It seems to be a fair request, in the name of compatibility with Arduino API.

However, it would break many sketches and libraries because of the significant WiFiServer::available() semantic change.

There will be no change for simple TCP server answering to a simple request then closing.
There will be an undefined behavior / a total mess when several clients can be managed in parallel: such code would believe to have several independent connections, while they would be in fact all the same one. I would call this a collapsing change.

We can't really fully deprecate WiFiServer::available(), can we ?

What could be done:

• in all case, ::accept() must be ::available() old implementation
• add an optional user #define like ESP8266_ARDUINO_WIFISERVER_OLD_API to keep up with current = old API
• without this define, make WiFiServer::available() unavailable
• another #define is needed, like ESP8266_ARDUINO_WIFISERVER_LEGAL_API would enable official Arduino implementation for enabling ::available() like the one proposed in this PR.

pros:

• current sketches/libraries would not compile (= will not silently break) after these changes, unless by using ESP8266_ARDUINO_WIFISERVER_OLD_API which is harmless with older versions of this core.
• Arduino compatibility is possible but requires another define.

cons:

• Arduino compatibility is not totally transparent and requires a define: using ESP8266_ARDUINO_WIFISERVER_LEGAL_API in a sketch / a library also requires from the user to check that this define is supported

#ifndef ESP8266_ARDUINO_WIFISERVER_LEGAL_API_SUPPORTED
#error must use core 3.0.0+
#else 
#define ESP8266_ARDUINO_WIFISERVER_LEGAL_API
#endif

This proposal raises several potential issues depending on how these defines are really defined by the user/sketch/library.
Before moving forward, we must all reach an agreement about the above.
Any other proposal is welcome. The condition is that an existing sketch must not silently break.

In the years esp8266 Arduino exists, missed someone the true Arduino behaviour of available()? I think not.
If it would be my call, it would add accept() as alias to available() and document available() as it is now.

now I realized that the Arduino implementation of available() is tied with the write-to-all-clients functionality of Server base class as Print implementation. this is not implemented in ESP8266WiFi library.

WiFiServer telnetLogging; where every telnetLogging.print goes to all connected clients (or is ignored if there is no connected client) could be passed to any function or class which would take Print& log.

maybe a WiFiArduinoServer could inherit from WiFiServer and add the Arduino style available() function and the write-to-all-clients functionality. (same for SSL server)

maybe a WiFiArduinoServer could inherit from WiFiServer and add the Arduino style available() function and the write-to-all-clients functionality. (same for SSL server)

That would be a very good first step.

maybe a WiFiArduinoServer could inherit from WiFiServer and add the Arduino style available() function and the write-to-all-clients functionality. (same for SSL server)

That would be a very good first step.

I rework this PR. Should I use WiFiClient or ClientContext to store the clients? (ClientContext with ref and unref)

I think you have noticed that WiFiClient is just a wrapper around ClientContext.
If you don't intend to indeed use but only store the client, just use the simple version.

this will be the example called PagerServer, a simple server that echoes any incoming messages to all connected telnet clients.

void loop() {
  WiFiClient client = server.available(); // returns first client which has data to read
  if (client) { // client is true only if it is connected and has data to read
    String s = client.readStringUntil('\n'); // read the message incoming from one of the clients
    Serial.println(s); // print the message to Serial Monitor
    client.print("echo: "); // this is only for sending client
    server.println(s); // send the message to all connected clients
    server.flush(); // flush the buffers
  }
}

I think it is Arduino way elegant.

https://github.com/jandrassy/WiFiEspAT/blob/master/examples/Basic/PagerServer/PagerServer.ino

next attempt #7612