As discussed, we want to make Server objects more usable in async/await code and more compatible with asyncio.run.

This is also needed to handle a use case when two or more servers are created and need to start listening at the same time.

We propose to:

1. Add a new bool flag defaulting to True to loop.create_server and loop.create_unix_server: start_serving.  By default, loop will return a server that is already accepting connections.  When start_serving is set to False, create_server and create_unix_server will return a server that will not listen on its sockets.

2. A new idempotent Server.start_serving() method can be used to make server listen on its sockets (useful when a server object was created with start_serving=False).

3. A new Server.serve_forever() method that calls start_serving() and blocks forever, until cancelled.  When cancelled, it closes its server object.

4. A new Server.is_serving() method.  This is useful to introspect a server object in unittests.

5. Server objects should be async context managers.  Server.__aexit__ should close the server and await on Server.wait_closed().

With these new APIs, the following pattern becomes possible:

    async def main():
        srv = await asyncio.start_server(...)
        async with srv:
            await srv.serve_forever()

    asyncio.run(main())

New changeset c9070d03f5169ad6e171e641b7fa8feab18bf229 by Yury Selivanov in branch 'master':
bpo-32662: Implement Server.start_serving() and Server.serve_forever() (#5312)
https://github.com/python/cpython/commit/c9070d03f5169ad6e171e641b7fa8feab18bf229

New changeset 4112c5b97d9c1c7b034653d0e017ffa894a45c74 by Yury Selivanov in branch 'master':
bpo-32662: Try making test_asyncio.test_server more reliable (#5338)
https://github.com/python/cpython/commit/4112c5b97d9c1c7b034653d0e017ffa894a45c74

test_asyncio.test_start_server_1() hangs on "s390x Debian 3.x" buildbot:
http://buildbot.python.org/all/#builders/13/builds/579

test_set_nodelay (test.test_asyncio.test_selector_events.TestSelectorUtils) ... ok
test_start_server_1 (test.test_asyncio.test_server.ProactorStartServerTests) ... skipped 'Windows only'
Exception in thread test-client:
Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/test_asyncio/functional.py", line 193, in run
    self._prog(TestSocketWrapper(self._sock))
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/test_asyncio/test_server.py", line 44, in <lambda>
    with self.tcp_client(lambda sock: client(sock, addr)):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/test_asyncio/test_server.py", line 19, in client
    sock.connect(addr)
ConnectionRefusedError: [Errno 111] Connection refused

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/test_asyncio/functional.py", line 195, in run
    self._test._abort_socket_test(ex)
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/test_asyncio/functional.py", line 122, in _abort_socket_test
    self.fail(ex)
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/case.py", line 680, in fail
    raise self.failureException(msg)
AssertionError: [Errno 111] Connection refused

Timeout (0:15:00)!
Thread 0x000003fffcdcd710 (most recent call first):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/selectors.py", line 468 in select
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/asyncio/base_events.py", line 1555 in _run_once
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/asyncio/base_events.py", line 450 in run_forever
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/asyncio/base_events.py", line 482 in run_until_complete
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/test_asyncio/test_server.py", line 45 in test_start_server_1
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/case.py", line 615 in run
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/case.py", line 663 in __call__
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/suite.py", line 122 in run
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/suite.py", line 84 in __call__
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/suite.py", line 122 in run
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/suite.py", line 84 in __call__
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/suite.py", line 122 in run
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/suite.py", line 84 in __call__
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/suite.py", line 122 in run
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/suite.py", line 84 in __call__
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/suite.py", line 122 in run
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/suite.py", line 84 in __call__
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/unittest/runner.py", line 176 in run
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/support/__init__.py", line 1861 in _run_suite
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/support/__init__.py", line 1951 in run_unittest
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/libregrtest/runtest.py", line 175 in test_runner
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/libregrtest/runtest.py", line 176 in runtest_inner
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/libregrtest/runtest.py", line 140 in runtest
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/libregrtest/main.py", line 291 in rerun_failed_tests
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/libregrtest/main.py", line 540 in _main
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/libregrtest/main.py", line 510 in main
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/libregrtest/main.py", line 585 in main
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/__main__.py", line 2 in <module>
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/runpy.py", line 85 in _run_code
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/runpy.py", line 193 in _run_module_as_main
test_start_server_1 (test.test_asyncio.test_server.SelectorStartServerTests) ... Makefile:1090: recipe for target 'buildbottest' failed
make: *** [buildbottest] Error 1
program finished with exit code 2
elapsedTime=2050.270461

The test fails differently on s390x SLES 3.x:
http://buildbot.python.org/all/#/builders/16/builds/576

