Issue #23347: send_signal(), kill() and terminate() methods of
BaseSubprocessTransport now check if the transport was closed and if the
process exited.

Issue #23347: Refactor creation of subprocess transports. Changes on
BaseSubprocessTransport:

* Add a wait() method to wait until the child process exit
* The constructor now accepts an optional waiter parameter. The _post_init()
  coroutine must not be called explicitly anymore. It makes subprocess
  transports closer to other transports, and it gives more freedom if we want
  later to change completly how subprocess transports are created.
* close() now kills the process instead of kindly terminate it: the child
  process may ignore SIGTERM and continue to run. Call explicitly terminate()
  and wait() if you want to kindly terminate the child process.
* close() now logs a warning in debug mode if the process is still running and
  needs to be killed
* _make_subprocess_transport() is now fully asynchronous again: if the creation
  of the transport failed, wait asynchronously for the process eixt. Before the
  wait was synchronous. This change requires close() to *kill*, and not
  terminate, the child process.
* Remove the _kill_wait() method, replaced with a more agressive close()
  method. It fixes _make_subprocess_transport() on error.
  BaseSubprocessTransport.close() calls the close() method of pipe transports,
  whereas _kill_wait() closed directly pipes of the subprocess.Popen object
  without unregistering file descriptors from the selector (which caused severe
  bugs).

These changes simplifies the code of subprocess.py.

is not explicitly closed. Close also explicitly transports in test_sslproto.

* Cleanup gather(): use cancelled() method instead of using private Future
  attribute
* Fix _UnixReadPipeTransport and _UnixWritePipeTransport. Only start reading
  when connection_made() has been called.
* Issue #23333: Fix BaseSelectorEventLoop._accept_connection(). Close the
  transport on error. In debug mode, log errors using call_exception_handler()

* _SelectorTransport constructor: extra parameter is now optional
* Fix _SelectorDatagramTransport constructor. Only start reading after
  connection_made() has been called.
* Fix _SelectorSslTransport.close(). Don't call protocol.connection_lost() if
  protocol.connection_made() was not called yet: if the SSL handshake failed or
  is still in progress. The close() method can be called if the creation of the
  connection is cancelled, by a timeout for example.

close() closes pipes, which is not None yet by _kill_wait().

only defining it when HAVE_MKNOD && HAVE_MAKEDEV are true.

This "oops" issue reported by John E. Malmberg on core-mentorship.
(what kinds of systems don't HAVE_MKNOD && HAVE_MAKEDEV?)

Only start reading when connection_made() has been called:
protocol.data_received() must not be called before protocol.connection_made().

* Rephrase also the comment explaining why the waiter is not awaken immediatly.
* SSLProtocol.eof_received() doesn't instanciate ConnectionResetError exception
  directly, it will be done by Future.set_exception(). The exception is not
  used if the waiter was cancelled or if there is no waiter.

Wake-up the waiter if it is not done yet.

no dynamically allocated bases.

* Remove unused SSLProtocol._closing attribute
* test_sslproto: skip test if ssl module is missing
* Python issue #23208: Don't use the traceback of the current handle if we
  already know the traceback of the source. The handle may be more revelant,
  but having 3 tracebacks (handle, source, exception) becomes more difficult to
  read. The handle may be preferred later but it requires more work to make
  this choice.

Patch by Mayank Tripathi.

If ReadFile() fails with ERROR_BROKEN_PIPE, the operation is not pending: don't
register the overlapped.

I don't know if WSARecv() can fail with ERROR_BROKEN_PIPE. Since
Overlapped.WSARecv() already handled ERROR_BROKEN_PIPE, let me guess that it
has the same behaviour than ReadFile().

If UnregisterWaitEx() fais with ERROR_IO_PENDING, it doesn't mean that the wait
is unregistered yet. We still have to wait until the wait is cancelled.

Use a coroutine with asyncio.sleep() instead of call_later() to ensure that the
schedule call is cancelled.

Add also a unit test cancelling connect_pipe().

If a client connected before the server was closed: drop the client (close the
pipe) and exit.

a function or a loop (e.g. "return" or "break").

In debug mode, BaseEventLoop._run_once() now sets the
BaseEventLoop._current_handle attribute to the handle currently executed.
In release mode or when no handle is executed, the attribute is None.

BaseEventLoop.default_exception_handler() displays the traceback of the current
handle if available.

The special case for connect_pipe() is not more needed. connect_pipe() doesn't
use overlapped operations anymore.

Fix create_datagram_endpoint(), connect_read_pipe() and connect_write_pipe():
close the transport if the task is cancelled or on error.

directory containing a backslash.

returned NotImplemented.  Removed incorrect implementations of __ne__().

Patch by Cory Benfield.

replacment string longer than mailformed input data.

This change parallels the language used in the unix man page.