asyncio: add a new "Develop with asyncio" section to the documentation

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+.. currentmodule:: asyncio
+
+Develop with asyncio
+====================
+
+Asynchronous programming is different than classical "sequential" programming.
+This page lists common traps and explain how to avoid them.
+
+
+Handle correctly blocking functions
+-----------------------------------
+
+Blocking functions should not be called directly. For example, if a function
+blocks for 1 second, other tasks are delayed by 1 second which can have an
+important impact on reactivity.
+
+For networking and subprocesses, the :mod:`asyncio` module provides high-level
+APIs like :ref:`protocols <protocol>`.
+
+An executor can be used to run a task in a different thread or even in a
+different process, to not block the thread of the event loop. See the
+:func:`BaseEventLoop.run_in_executor` function.
+
+
+.. _asyncio-logger:
+
+Logger
+------
+
+.. data:: asyncio.logger.log
+
+   :class:`logging.Logger` instance used by :mod:`asyncio` to log messages.
+
+The logger name is ``'asyncio'``.
+
+.. _asyncio-coroutine-not-scheduled:
+
+Detect coroutine objects never scheduled
+----------------------------------------
+
+When a coroutine function is called but not passed to :func:`async` or to the
+:class:`Task` constructor, it is not scheduled and it is probably a bug.
+
+To detect such bug, set :data:`asyncio.tasks._DEBUG` to ``True``. When the
+coroutine object is destroyed by the garbage collector, a log will be emitted
+with the traceback where the coroutine function was called. See the
+:ref:`asyncio logger <asyncio-logger>`.
+
+The debug flag changes the behaviour of the :func:`coroutine` decorator. The
+debug flag value is only when then coroutine function is defined, not when it
+is called.  Coroutine functions defined before the debug flag is set to
+``True`` will not be tracked. For example, it is not possible to debug
+coroutines defined in the :mod:`asyncio` module, because the module must be
+imported before the flag value can be changed.
+
+Example with the bug::
+
+    import asyncio
+    asyncio.tasks._DEBUG = True
+
+    @asyncio.coroutine
+    def test():
+        print("never scheduled")
+
+    test()
+
+Output in debug mode::
+
+    Coroutine 'test' defined at test.py:4 was never yielded from
+
+The fix is to call the :func:`async` function or create a :class:`Task` object
+with this coroutine object.
+
+
+Detect exceptions not consumed
+------------------------------
+
+Python usually calls :func:`sys.displayhook` on unhandled exceptions. If
+:meth:`Future.set_exception` is called, but the exception is not consumed,
+:func:`sys.displayhook` is not called. Instead, a log is emitted when the
+future is deleted by the garbage collector, with the traceback where the
+exception was raised. See the :ref:`asyncio logger <asyncio-logger>`.
+
+Example of unhandled exception::
+
+    import asyncio
+
+    @asyncio.coroutine
+    def bug():
+        raise Exception("not consumed")
+
+    loop = asyncio.get_event_loop()
+    asyncio.async(bug())
+    loop.run_forever()
+
+Output::
+
+    Future/Task exception was never retrieved:
+    Traceback (most recent call last):
+      File "/usr/lib/python3.4/asyncio/tasks.py", line 279, in _step
+        result = next(coro)
+      File "/usr/lib/python3.4/asyncio/tasks.py", line 80, in coro
+        res = func(*args, **kw)
+      File "test.py", line 5, in bug
+        raise Exception("not consumed")
+    Exception: not consumed
+
+There are different options to fix this issue. The first option is to chain to
+coroutine in another coroutine and use classic try/except::
+
+    @asyncio.coroutine
+    def handle_exception():
+        try:
+            yield from bug()
+        except Exception:
+            print("exception consumed")
+
+    loop = asyncio.get_event_loop()
+    asyncio.async(handle_exception())
+    loop.run_forever()
+
+Another option is to use the :meth:`BaseEventLoop.run_until_complete`
+function::
+
+    task = asyncio.async(bug())
+    try:
+        loop.run_until_complete(task)
+    except Exception:
+        print("exception consumed")
+
+See also the :meth:`Future.exception` method.
+
+
+Chain correctly coroutines
+--------------------------
+
+When a coroutine function calls other coroutine functions and tasks, they
+should chained explicitly with ``yield from``. Otherwise, the execution is no
+more guaranteed to be sequential.
+
+Example with different bugs using sleep to simulate slow operations::
+
+    import asyncio
+
+    @asyncio.coroutine
+    def create():
+        yield from asyncio.sleep(3.0)
+        print("(1) create file")
+
+    @asyncio.coroutine
+    def write():
+        yield from asyncio.sleep(1.0)
+        print("(2) write into file")
+
+    @asyncio.coroutine
+    def close():
+        print("(3) close file")
+
+    @asyncio.coroutine
+    def test():
+        asyncio.async(create())
+        asyncio.async(write())
+        asyncio.async(close())
+        yield from asyncio.sleep(2.0)
+        loop.stop()
+
+    loop = asyncio.get_event_loop()
+    asyncio.async(test())
+    loop.run_forever()
+    print("Pending tasks at exit: %s" % asyncio.Task.all_tasks(loop))
+
+Expected output::
+
+    (1) create file
+    (2) write into file
+    (3) close file
+    Pending tasks at exit: set()
+
+Actual output::
+
+    (3) close file
+    (2) write into file
+    Pending tasks at exit: {Task(<create>)<PENDING>}
+
+The loop stopped before the ``create()`` finished, ``close()`` has been called
+before ``write()``, whereas coroutine functions were called in this order:
+``create()``, ``write()``, ``close()``.
+
+To fix the example, tasks must be marked with ``yield from``::
+
+    @asyncio.coroutine
+    def test():
+        yield from asyncio.async(create())
+        yield from asyncio.async(write())
+        yield from asyncio.async(close())
+        yield from asyncio.sleep(2.0)
+        loop.stop()
+
+Or without ``asyncio.async()``::
+
+    @asyncio.coroutine
+    def test():
+        yield from create()
+        yield from write()
+        yield from close()
+        yield from asyncio.sleep(2.0)
+        loop.stop()
+
+.. XXX: Document "poll xxx" log message?
+