CHANGES

CHANGES
-------

Version 0.10 : In Progress
--------------------------

**** SPECIAL NOTE ****: For the past few years, Curio has been been an
experimental project.  However, as it moves towards having a more
"stable" release, I feel that it is better served by being small as
opposed to providing every possible feature. Thus, a wide range of
features are being removed or refactored a bit.  If a feature you are
using gets removed and you'd like to request its return, please submit
a bug report!  - Dave

10/15/2019 Refactored task.py into separate files for tasks and time
           management.

09/29/2019 Removed Promise.  Not documented, but also want to rethink the
           whole design/implementation of it. The "standard" way that
           Python implements "promises" is through the Future class
           as found in the concurrent.futures module.  However, Futures
           are tricky.  They often have callback functions attached to 
           them and they can be cancelled.  Some further thought needs
           to be given as to how such features might integrate with the
           rest of Curio.  Code for the old Promise class can be 
           found the examples directory.

09/26/2019 Removed support for the Asyncio bridge. It wasn't documented
           and there are many possible ways in which Curio might 
           potentially interact with an asyncio event loop. For example,
           using queues.  Asyncio interaction may be revisited in the
           future.  

09/13/2019 Support for context-manager use of spawn_thread() has been
           withdrawn.  It's a neat feature, but the implementation
           is pretty hairy.  It also creates a situation where async
           functions partially run in coroutines and partially in threads.
           All things equal, it's probably more sane to make this 
           kind of distinction at the function level, not at the level
           of code blocks.   

09/11/2019 Removed AsyncObject and AsyncInstanceType. Very specialized.
           Not used elsewhere in Curio. Involves metaclasses. One
           less thing to maintain.

09/11/2019 I want to use f-strings. Now used for string formatting
           everywhere except in logging messages. Sorry Python 3.5.

09/11/2019 Removed the allow_cancel optional argument to spawn().
           If a task wants to disable cancellation, it should 
           explicitly manage that on its own. 

09/11/2019 Removed the report_crash option to spawn(). Having
           it as an optional argument is really the wrong place for
           this.  On-by-default crash logging is extremely useful for
           debugging.  However, if you want to disable it, it's
           annoying to have to go change your source code on a task-by-task
           basis.  A better option is to suppress it via configuration 
           of the logging module.  Better yet: write your code so that
           it doesn't crash.

09/10/2019 Some refactoring of some internal scheduling operations.
           The SchedFIFO and SchedBarrier classes are now available
           for more general use by any code that wants to implement
           different sorts of synchronization primitives.

09/10/2019 Removed the abide() function.  This feature was from the
           earliest days of Curio when there was initial thinking 
           about the interaction of async tasks and existing threads.
           The same functionality can still be implemented using run_in_thread()
           or block_in_thread() instead.  In the big picture, the problem
           being solved might not be that common.  So, in the interest
           of making Curio smaller, abide() has ridden off into the sunset.

09/08/2019 Removed BoundedSemaphore.

09/03/2019 Removed the experimental aside() functionality.  Too much 
           magic.  Better left to others.

09/03/2019 Removed the gather() function.  Use TaskGroup instead.

09/03/2019 Removed get_all_tasks() function.

09/03/2019 Removed the Task.pdb() method.

09/03/2019 Removed the Task.interrupt() method.

09/03/2019 The pointless (and completely unused) name argument to TaskGroup() 
           has been removed. 

08/09/2019 Exceptions raised inside the Curio kernel itself are no longer 
           reported to tasks.  Instead, they cause Curio to die. The
           kernel is never supposed to raise exceptions on its own--any
           exception that might be raised is an internal programming error.
           This change should not impact user-level code, but might affect
           uncontrolled Ctrl-C handling. If a KeyboardInterrupt occurs
           in the middle of kernel-level code, it will cause an uncontrolled
           death. If this actually matters to you, then modify your code to
           properly handle Ctrl-C via signal handling.

04/14/2019 The Channel.connect() method no longer implements auto-retry. 
           In practice, this kind of feature can cause interference. Better
           to let the caller do the retry if they want.

04/14/2019 Simplified the implementation and semantics of cancellation control.
           The enable_cancellation() function has been removed.  It is now
           only possible to disable cancellation.  Nesting is still allowed.
           Pending cancellation exceptions are raised on the first blocking
           call executed when reenabled.  The check_cancellation() function 
           can be used to explicitly check for cancellation as before.
           
03/09/2019 Fixed a bug in network.open_connection() that was passing arguments
           incorrectly.  Issue #291.

11/18/2018 Removed code that attempted to detect unsafe async generator
           functions.  Such code is now executed without checks or 
           warnings.  It's still possible that code in finally blocks
           might not execute unless you use curio.meta.finalize() or
           a function such as async_generator.aclosing() (third party).
           The @safe_generator decorator is now also unnecessary.

11/11/2018 Removed the wait argument to TaskGroup.join().  The waiting policy
           for task groups is specified in the TaskGroup constructor.  For
           example:

               with TaskGroup(wait=any) as group:
                    ...

09/05/2018 Tasks submitted to Kernel.run() no longer log exceptions should they
           crash.  Such tasks already return immediately to the caller with the
           exception raised. 

09/05/2018 Refinement to Kernel.__exit__() to make sure the kernel shuts down
           regardless of any exceptions that have occurred.  See Issue #275.

04/29/2018 New task-related function.  get_all_tasks() returns a list of all active
           Tasks. For example:

               tasks = await get_all_tasks()

           Tasks also have a where() method that returns a (filename, lineno) tuple
           indicating where they are executing.

04/29/2018 Curio now properly allows async context managers to be defined using
           context.asynccontextmanager.  Issue #247.

