ClientContext (tcp) updates (#5089)

* +sync, get/set default nodelay, sync

* default nodelay=1

* update flush()

* fix return value

* ClientContext: put things together

* ClientContext: fix debugging messages

* WiFiClient: move static members out of the class, add comments

* remove circular dependency

* parameter and return value for Client::flush&stop, flush timeout raised to 300ms

* tcp flush: restart timer on ack receive

* OTA protocol needs setNoDelay(true)

* fix Ethernet with Client changes

* 1 line unredable -> 5 lines readable code

* doc

* Update client-class.rst

* Added details for getters

doc/esp8266wifi/client-class.rst

@@ -18,6 +18,17 @@ Methods documented for `Client <https://www.arduino.cc/en/Reference/WiFiClientCo
 
 Methods and properties described further down are specific to ESP8266. They are not covered in `Arduino WiFi library <https://www.arduino.cc/en/Reference/WiFi>`__ documentation. Before they are fully documented please refer to information below.
 
+flush and stop
+~~~~~~~~~~~~~~
+
+``flush(timeoutMs)`` and ``stop(timeoutMs)`` both have now an optional argument: ``timeout`` in millisecond, and both return a boolean.
+
+Default input value 0 means that effective value is left at the discretion of the implementer.
+
+``flush()`` returning ``true`` indicates that output data have effectively been sent, and ``false`` that a timeout has occurred.
+
+``stop()`` returns ``false`` in case of an issue when closing the client (for instance a timed-out ``flush``). Depending on implementation, its parameter can be passed to ``flush()``.
+
 setNoDelay
 ~~~~~~~~~~
 
@@ -35,6 +46,47 @@ This algorithm is intended to reduce TCP/IP traffic of small packets sent over t
 
     client.setNoDelay(true);
 
+getNoDelay
+~~~~~~~~~~
+
+Returns whether NoDelay is enabled or not for the current connection.
+
+setSync
+~~~~~~~
+
+This is an experimental API that will set the client in synchronized mode.
+In this mode, every ``write()`` is flushed.  It means that after a call to
+``write()``, data are ensured to be received where they went sent to (that is
+``flush`` semantic).
+
+When set to ``true`` in ``WiFiClient`` implementation,
+
+- It slows down transfers, and implicitely disable the Nagle algorithm.
+
+- It also allows to avoid a temporary copy of data that otherwise consumes
+  at most ``TCP_SND_BUF`` = (2 * ``MSS``) bytes per connection,
+
+getSync
+~~~~~~~
+
+Returns whether Sync is enabled or not for the current connection.
+
+setDefaultNoDelay and setDefaultSync
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+These set the default value for both ``setSync`` and ``setNoDelay`` for
+every future instance of ``WiFiClient`` (including those coming from
+``WiFiServer.available()`` by default).
+
+Default values are false for both ``NoDelay`` and ``Sync``.
+
+This means that Nagle is enabled by default *for all new connections*.
+
+getDefaultNoDelay and getDefaultSync
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Return the values to be used as default for NoDelay and Sync for all future connections.
+
 Other Function Calls
 ~~~~~~~~~~~~~~~~~~~~
 
@@ -54,7 +106,6 @@ Other Function Calls
     uint16_t  remotePort () 
     IPAddress  localIP () 
     uint16_t  localPort () 
-    bool  getNoDelay () 
 
 Documentation for the above functions is not yet prepared.
 

doc/esp8266wifi/server-class.rst

@@ -32,6 +32,10 @@ This algorithm is intended to reduce TCP/IP traffic of small packets sent over t
     server.begin();
     server.setNoDelay(true);
 
+By default, ``nodelay`` value will depends on global ``WiFiClient::getDefaultNoDelay()`` (currently false by default).
+
+However, a call to ``wiFiServer.setNoDelay()`` will override ``NoDelay`` for all new ``WiFiClient`` provided by the calling instance (``wiFiServer``).
+
 Other Function Calls
 ~~~~~~~~~~~~~~~~~~~~