WiFiClientSecure readString function slow - arduino

Question

I have a sketch that requests a device token from Facebook in order to authenticate a Wemos D1 Mini.
All in all, I want to reproduce the following, which gives me a device code in less than a second:
curl https://graph.facebook.com/v2.7/device/login -d
"type=device_code&access_token=MYTOKEN"
I set up the following sketch which works albeit very slowly. It fetches the device token in about 17 seconds.
It seems that String response = client.readString(); is the culprit.
Can any offer advice on why that could be, and how to possibly remedy it?
Thank you kindly,
Nate
#include <ESP8266WiFi.h>
#include <WiFiClientSecure.h>
#include <ArduinoHttpClient.h>
#include <ArduinoJson.h>
WiFiClientSecure client; //edited
void setup() {
connect(WIFI_SSID, WIFI_PWD); //edited
Serial.println(getFBDeviceToken());
}
void connect(const char* WIFI_SSID, const char* WIFI_PWD) {
delay(1000);
WiFi.persistent(false);
delay(1000);
WiFi.mode(WIFI_STA);
if (WiFi.status() == WL_DISCONNECTED) {
WiFi.begin(WIFI_SSID, WIFI_PWD);
while(WiFi.status() != WL_CONNECTED){
delay(1000);
Serial.println("connecting...");
}
Serial.println("\r\n"+WiFi.localIP());
}
}
String getFBDeviceToken(){
//Connect to FB SSL
if(!client.connect(host, httpPort)){
return "** Failed to connect **";
}
client.println("POST " + path + " HTTP/1.1");
client.println("Host: " + String(host));
client.println("User-Agent: ESP8266/1.0");
client.println("Connection: close");
client.println("Content-Type: application/x-www-form-urlencoded;");
client.print("Content-Length: ");
client.println(data.length());
client.println();
client.println(data);
String response = client.readString();
int bodypos = response.indexOf("\r\n\r\n") + 4;
DynamicJsonBuffer jsonBuffer;
JsonObject& root = jsonBuffer.parseObject(response.substring(bodypos));
String device_token = root[String("user_code")];
return device_token;
}

Answer 1

I am not sure if this is still relevant for you but I faced the same situation and I believe I found a solution. If it is not relevant to you, perhaps it will help some other, especially given that the default WiFiClientSecure library will soon be switched to BearSSL and no support is currently being given for the existing implementation.
Although I did not manage to speed up readString function, I used the WiFiClientSecure::read(uint8_t *buf, size_t size) function to get the data from the server:
// Buffer size, 128 bytes in this case
#define RESP_BUFFER_LENGTH 128
// Pointer to actual buffer
uint8_t * _buffer = new uint8_t[RESP_BUFFER_LENGTH];
// String to hold the final response
String _responseString = "";
// If info is still available
while (wifiClient.available())
{
// Fill the buffer and make a note of the total read length
int actualLength = wifiClient.read(_buffer, RESP_BUFFER_LENGTH);
// If it fails for whatever reason
if(actualLength <= 0)
{
// Handle as you see fit
return -1;
}
// Concatenate the buffer content to the final response string
// I used an arduino String for convenience
// but you can use strcat or whatever you see fit
_responseString += String((char*)_buffer).substring(0, actualLength);
}
// Clear buffer memory
delete[] _buffer;
I don't know why the regular readString() is so slow, but this method is considerably faster, with my relatively small messages (~50 bytes) the response is being read almost instantly.

Answer 2

If look under hood, readString() always reads all data and when it reads the last symbol it stucks with server communication.
To avoid it you should not read last one symbol somehow.
I read headers by using readStringUntil('\n') and searching Content-Length header.
When headers has been read, just use readBytes with content-length.
HTTP request now took around 150ms.