04/29/2018 Removed the cancel_remaining keyword argument from TaskGroup.next_done()

04/28/2018 Added new "object" wait mode to TaskGroup.  It waits for the
           first non-None result to be returned by a task.  Then all 
           remaining tasks are cancelled.  For example:

               async def coro1():
                   await sleep(1)
                   return None

               async def coro2():
                   await sleep(2)
                   return 37

               async def coro3():
                   await sleep(3)
                   return 42
 
               async with TaskGroup(wait=object) as tg:
                   await tg.spawn(coro1)      # Ignored (returns None)
                   await tg.spawn(coro2)      # Returns result
                   await tg.spawn(coro3)      # Cancelled

               print(tg.completed.result)  # -> 37

04/27/2018 Removed the ignore_result keyword argument to TaskGroup.spawn().
           It's not really needed and the extra complexity isn't worth it.

04/27/2018 Added TaskGroup.next_result() function.  It's mostly a convenience
           function for returning the result of the next task that completes.
           If the task failed with an exception, that exception is raised.

04/14/2018 Changed the method of spawning processes for run_in_process to
           use the "spawn" method of the multiprocessing module. This 
	   prevents issues of having open file-descriptors cloned by
           accident via fork().  For example, as in Issue #256.

Version 0.9 : March 10, 2018
----------------------------
02/24/2018 Refinements to Task crash reporting.  By default all Tasks
           that terminate with an uncaught exception log that exception
           as soon as it is detected.  Although there is a risk that this
           creates extra output, silencing the output makes it almost
           impossible to debug programs.  This is because errors get
           deferred to a later join() method---and you often don't know
           when that's going to take place.  You might just be staring
           a program wondering why it's not working.    If you really
           want errors to be silent, use spawn(coro, report_crash=False).
           
02/23/2018 Some refinements to the AWAIT() function.  You can now call it as 
           follows:

               result = AWAIT(callable, *args, **kwargs)

           If the passed callable is a coroutine function or a function that
           produces an awaitable object, it will be passed to Curio and executed
           in an asynchronous context.   If callable is just a normal function,
           then callable(*args, **kwargs) is returned.

           This change is made to make it slightly easier to use objects
           as UniversalQueue within the context of an async thread.  For example,
           if you do this::

               q = UniversalQueue()
               ...
               @async_thread
               def consumer():
                   while True:
                       item = AWAIT(q.get)
                       print("Got:", item)
 
            The AWAIT operation will run the asynchronous version of q.get()
            instead of the normal synchronous version.  You want this--the
            async version supports cancellation and other nice features.

02/22/2018 The async_thread() function is now meant to be used as a decorator only.

              @async_thread
              def func(args):
                  ...

           When the decorated function is called from asynchronous code using
           await func(args), it will seamlessly run in a separate execution thread.
           However, the function can also be called from synchronous code using
           func(args).  In that case, the function runs as it normally does.
           It's subtle, but the @async_thread decorator allows a function to adapt
           to either a synchronous or asynchronous environment depending on how 
           it has been called. 
 
02/22/2018 New function spawn_thread() can be used to launch asynchronous threads.
           It mirrors the use of the spawn() function. For example:

              def func(args):
                  ...

              t = await spawn_thread(func, args)
              result = await t.join()

           Previously, async threads were created using the AsyncThread class.
	   That still works, but the spawn_thread() function is probably easier.

           The launched function must be a normal synchronous function. 
           spawn_thread() can also be used as a context manager (see below).

02/19/2018 Added ability of async threads to be created via context manager.
           For example:

              async with spawn_thread():
                  # Various blocking/synchronous operations
                  # Executes in a separate thread
                  ...

           When used, the body of the context manager runs in a
           separate thread and may involve blocking operations.
           However, be aware that any use of async/await is NOT
           allowed in such a block.  Any attempt to await on an async
           function inside the block will result in a RuntimeError
           exception. 

02/06/2018 Refinements to the schedular activation API and debugging
           features.

02/04/2018 The ZMQ module has been removed from Curio core and put into
           the examples directory.   This should be spun into a separate
           package maintained independently of Curio.

02/03/2018 Local() objects have been removed.  The idea of having a
           thread-local style object for storing attributes is fraught
           with problems--especially given the potential mix of tasks,
           threads, and processes that is possible in Curio.  The
           implementation of this has been moved into the examples
           directory where it can be adapted/copied into your code if
           it's still needed.   Better yet, maybe take a look at PEP 567.

02/02/2018 Some refactoring and simplification of the kernel run() 
	   method.  First, run() only executes as long as the
           submitted coroutine is active.  Upon termination, run()
           immediately returns regardless of whether or not other
           tasks are still running.  Curio was already generating
           warning messages for orphaned tasks--tasks for which
           Task.join() is never invoked.  As such, this change should
           not affect most properly written applications.

           Second, the timeout argument is no longer supported. If
           you want a timeout on what's happening, put it into the 
           supplied async function itself.

           Finally, if Kernel.run() is called with no arguments, it
           causes the kernel to process any activity that might be
           pending at that moment in time before returning.  This is
           something that might be useful should it be necessary
           to integrate Curio with a foreign event-loop.

01/31/2018 If the main task crashes with an exception, the kernel
           now returns immediately--even if child tasks are still
           in progress.   This prevents a problem where the main task
           crashes, but it's not reported for an extended period due
           to child tasks continuing to run.  In addition, if the
           main task crashes, the kernel does not perform a shutdown--
           leaving tasks as they were at the time of the crash.  This
           might facilitate debugging.

01/31/2018 Printing a Task object will now show the file and line number
           of where it's currently waiting.

