AsyncAbstractResponse: Use packet buffer space logic on ESP32

When under memory pressure, the ESP32 TCP stack can also stop operating,
resulting in hung requests.  Use the logic developed for ESP8266 to
avoid allocating buffers too large for the system to actually send.

Fixes stalls on low-RAM ESP32 devices, such as the S2.

Author: willmmiles

src/WebResponses.cpp

@@ -33,6 +33,8 @@
 #else
 #define GET_MAX_BLOCK_SIZE getMaxAllocHeap
 #endif
+// When looking up available memory, leave some slack
+#define BLOCK_SIZE_SLACK 128
 
 // Since ESP8266 does not link memchr by default, here's its implementation.
 void* memchr(void* ptr, int ch, size_t count)
@@ -358,29 +360,25 @@ size_t AsyncAbstractResponse::_ack(AsyncWebServerRequest *request, size_t len, u
     } else {
       outLen = std::min(space, _contentLength - _sentLength);
     }
-#ifdef ESP8266
+
     // Limit outlen based on available memory
     // We require two packet buffers - one allocated here, and one belonging to the TCP stack
     {      
       auto old_space = _packet.capacity();
-      auto max_block_size = ESP.getMaxFreeBlockSize() - 128;
+      auto max_block_size = ESP.GET_MAX_BLOCK_SIZE() - BLOCK_SIZE_SLACK;
       if ((old_space < outLen) || (outLen > max_block_size)) {
         DEBUG_PRINTFP("(%d) Space adjustment, have %d, want %d, avail %d\n", (intptr_t)this, old_space, outLen, max_block_size);
         do { 
           dealloc_vector(_packet);
           outLen = std::min(outLen, max_block_size);
           _packet.reallocate(outLen);
-          max_block_size = ESP.getMaxFreeBlockSize() - 128;
+          max_block_size = ESP.GET_MAX_BLOCK_SIZE() - BLOCK_SIZE_SLACK;
           DEBUG_PRINTFP("(%d) Checking %d vs %d\n", (intptr_t)this, outLen, max_block_size);
         } while (max_block_size < outLen);
       } else {
         _packet.reallocate(outLen);
       }
     }
-#else
-    //Serial.printf("[%u] %d/%d -> %d\n", (unsigned) millis(),  ESP.getMaxAllocHeap(), ESP.getFreeHeap(), outLen);
-    _packet.reallocate(outLen);
-#endif
 
     if(_chunked){      
       // HTTP 1.1 allows leading zeros in chunk length. Or spaces may be added.