void* arg, tcp_pcb* pcb, int8_t err) {
 
 static int8_t _tcp_poll(void* arg, struct tcp_pcb* pcb) {
   // throttle polling events queing when event queue is getting filled up, let it handle _onack's
+  //log_d("qs:%u", uxQueueMessagesWaiting(_async_queue));
   if (uxQueueMessagesWaiting(_async_queue) > (rand() % CONFIG_ASYNC_TCP_QUEUE_SIZE / 2 + CONFIG_ASYNC_TCP_QUEUE_SIZE / 4)) {
     log_d("throttling");
     return ERR_OK;
@@ -612,7 +646,7 @@ AsyncClient::AsyncClient(tcp_pcb* pcb)
     tcp_recv(_pcb, &_tcp_recv);
     tcp_sent(_pcb, &_tcp_sent);
     tcp_err(_pcb, &_tcp_error);
-    tcp_poll(_pcb, &_tcp_poll, 1);
+    tcp_poll(_pcb, &_tcp_poll, CONFIG_ASYNC_TCP_POLL_TIMER);
     if (!_allocate_closed_slot()) {
       _close();
     }