01/23/2018 A refinement in task crash reporting.  Previously, all 
           task crashes were logged.  However, Curio was also logging
           crashes in all unjoined tasks.  Sometimes this would result
           in duplicate tracebacks.  It's been modified to now only report
           crashes in unjoined tasks.  If joining, it's assumed that the
           exception would be noticed there.

01/22/2018 Semaphore and BoundedSemaphore now exposes a read-only
	   property ".value" that gives the current Semaphore value.
           BoundedSemaphore objects expose a ".bound" property that
           gives the upper bound.
           
01/03/2018 The Local() object has been officially deprecated in the 
           documentation and will be removed at some point.
           Implementing task-level locals is much more complicated
           than it seems at first glance and there is discussion of a
           more general solution in PEP 567.   If you need this kind
           of functionality, you should copy the task local code into
           your own application.

12/22/2017 writeable() method of Socket removed.  Use of this was highly
           specialized and potentially confusing since it's not normally
           needed.   Use await _write_wait(sock) if you need to wait
           for a socket to be writable before calling send().

12/19/2017 Slight change to Task Groups that allow tasks to spawn
           other tasks into the same group.  See Issue #239.

09/15/2017 Added the .readinto() method onto async files.

09/15/2017 Made the @async_thread decorator return the actual
           exception that gets raised in the event of a failure.
	   This makes it more like a normal function call.  Previously,
	   it was returning a TaskError exception for any crash.
           That's a bit weird since the use of threads or spawning
           of an external task is meant to be more implicit.

Version 0.8 : August 27, 2017
-----------------------------
07/01/2017 New time queue implementation. For timeout handling, it's
           faster and far more space efficient.

05/11/2017 Fixed Issue #212, "never joined" message when add terminated
           tasks to a task group and using task.result to obtain the 
           result.

05/11/2017 Added a new keyword argument to Task.cancel() to allow a different
           exception to be raised.  For example:

              t.cancel(exc=SomeException)

           The default exception is still TaskCancelled.

04/23/2017 Change to ssl.wrap_socket() function and method to make it an
           async method.   If applied to an already connected socket, it
           needs to run the handshake--which needs to be async.  See
	   Issue #206.

04/14/2017 Refinement to SignalEvent to make it work better on Windows.
           It's now triggered via the same file descriptor used for SignalQueue
           objects.

03/26/2017 Added a Task.interrupt() method.  Cancels the task's current
	   blocking operation with an 'TaskInterrupted' exception.
	   This is really just a more nuanced form of cancellation,
	   similar to a timeout.  However, it's understood that an 
	   interrupted operation doesn't necessarily mean that the
	   task should quit.   Instead, the task might coordinate with 
	   other tasks in some way and retry later. 

Version 0.7 : March 17, 2017
----------------------------

03/15/2017 The undocumented wait() function for waiting on multiple
           tasks has evolved into a more general TaskGroup object.
           To replicate the old wait(), do this:

              t1 = spawn(coro1, args)
              t2 = spawn(coro2, args)
              t3 = spawn(coro3, args)

              async with TaskGroup([t1,t2,t3]) as g:
                  first_done = await g.next_done()
                  await g.cancel_remaining()

           TaskGroups have more functionality such as the ability
           to spawn tasks, report multiple errors and more.

           For example, the above code could also be written as follows:

              async with TaskGroup() as g:
                  await g.spawn(coro1, args)
                  await g.spawn(coro2, args)
                  await g.spawn(coro3, args)
                  first_done = await g.next_done()
                  await g.cancel_remaining()

03/12/2017 Added a .cancelled attribute to the context manager used
	   by the ignore_after() and ignore_at() functions.  It
	   can be used to determine if a timeout fired.  For example:

              async with ignore_after(10) as context:
                  await sleep(100)

              if context.cancelled:
                  print('Cancelled!')

03/10/2017 SignalSet is gone.   Use a SignalQueue instead.  Usage
           is almost identical:

               async with SignalQueue(signal.SIGUSR1) as sq:
                   while True:
                        signo = await sq.get()
                        ...

03/08/2017 More work on signal handling. New objects: SignalQueue 
	   and SignalEvent.  SignalEvents are neat:

               import signal
               from curio import SignalEvent

               ControlC = SignalEvent(signal.SIGINT)

               async def coro():
                   await ControlC.wait()
                   print("Goodbye")

03/08/2017 UniversalEvent object added.  An event that's safe for use in
	   Curio and threads.

03/08/2017 Further improvement to signal handling.  Now handled by a backing
           thread.  Supports signal delivery to multiple threads, 
           multiple instances of Curio running, asyncio, and all sorts
           of stuff.

03/08/2017 Removed signal handling from the kernel up into Curio
	   "user-space".  No existing code the uses signals should break.

03/07/2017 Refined error reporting and warnings related to Task
           termination.  If any non-daemonic task is garbage collected
           and it hasn't been explicitly joined or cancelled, a
           warning message is logged.  This warning means that a task
           was launched, but that nobody ever looked at its result.
           If any unjoined task is garbage collected and it has
           crashed with an uncaught exception, that exception is
           logged as an error.

	   This change has a few impacts.  First, if a task crashes,
           but is joined, you won't get a spurious output message
           showing the crash.  The exception was delivered to someone
           else.   On the other hand, you might get more warning
           messages if you've been launching tasks without paying
           attention to their result. 

03/06/2017 A lot of cleanup of the kernel. Moved some functionality 
	   elsewhere. Removed unneeded traps.  Removed excess
           abstraction in the interest of readability.  The different
           trap functions in Curio are almost all quite small.
           However, details concerning their execution behavior was
           split across a few different places in the code and wrapped
           with decorators--making it somewhat hard to piece together
           how they worked looking at them in isolation.  Each trap 
	   is now basically self-contained.  You can look at the code and 
	   see exactly what it does.  Each trap is also afforded more
	   flexibility about how it could work in the future (e.g.,
	   scheduling behavior, cancellation, etc.).  

