Several fixes around Chunk processing (Json) of a large Json document and available length passed to fill send buffer

src/AsyncJson.cpp

@@ -61,7 +61,7 @@ size_t AsyncJsonResponse::_fillBuffer(uint8_t *data, size_t len) {
 #else
   serializeJson(_root, dest);
 #endif
-  return len;
+  return dest.written();
 }
 
 #if ARDUINOJSON_VERSION_MAJOR == 6
@@ -89,7 +89,7 @@ size_t PrettyAsyncJsonResponse::_fillBuffer(uint8_t *data, size_t len) {
 #else
   serializeJsonPretty(_root, dest);
 #endif
-  return len;
+  return dest.written();
 }
 
 // MessagePack content type response
@@ -106,7 +106,7 @@ size_t AsyncMessagePackResponse::setLength() {
 size_t AsyncMessagePackResponse::_fillBuffer(uint8_t *data, size_t len) {
   ChunkPrint dest(data, _sentLength, len);
   serializeMsgPack(_root, dest);
-  return len;
+  return dest.written();
 }
 
 #endif

src/ChunkPrint.cpp

@@ -3,16 +3,21 @@
 
 #include <ChunkPrint.h>
 
-ChunkPrint::ChunkPrint(uint8_t *destination, size_t from, size_t len) : _destination(destination), _to_skip(from), _to_write(len), _pos{0} {}
-
 size_t ChunkPrint::write(uint8_t c) {
-  if (_to_skip > 0) {
-    _to_skip--;
+  // handle case where len is zero
+  if (!_len) {
+    return 0;
+  }
+  // skip first bytes until from is zero (bytes were already sent by previous chunk)
+  if (_from) {
+    _from--;
     return 1;
-  } else if (_to_write > 0) {
-    _to_write--;
-    _destination[_pos++] = c;
+  }
+  // write a maximum of len bytes
+  if (_len - _index) {
+    _destination[_index++] = c;
     return 1;
   }
+  // we have finished writing len bytes, ignore the rest
   return 0;
 }

src/ChunkPrint.h

@@ -8,14 +8,17 @@
 class ChunkPrint : public Print {
 private:
   uint8_t *_destination;
-  size_t _to_skip;
-  size_t _to_write;
-  size_t _pos;
+  size_t _from;
+  size_t _len;
+  size_t _index;
 
 public:
-  ChunkPrint(uint8_t *destination, size_t from, size_t len);
+  ChunkPrint(uint8_t *destination, size_t from, size_t len) : _destination(destination), _from(from), _len(len), _index(0) {}
   size_t write(uint8_t c);
   size_t write(const uint8_t *buffer, size_t size) {
     return this->Print::write(buffer, size);
   }
+  size_t written() const {
+    return _index;
+  }
 };

src/WebResponses.cpp

@@ -470,7 +470,8 @@ size_t AsyncAbstractResponse::write_send_buffs(AsyncWebServerRequest *request, s
           }
         }
       } else {
-        size_t const readLen = _fillBufferAndProcessTemplates(_send_buffer->data(), std::min(_send_buffer->size(), tcp_win));
+        size_t const readLen =
+          _fillBufferAndProcessTemplates(_send_buffer->data(), std::min(std::min(_send_buffer->size(), tcp_win), _contentLength - _sentLength));
         if (readLen == 0) {
           // no more data to send
           _state = RESPONSE_END;