implement in-flight buffer credits and event moderation for large/chunked responses

Referer to https://github.com/mathieucarbou/ESPAsyncWebServer/discussions/165
Relates to #169

in-flight buffer credits are intended to moderate buffer fill callbacks in AsyncAbstractResponse
it could prevent bad designed slow user-callbacks to flood the queue in chunked responces.

for response data we need to control the queue and in-flight fragmentation. Sending small chunks could give low latency,
but flood asynctcp's queue and fragment socket buffer space for large responses.
Let's ignore polled acks and acks in case when we have more in-flight data then the available socket buff space.
That way we could balance on having half the buffer in-flight while another half is filling up, while minimizing events in asynctcp q

src/WebResponseImpl.h

@@ -47,6 +47,10 @@ class AsyncBasicResponse : public AsyncWebServerResponse {
 
 class AsyncAbstractResponse : public AsyncWebServerResponse {
   private:
+    // amount of responce data in-flight, i.e. sent, but not acked yet
+    size_t _in_flight{0};
+    // in-flight queue credits
+    size_t _in_flight_credit{2};
     String _head;
     // Data is inserted into cache at begin().
     // This is inefficient with vector, but if we use some other container,

src/WebResponses.cpp

@@ -352,7 +352,21 @@ size_t AsyncAbstractResponse::_ack(AsyncWebServerRequest* request, size_t len, u
     request->client()->close();
     return 0;
   }
+  // return a credit for each chunk of acked data (polls does not give any credits)
+  if (len)
+    ++_in_flight_credit;
+
+  // for chunked responses ignore acks if there are no _in_flight_credits left
+  if (_chunked && !_in_flight_credit){
+#ifdef ESP32
+    log_d("(chunk) out of in-flight credits");
+#endif
+    return 0;
+  }
+
   _ackedLength += len;
+  _in_flight -= (_in_flight > len) ? len : _in_flight;
+  // get the size of available sock space
   size_t space = request->client()->space();
 
   size_t headLen = _head.length();
@@ -364,16 +378,31 @@ size_t AsyncAbstractResponse::_ack(AsyncWebServerRequest* request, size_t len, u
       String out = _head.substring(0, space);
       _head = _head.substring(space);
       _writtenLength += request->client()->write(out.c_str(), out.length());
+      _in_flight += out.length();
+      --_in_flight_credit;  // take a credit
       return out.length();
     }
   }
 
   if (_state == RESPONSE_CONTENT) {
+    // for response data we need to control the queue and in-flight fragmentation. Sending small chunks could give low latency,
+    // but flood asynctcp's queue and fragment socket buffer space for large responses.
+    // Let's ignore polled acks and acks in case when we have more in-flight data then the available socket buff space.
+    // That way we could balance on having half the buffer in-flight while another half is filling up, while minimizing events in asynctcp q
+    if (_in_flight > space){
+      //log_d("defer user call %u/%u", _in_flight, space);
+      // take the credit back since we are ignoring this ack and rely on other inflight data
+      if (len)
+        --_in_flight_credit;
+      return 0;
+    }
+
     size_t outLen;
     if (_chunked) {
       if (space <= 8) {
         return 0;
       }
+
       outLen = space;
     } else if (!_sendContentLength) {
       outLen = space;
@@ -422,6 +451,8 @@ size_t AsyncAbstractResponse::_ack(AsyncWebServerRequest* request, size_t len, u
 
     if (outLen) {
       _writtenLength += request->client()->write((const char*)buf, outLen);
+      _in_flight += outLen;
+      --_in_flight_credit;  // take a credit
     }
 
     if (_chunked) {