	   Debug logging features have been removed from the kernel and
	   placed into a new subsystem.  See the file curio/debug.py.
	   This is still in progress.

03/05/2017 Change to how the debugging monitor is invoked.  It's still
           an option on the run() function.  However, it is not a
           option on the Kernel class itself.  If you need to do that,
           use this:

                from curio import Kernel
                from curio.monitor import Monitor

                k = Kernel()
                m = Monitor(k)

03/04/2017 Support for using Event.set() from a synchronous context has
           been withdrawn.   This was undocumented and experimental.
	   There are other mechanisms for achieving this.  For example,
	   communicating through a UniversalQueue.

03/03/2017 timeout_after() and related functions now accept
           coroutine functions and arguments such as this:

              async def coro(x, y):
                  pass

              async def main():
                  try:
                      await timeout_after(5, coro, 2, 3) 
                  except TaskTimeout:
                      pass
            
03/03/2017 spawn() and run() have been made consistent in their
           calling conventions compared to worker related functions.
           For example:

              async def coro(x, y):
                  pass

              async def main():
                  t = await spawn(coro, 2, 3)   # Instead of spawn(coro(2,3))

           The old approach still works, but the new one will be preferred
           going forward.

03/03/2017 Support for keyword arguments on many task-related worker
           functions (run_in_thread, run_in_process, block_in_thread, etc.)
           has been rescinded.   If you need keyword arguments, use
           functools.partial.   For example:

              await run_in_thread(partial(foo, kw=some_value))

03/03/2017 Functionality for using Queue.put() in a synchronous context
           has been withdrawn. This was always experimental and undocumented.
	   There are better alternatives for doing this.  For example, use a
           UniversalQueue.

03/01/2017 Addition of an asyncio bridge.  You can instantiate a separate
           asyncio loop and submit tasks to it.  For example:

              async def coro():
                  # Some coroutine that runs on asyncio
                  ...
              async with AsyncioLoop() as loop:
                  await loop.run_asyncio(coro)

           The same loop can be used by any number of Curio tasks and
           requests can run concurrently.  The asyncio loop runs in 
           a separate thread than Curio.

           Original idea contributed by Laura Dickinson and adapted a
           a bit into the AsyncioLoop class. 

02/26/2017 Modified the gather() function so that it also cancels all tasks
           if it is cancelled by timeout or other means.  See issue #186.
	   The resulting exception has a .results attribute set with
	   the results of all tasks at the time of cancellation.

02/19/2017 Added new curio.zmq module for supporting ZeroMQ.

Version 0.6 : February 15, 2017
-------------------------------

02/13/2017 Added a withfd=True option to UniversalQueue.  For example:

              q = UniversalQueue(withfd=True)

           If added, the queue internally sets up an I/O loopback
           where putting items on the queue write bytes to an I/O
           channel.  The queue then spouts a fileno() method and
           becomes pollable in other event loops.  This is potentially
           useful strategy for integrating Curio with GUIs and other
           kinds of foreign event loops.  

02/11/2017 Added a guard for proper use of asynchronous generators
           involving asynchronous finalization.  Must be wrapped by finalize(). 
	   For example:

           async def some_generator():
               ...
               try:
                   yield val
               finally:
                   await action()

           async def coro():
               ...
               async with finalize(some_generator()) as agen:
                   async for item in agen:
                       ...

           Failure to do this results in a RuntimeError if an
           asynchronous generator is iterated.   This is not needed for
           generators that don't perform finalization steps involving
           async code.

02/08/2017 New Kernel.run() method implementation.  It should be backwards
           compatible, but there are two new ways of using it:

               kernel = Kernel()
               ...
               # Run a coroutine with a timeout/deadline applied to it
               try:
                   result = kernel.run(coro, timeout=secs)
               except TaskTimeout:
                   print('Timed out')

               # Run all daemonic tasks through a single scheduling cycle
               # with no blocking
               kernel.run()

               # Run all daemonic tasks through a cycle, but specify a
               # timeout on internal blocking
               kernel.run(timeout=secs)
                
02/06/2017 New aside() function for launching a Curio task in an
           independent process.  For example:

           async def child(name, n):
               print('Hello from', name)
               for i in range(n):
                   print('name says', i)
                   await sleep(1)

           async def main():
               t = await aside(child, 'Spam', 10)   # Runs in subprocess
	       await t.join()

           run(main())

           In a nutshell, aside(coro, *args, **kwargs) creates a clean
	   Python interpreter and invokes curio.run(coro(*args,
	   **kwargs)) on the supplied coroutine.  The return value of
	   aside() is a Task object.  Joining with it returns the
	   child exit code (normally 0).  Cancelling it causes a
	   TaskCancelled exception to be raised in the child.

           aside() does not involve a process fork or pipe. There
           is no underlying communication between the child and parent
           process.  If you want communication, use a Channel object 
           or set up some other kind of networking.
      
02/06/2017 Some improvements to message passing and launching tasks in
           subprocesses.  A new Channel object makes it easy
           to establish message passing between two different interpreters.
           For example, here is a producer program:

           # producer.py
           from curio import Channel, run

           async def producer(ch):
               while True:
                   c = await ch.accept(authkey=b'peekaboo')
                   for i in range(10):
                       await c.send(i)
                   await c.send(None)   # Sentinel

           if __name__ == '__main__':
               ch = Channel(('localhost', 30000))
               run(producer(ch))

           Here is a consumer program::

           # consumer.py
           from curio import Channel, run

           async def consumer(ch):
               c = await ch.connect(authkey=b'peekaboo')
               while True:
                   msg = await c.recv()
                   if msg is None:
                      break
                   print('Got:', msg)

           if __name__ == '__main__':
              ch = Channel(('localhost', 30000))
              run(consumer(ch))

           A Channel is a lot like a socket except that it sends discrete
           messages.   Any picklable Python compatible object can be
           passed.