...
test_force_close (test.test_asyncio.test_selector_events.SelectorTransportTests) ... ok
test_set_nodelay (test.test_asyncio.test_selector_events.TestSelectorUtils) ... ok
test_start_server_1 (test.test_asyncio.test_server.ProactorStartServerTests) ... skipped 'Windows only'
test_start_server_1 (test.test_asyncio.test_server.SelectorStartServerTests) ... Exception in thread test-client:
Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/test/test_asyncio/functional.py", line 193, in run
    self._prog(TestSocketWrapper(self._sock))
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/test/test_asyncio/test_server.py", line 44, in <lambda>
    with self.tcp_client(lambda sock: client(sock, addr)):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/test/test_asyncio/test_server.py", line 19, in client
    sock.connect(addr)
ConnectionRefusedError: [Errno 111] Connection refused

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/test/test_asyncio/functional.py", line 195, in run
    self._test._abort_socket_test(ex)
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/test/test_asyncio/functional.py", line 122, in _abort_socket_test
    self.fail(ex)
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/unittest/case.py", line 680, in fail
    raise self.failureException(msg)
AssertionError: [Errno 111] Connection refused

/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/test/test_asyncio/test_server.py:45: ResourceWarning: unclosed <socket.socket fd=8, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('127.0.0.1', 56340)>
  self.loop.run_until_complete(main_task)
ERROR
test_start_unix_server_1 (test.test_asyncio.test_server.SelectorStartServerTests) ... ok
test_start_tls_client_1 (test.test_asyncio.test_sslproto.ProactorStartTLSTests) ... skipped 'Windows only'
test_start_tls_server_1 (test.test_asyncio.test_sslproto.ProactorStartTLSTests) ... skipped 'Windows only'
test_start_tls_wrong_args (test.test_asyncio.test_sslproto.ProactorStartTLSTests) ... skipped 'Windows only'
test_start_tls_client_1 (test.test_asyncio.test_sslproto.SelectorStartTLSTests) ... /home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/collections/__init__.py:404: ResourceWarning: unclosed <socket.socket fd=7, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('127.0.0.1', 42302)>
  result = _self._make(map(kwds.pop, field_names, _self))
Unhandled error in exception handler
context: {'message': 'Task was destroyed but it is pending!', 'task': <Task pending coro=<BaseStartServer.test_start_server_1.<locals>.main() done, defined at /home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/test/test_asyncio/test_server.py:31> wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x3fff979bbf8>()]>>}
Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/asyncio/base_events.py", line 1459, in call_exception_handler
    self._exception_handler(self, context)
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/test/test_asyncio/functional.py", line 22, in loop_exception_handler
    self.loop.default_exception_handler(context)
AttributeError: 'NoneType' object has no attribute 'default_exception_handler'
ok
test_start_tls_server_1 (test.test_asyncio.test_sslproto.SelectorStartTLSTests) ... ok
test_start_tls_wrong_args (test.test_asyncio.test_sslproto.SelectorStartTLSTests) ... ok
test_cancel_handshake (test.test_asyncio.test_sslproto.SslProtoHandshakeTests) ... ok
test_close_during_handshake (test.test_asyncio.test_sslproto.SslProtoHandshakeTests) .
...

======================================================================
ERROR: test_start_server_1 (test.test_asyncio.test_server.SelectorStartServerTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/test/test_asyncio/test_server.py", line 45, in test_start_server_1
    self.loop.run_until_complete(main_task)
  File "/home/dje/cpython-buildarea/3.x.edelsohn-sles-z/build/Lib/asyncio/base_events.py", line 493, in run_until_complete
    raise RuntimeError('Event loop stopped before Future completed.')
RuntimeError: Event loop stopped before Future completed.
----------------------------------------------------------------------

Just to be complete, the test also fails on x86 Gentoo Non-Debug with X 3.x:
http://buildbot.python.org/all/#/builders/99/builds/564

...
test_start_server_1 (test.test_asyncio.test_server.SelectorStartServerTests) ... Exception in thread test-client:
Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/test/test_asyncio/functional.py", line 193, in run
    self._prog(TestSocketWrapper(self._sock))
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/test/test_asyncio/test_server.py", line 44, in <lambda>
    with self.tcp_client(lambda sock: client(sock, addr)):
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/test/test_asyncio/test_server.py", line 19, in client
    sock.connect(addr)
ConnectionRefusedError: [Errno 111] Connection refused

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/test/test_asyncio/functional.py", line 195, in run
    self._test._abort_socket_test(ex)
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/test/test_asyncio/functional.py", line 122, in _abort_socket_test
    self.fail(ex)
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/unittest/case.py", line 680, in fail
    raise self.failureException(msg)
AssertionError: [Errno 111] Connection refused

Timeout (0:15:00)!
Thread 0xb7d69700 (most recent call first):
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/selectors.py", line 468 in select
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/asyncio/base_events.py", line 1555 in _run_once
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/asyncio/base_events.py", line 450 in run_forever
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/asyncio/base_events.py", line 482 in run_until_complete
  File "/buildbot/buildarea/3.x.ware-gentoo-x86.nondebug/build/Lib/test/test_asyncio/test_server.py", line 45 in test_start_server_1
...

I think these failures were on the previous version of the test; i've fixed it last night and it shouldn't timeout this way anymore.