Description:

WiFiClient.connected() is broken since merging of #1388
connected() won't change to false even when with no data in receive buffer and FIN packet received (indication from other side no more data is incomming, normally via shutdown(SHUT_WR) call).

So any simple code like this won't work properly for now (well, unless TCP will timeout sooner or later):

while (client.connected()) 
          if (client.available() > 0)
            client.readBytes(buf, min(256, client.available()));

Please find the minimal sketch below, connect to your esp32 with telnet ("telnet 192.168.0.66 8080" for example) and close telnet window (if I kill telnet process instead of closing its window it will send RST packet and any build will behave the same). Sketch built with espressif32@~0.12.0 will drop from 'connected' cycle immediately, sketch built with current release or staging will wait for TCP timeout.

After inserting log_e into WiFiClient::connected():
espressif32@~0.12.0:

[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 11
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 11
---Connected to client---
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 11
....
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 11
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res 0 errno 128
---Client closed connection---

current release or staging:

[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 11
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 11
---Connected to client---
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 11
....
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 11
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res 0 errno 128
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 11
.....
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 11
[E][WiFiClient.cpp:316] connected(): wifi_connected: recv res -1 errno 104
---Client closed connection---

Error codes:
#define ECONNRESET 104 /* Connection reset by peer /
#define ENOTCONN 128 / Socket is not connected /
#define EAGAIN 11 / No more processes */

I did no research if recv res = 0 and error code ENOTCONN is a valid behavior on idf side, that issue might be a manifestation of bug in it.

Screenshots from wireshark (top - current release, bottom espressif32@~0.12.0, each time first connection ended by closing telnet (shutdown/FIN packet), second connection terminated (close/RST packet).

Sketch:

#include <Arduino.h>
#include <WiFi.h>
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>

WiFiServer server(8080);

void setup() {
  Serial.begin(115200);

   WiFi.mode(WIFI_STA);
   WiFi.begin("111", "111");
   while (WiFi.waitForConnectResult() != WL_CONNECTED) {
     Serial.println("Connection Failed! Rebooting...");
     delay(5000);
     ESP.restart();
   }
    Serial.print("Connected to wifi. My address:");
    IPAddress myAddress = WiFi.localIP();
    Serial.println(myAddress);

    server.begin();
}

void loop() {
    WiFiClient client = server.available();
    if (client) {
      if (client.connected()) 
        Serial.println("---Connected to client---");

      unsigned char buf[256];
      while (client.connected()) {
          if (client.available() > 0) {
            int l = client.available();
            if (l > 256)  l = 256;
            client.readBytes(buf, l);
            Serial.write(buf, l);
          } else
            delay(300);
      }
      Serial.println("---Client closed connection---");
      client.stop();
    }
}