02/03/2017 Fixed a few regressions in SSL sockets and the Kernel.run() method.

Version 0.5 : February 2, 2017
------------------------------

01/08/2017 Some refinements to the abide() function.   You can now have it
           reserve a dedicated thread.  This allows it to work with things
           like Condition variables.   For example::

               cond = threading.Condition()    # Foreign condition variable

               ...
               async with abide(code, reserve_thread=True) as c:
                   # c is an async-wrapper around (code)
                   # The following operation uses the same thread that was
                   # used to acquire the lock.
                   await c.wait()             
               ...

           abide() also prefers to use the block_in_thread() function that
           makes it much more efficient when synchronizing with basic locks
           and events.
 
01/08/2017 Some reworking of internals related to thread/process workers and 
           task cancellation. One issue with launching work into a thread
           worker is that threads have no mechanism for cancellation.  They
           run fully to completion no matter what.  Thus, if you perform some
           work like this:

                await run_in_thread(callable, args)

           and the calling task gets cancelled, it's impossible to find out
           what happened with the thread.  Basically, it's lost to the sands
           of time.   However, you can now supply an optional call_on_cancel
           argument to the function and use it like this:

                def cancelled_result(future):
                    result = future.result()
                    ...
                    
                await run_in_thread(callable, args, call_on_cancel=cancelled_result)

           The call_on_cancel function is a normal synchronous
           function. It receives the Future instance that was being used
           to receive the result of the threaded operation.  This
           Future is guaranteed to have the result/exception set.

	   Be aware that there is no way to know when the call_on_cancel 
           function might be triggered.  It might be far in the future.
           The Curio kernel might not even be running.   Thus, it's 
           generally not safe to make too many assumptions about it.
           The only guarantee is that the call_on_cancel function is
           called after a result is computed and it's called in the
           same thread.

           The main purpose of this feature is to have better support
           for cleanup of failed synchronization operations involving
           threads.

01/06/2017 New function.  block_in_thread().   This works like run_in_thread()
           except that it's used with the expectation that whatever operation
           is being performed is likely going to block for an undetermined
           time period.  The underlying operation is handled more efficiently.
           For each unique callable, there is at most 1 background thread
           being used regardless of how many tasks might be trying to
           perform the same operation.  For example, suppose you were
           trying to synchronize with a foreign queue:

               import queue
               
               work_q = queue.Queue()     # Standard thread queue

               async def worker():
                   while True:
                       item = await block_in_thread(work_q.get)
                       ...

               # Spin up a huge number of workers
               for n in range(1000):
                   await spawn(worker())

           In this code, there is one queue and 1000 worker tasks trying to
           read items.  The block_in_thread() function only uses 1 background
           thread to handle it.   If you used run_in_thread() instead, it
           consume all available worker threads and you'd probably deadlock.

01/05/2017 Experimental new feature--asynchronous threads!  An async thread
           is an actual real-life thread where it is safe to call Curio
           coroutines and use its various synchronization features. 
           As an example, suppose you had some code like this:

               async def handler(client, addr):
                   async with client:
                       async for data in client.as_stream():
                           n = int(data)
                           time.sleep(n)
                           await client.sendall(b'Awake!\n')
                   print('Connection closed')

               run(tcp_server('', 25000, handler))

           Imagine that the time.sleep() function represents some kind of
           synchronous, blocking operation.  In the above code, it would
           block the Curio kernel, prevents all other tasks from running.

           Not a problem, change the handler() function to an async thread
           and use the await() function like this:

               from curio.thread import await, async_thread

               @async_thread
               def handler(client, addr):
                   with client:
                       for data in client.as_stream():
                           n = int(data)
                           time.sleep(n)
                           await(client.sendall(b'Awake!\n'))
                   print('Connection closed')

              run(tcp_server('', 25000, handler))


           You'll find that the above code works fine and doesn't block
           the kernel.

           Asynchronous threads only work in the context of Curio.  They
           may use all of Curio's features.  Everywhere you would normally
           use await, you use the await() function. with and for statements
           will work with objects supporting asynchronous operation. 

01/04/2017 Modified enable_cancellation() and disable_cancellation() so that
           they can also be used as functions.  This makes it easier to
           shield a single operation. For example:

              await disable_cancellation(coro())

           Functionally, it is the same as this:

              async with disable_cancellation():
                  await coro()

           This is mostly a convenience feature.  

01/04/2017 Two tasks that attempt to wait on the same file descriptor
           now results in an exception.  Closes issue #104.

01/04/2017 Modified the monitor so that killing the Curio process also
           kills the monitor thread and disconnects any connected clients.
           Addresses issue #108.

01/04/2017 Modified task.cancel() so that it also cancels any pending
           timeout. This prevents the delivery of a timeout exception
           (if any) in code that might be executing to cleanup from
           task cancellation.

01/03/2017 Added a TaskCancelled exception.  This is now what gets 
           raised when tasks are cancelled using the task.cancel()
           method.  It is a subclass of CancelledError.   This change 
	   makes CancelledError more of an abstract exception class
           for cancellation.  The TaskCancelled, TaskTimeout, and 
           TimeoutCancellationError exceptions are more specific
           subclasses that indicates exactly what has happened.

01/02/2017 Major reorganization of how task cancellation works. There
	   are two major parts to it.

	   Kernel:

           Every task has a boolean flag "task.allow_cancel" that
           determines whether or not cancellation exceptions (which
           includes cancellation and timeouts) can be raised in the
           task or not.  The flag acts as a simple mask. If set True,
           a cancellation results in an exception being raised in the
           task.  If set False, the cancellation-related exception is
           placed into "task.cancel_pending" instead.  That attribute
           holds onto the exception indefinitely, waiting for the task
           to re-enable cancellations.  Once re-enabled, the exception
           is raised immediately the next time the task performs a 
	   blocking operation.

	   Coroutines:

	   From coroutines, control of the cancellation flag is
	   performed by two functions which are used as context
	   managers:

	   To disable cancellation, use the following construct:

               async def coro():
                   async with disable_cancellation():
                       ...
                       await something1()
                       await something2()
                       ...

                   await blocking_op()   # Cancellation raised here (if any)
           
           Within a disable_cancellation() block, it is illegal for
           a CancelledError exception to be raised--even manually. Doing
           so causes a RuntimeError.  

	   To re-enable cancellation in specific regions of code, use
	   enable_cancellation() like this:

               async def coro():
                   async with disable_cancellation():
                       while True:
                           await something1()
                           await something2()
                           async with enable_cancellation() as c:
                               await blocking_op()

                           if c.cancel_pending:
                               # Cancellation is pending right now. Must bail out.
      			       break

                   await blocking_op()   # Cancellation raised here (if any)

           Use of enable_cancellation() is never allowed outside of an
           enclosing disable_cancellation() block.  Doing so will
           cause a RuntimeError exception. Within an
           enable_cancellation() block, all of the normal cancellation
           rules apply.  This includes raising of exceptions,
           timeouts, and so forth.  However, CancelledError exceptions
           will never escape the block.  Instead, they turn back into
           a pending exception which can be checked as shown above.

	   Normally cancellations are only delivered on blocking operations.
           If you want to force a check, you can use check_cancellation()
           like this:

                if await check_cancellation():
                    # Cancellation is pending, but not allowed right now
                    ...

           Depending on the setting of the allow_cancel flag, 
           check_cancellation() will either raise the cancellation 
           exception immediately or report that it is pending.
           
12/27/2016 Modified timeout_after(None) so that it leaves any prior timeout
           setting in place (if any).  However, if a timeout occurs, it
           will appear as a TimeoutCancellationError instead of the usual
           TaskTimeout exception.  This is subtle, but it means that the
           timeout occurred to due to an outer timeout setting.   This
           change makes it easier to write functions that accept optional
           timeout settings.  For example:

              async def func(args, timeout=None):
                  try:
                      async with timeout_after(timeout):
                          statements
                          ...
                  except TaskTimeout as e:
                       # Timeout specifically due to timeout setting supplied
                       ...
                  except CancelledError as e:
                       # Function cancelled for some other reason
                       # (possibly an outer timeout)
                       ...
           
12/23/2016 Added further information to cancellation/timeout exceptions
           where partial I/O may have been performed. For readall() and
           read_exactly() methods, the bytes_read attribute contains
           all data read so far.  The readlines() method attaches a 
           lines_read attribute.  For write() and writelines(), a bytes_written
           attribute is added to the exception.   For example:

               try:
                  data = timeout_after(5, s.readall())
               except TimeoutError as e:
                  data = e.bytes_read     # Data received prior to timeout

           Here is a sending example:

               try:
                   timeout_after(5, s.write(data))
               except TimeoutError as e:
                   nwritten = e.bytes_written

           The primary purpose of these attributes is to allow more
           robust recovery in the event of cancellation. 

12/23/2016 The timeout arguments to subprocess related functions have been
           removed.  Use the curio timeout_after() function instead to deal
           with this case.  For example:

               try:
                   out = timeout_after(5, subprocess.check_output(args))
               except TaskTimeout as e:
                   # Get the partially read output
                   partial_stdout = e.stdout
                   partial_stderr = e.stderr
                   ... other recovery ...

           If there is an exception, the stdout and stderr
           attributes contain any partially read data on standard output
           and standard error.  These attributes mirror those present
           on the CalledProcessError exception raised if there is an error.

12/03/2016 Added a parentid attribute to Task instances so you can find parent
	   tasks.   Nothing else is done with this internally.

12/03/2016 Withdrew the pdb and crash_handler arguments to Kernel() and the
           run() function.  Added a pdb() method to tasks that can be used
	   to enter the debugger on a crashed task.  High-level handling
	   of crashed/terminated tasks is being rethought.   The old
	   crash_handler() callback was next to useless since no useful
	   actions could be performed (i.e., there was no ability to respawn
	   tasks or execute any kind of coroutine in response to a crash).

11/05/2016 Pulled time related functionality into the kernel as a new call.
           Use the following to get the current value of the kernel clock:
            
                await curio.clock()

           Timeout related functions such as timeout_after() and ignore_after()
           now rely on the kernel clock instead of using time.monotonic().
           This changes consolidates all use of the clock into one place
           and makes it easier (later) to reconfigure timing should it be
           desired.  For example, perhaps changing the scale of the clock
           to slow down or speed up time handling (for debugging, testing, etc.)

10/29/2016 If the sendall() method of a socket is aborted with a CancelledError,
           the resulting exception object now gets a bytes_sent attribute set to
           indicate how much progress was made.   For example:

           try:
               await sock.sendall(data)
           except CancelledError as e:
               print(e.bytes_sent, 'bytes sent')

10/29/2016 Added timeout_at() and ignore_at() functions that allow timeouts
           to be specified at absolute clock values.  The usage is the
           same as for timeout_after() and ignore_after().
           
10/29/2016 Modified TaskTimeout exception so that it subclasses CancelledError.
           This makes it easier to write code that handles any kind of 
	   cancellation (explicit cancellation, cancellation by timeout, etc.)

10/17/2016 Added shutdown() method to sockets.  It is an async function
           to mirror async implementation of close()

           await sock.shutdown(how)

10/17/2016 Added writeable() method to sockets.  It can be used to
           quickly test if a socket will accept more data before
           doing a send().  See Issue #83.

           await sock.writeable()
           nsent = await sock.send(data)

10/17/2016 More precisely defined the semantics of nested timeouts
           and exception handling.  Consider the following arrangement
           of timeout blocks:

           # B1
           async with timeout_after(time1):
                # B2
                async with timeout_after(time2):
                    await coro()

           Here are the rules:

           1. If time2 expires before time1, then block B2 receives
              a TaskTimeout exception.

           2. If time1 expires before time2, then block B2 receives
              a TimeoutCancellationError exception and block B1
              receives a TaskTimeout exception.  This reflects the
              fact that the inner timeout didn't actually occur
              and thus it shouldn't be reported as such.  The inner
              block is still cancelled however in order to satisfy
              the outer timeout.

           3. If time2 expires before time1 and the resulting
              TaskTimeout is NOT caught, but allowed to propagate out
              to B1, then block B1 receives an UncaughtTimeoutError
              exception.  A block should never report a TaskTimeout
              unless its specified time interval has actually expired.
              Reporting a timeout early because of an uncaught 
              exception in an inner block should be considered to be
              an operational error. This exception reflects that.

           4. If time1 and time2 both expire simultaneously, the
              outer timeout takes precedence and time1 is considered
              to have expired first.

           See Issue #82 for further details about the rationale for
           this change. https://github.com/dabeaz/curio/issues/82
           
           
08/16/2016 Modified the Queue class so that the put() method can be used from either
           synchronous or asynchronous code.  For example:

              from curio import Queue
              queue = Queue()

              def spam():
                  # Create some item
                  ...
                  queue.put(item)

              async def consumer():
                  while True:
                       item = await queue.get()
                       # Consume the item
                       ...

              async def coro():
                    ...
                    spam()       # Note: Normal synchronous function call
                    ...

              async def main():
                  await spawn(coro())
                  await spawn(consumer())

              run(main())

           The main purpose of adding this is to make it easier for normal
           synchronous code to communicate to async tasks without knowing
           too much about what they are.  Note:  The put() method is never
           allowed to block in synchronous mode.  If the queue has a bounded
           size and it fills up, an exception is raised.

08/16/2016 Modified the Event class so that events can also be set from synchronous
           code.  For example:

               from curio import Event
               evt = Event()

               async def coro():
                   print('Waiting for something')
                   await evt.wait()
                   print('It happened')

               # A normal synchronous function. No async/await here.
               def spam():
                   print('About to signal')
                   evt.set()

               async def main():
                   await spawn(coro())
                   await sleep(5)
                   spam()         # Note: Normal synchronous function call

               run(main())

           The main motivation for adding this is that is very easy for
           control flow to escape the world of "async/await".  However,
           that code may still want to signal or coordinate with async
           tasks in some way.   By allowing a synchronous set(), it
           makes it possible to do this.    It should be noted that within
           a coroutine, you have to use await when triggering an event.
           For example:

               evt = Event()

               def foo():
                   evt.set()           # Synchronous use

               async def bar():
                   await evt.set()     # Asynchronous use

08/04/2016 Added a new KernelExit exception that can be used to 
           make the kernel terminate execution.  For example:

              async def coro():
                  ...
                  if something_bad:
                      raise KernelExit('Something bad')

           This causes the kernel to simply stop, aborting the
           currently executing task.   The exception will propagate
           out of the run() function so if you need to catch it, do
           this:

               try:
                   run(coro())
               except KernelExit as e:
                   print('Going away because of:', e)

           KernelExit by itself does not do anything to other 
	   running tasks.  However, the run() function will
           separately issue a shutdown request causing all
           remaining tasks to cancel.

08/04/2016 Added a new TaskExit exception that can be used to make a 
           single task terminate.  For example:

               async def coro():
                   ...
                   if something_bad:
                       raise TaskExit('Goodbye')
                   ...

           Think of TaskExit as a kind of self-cancellation. 

08/04/2016 Some refinements to kernel shutdown.   The shutdown process is
           more carefully supervised and fixes a few very subtle errors
           related to task scheduling. 

07/22/2016 Added support for asynchronous access to files as might be
           opened by the builtin open() function.  Use the new aopen()
           function with an async-context manager like this:

             async with aopen(filename, 'r') as f:
                 data = await f.read()

           Note: a file opened in this manner provides an asynchronous API
           that will prevent the Curio kernel from blocking on things
           like disk seeks.  However, the underlying implementation is
           not specified.  In the initial version, thread pools are
           used to carry out each I/O operation.
           
07/18/2016 Some changes to Kernel cleanup and resource management.  The
           proper way to shut down the kernel is to use Kernel.run(shutdown=True).
           Alternatively, the kernel can now been used as a context manager:

             with Kernel() as kern:
                  kern.run(coro())

           Note: The plain run() method properly shuts down the Kernel
           if you're only running a single coroutine.

           The Kernel.__del__() method now raises an exception if the
           kernel is deleted without being properly shut down. 

06/30/2016 Added alpn_protocols keyword argument to open_connection()
           function to make it easier to use TLS ALPN with clients.  For 
           example to open a connection and have it negotiate HTTP/2 
           or HTTP/1.1 as a protocol, you can do this:

           sock = await open_connection(host, port, ssl=True, 
                                        server_hostname=host,
                                        alpn_protocols=['h2', 'http/1.1'])

           print('selected protocol:', sock.selected_alpn_protocol())

06/30/2016 Changed internal clock handling to use absolute values of
           the monotonic clock.  New wakeat() function utilizes this
           to allow more controlled sleeping for periodic timers
           and other applications.  For example, here is a loop that
           precisely wakes up on a specified time interval:

           import time
           from curio import wakeat

           async def pulse(interval):
               next_wake = time.monotonic()
               while True:
                    await wake_at(next_wake)
                    print('Tick', time.asctime())
                    next_wake += interval
                
06/16/2016 Fixed Issue #55.  Exceptions occurring in code executed by
           run_in_process() now include a RemoteTraceback exception 
           that shows the traceback from the remote process. This
           should make debugging a big easier. 

06/11/2016 Fixed Issue #53.  curio.run() was swallowing all exceptions.  It now
           reports a TaskError exception if the given coroutine fails.  This is
           a chained exception where __cause__ contains the actual cause of
           failure.   This is meant to be consistent with the join() method 
           of Tasks.

06/09/2016 Experimental new wait() function added.  It can be used to wait for
           more than one task at a time and to return them in completion order.
           For example:

           task1 = await spawn(coro())
           task2 = await spawn(coro())
           task3 = await spawn(coro())

           # Get results from all tasks as they complete
           async for task in wait([task1, task2, task3]):
               result = await task.join()

           # Get the first result and cancel remaining tasks
           async with wait([task1, task2, task3]) as w:
               task = await w.next_done()
               result = await task.join()
               # Other tasks cancelled here

06/09/2016 Refined the behavior of timeouts.  First, a timeout is not allowed
           to extend the time expiration of a previously set timeout. For
           example, if code previously set a 5 second timeout, an attempt
           to now set a 10 second timeout still results in a 5 second timeout.
           Second, when restoring a previous timeout, if the timeout period has
           expired, Curio arranges for a TaskTimeout exception to be raised on
           the next blocking call.   Without this, it's too easy for timeouts
           to disappear or not have any effect.   Setting a timeout of None
           disables timeouts regardless of any prior setting.

06/07/2016 Changed trap names (e.g., '_trap_io') to int enums. This is
           low-level change that shouldn't affect existing code.

05/23/2016 Fixed Issue #52 (Problem with ignore_after context manager).
           There was a possibility that a task would be marked for
           timeout at precisely the same time some other operation had
           completed and the task was sitting on the ready queue. To fix,
           the timeout is deferred and retried the next time the kernel
           blocks. 

05/20/2016 Added asyncobject class to curio/meta.py.  This allows you
           to write classes with an asynchronous __init__ method. For example:

           from curio.meta import asyncobject
           class Spam(asyncobject):
               async def __init__(self):
                   ...
                   self.value = await coro()
                   ...

           Instances can only be created via await.  For example:

              s = await Spam()

05/15/2016 Fixed Issue #50. Undefined variable n in io.py 
	   Reported by Wolfgang Langner

Version 0.4 : May 13, 2016
--------------------------
05/13/2016 Fixed a subtle bug with futures/cancellation.

Version 0.3 : May 13, 2016
--------------------------
05/13/2016 Bug fixes to the run_in_process() and run_in_thread() 
           functions so that exceptions are reported properly.
           Also fixed logic bug on related to kernel task initialization.

05/13/2016 Modification to the abide() function to allow it to work
           with RLocks from the threading module.  One caveat: Such
	   locks are NOT reentrant from within curio itself. 

Version 0.2 : May 11, 2016
--------------------------
05/05/2016 Refactoring of stream I/O classes. There is now FileStream
           and SocketStream.   The old Stream class is gone.

04/30/2016 The run_blocking() and run_cpu_bound() calls are now
           called run_in_thread() and run_in_process().

04/23/2016 Changed the new_task() function to spawn().

04/22/2016 Removed parent/child task relationship and associated
           tracking.  It's an added complexity that's not really
	   needed in the kernel and it can be done easily enough by
	   the user in cases where it might be needed.

04/18/2016 Major refactoring of timeout handling.  Virtually all
           operations in curio support cancellation and timeouts.
           However, putting an explicit "timeout" argument on
           every API function/method greatly complicates the 
           underlying implementation (and introduces performance
           overhead in cases where timeouts aren't used). To
           put a timeout on an operation, use the timeout_after()
           function instead.  For example:

               await timeout_after(5, sock.recv(1024))

           This will cause a timeout to be raised after the
           specified time interval.  

04/01/2016 Improved management of the I/O selector.  The number of
           register/unregister operations are reduced for tasks 
           that constantly perform I/O on the same resources.  This
           could offer a nice performance boost in certain cases.

03/31/2016 Switched test suite to py.test. All of the tests are in the
           top-level tests directory.  Use 'python3 -m pytest' to test.

03/30/2016 Improved the curio monitor.  Instead of relying on the
           console TTY (and invoked via Ctrl-C), it now uses a socket
           to which you must connect via a different session. To
           enable the monitor either use:

               kernel = Kernel(with_monitor=True)

           or run with an environment variable

               env CURIOMONITOR=TRUE python3 yourprogram.py

           To connect to the monitor, use the following command:

               python3 -m curio.monitor
          
02/15/2016 Fixed Issue #37 where scheduling multiple tasks for sleeping
           could potentially cause a crash in rare circumstances.

Version 0.1 : October 31, 2015
------------------------------
Initial version