#!/usr/bin/env python
#
# Copyright 2012 Facebook
#
# Licensed under the Apache License, Version 2.0 (the "License"); you may
# not use this file except in compliance with the License. You may obtain
# a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations
# under the License.
"""Utilities for working with threads and ``Futures``.
``Futures`` are a pattern for concurrent programming introduced in
Python 3.2 in the `concurrent.futures` package. This package defines
a mostly-compatible `Future` class designed for use from coroutines,
as well as some utility functions for interacting with the
`concurrent.futures` package.
"""
from __future__ import absolute_import, division, print_function, with_statement
import functools
import platform
import textwrap
import traceback
import sys
from tornado.log import app_log
from tornado.stack_context import ExceptionStackContext, wrap
from tornado.util import raise_exc_info, ArgReplacer
try:
from concurrent import futures
except ImportError:
futures = None
# Can the garbage collector handle cycles that include __del__ methods?
# This is true in cpython beginning with version 3.4 (PEP 442).
_GC_CYCLE_FINALIZERS = (platform.python_implementation() == 'CPython' and
sys.version_info >= (3, 4))
class ReturnValueIgnoredError(Exception):
pass
# This class and associated code in the future object is derived
# from the Trollius project, a backport of asyncio to Python 2.x - 3.x
class _TracebackLogger(object):
"""Helper to log a traceback upon destruction if not cleared.
This solves a nasty problem with Futures and Tasks that have an
exception set: if nobody asks for the exception, the exception is
never logged. This violates the Zen of Python: 'Errors should
never pass silently. Unless explicitly silenced.'
However, we don't want to log the exception as soon as
set_exception() is called: if the calling code is written
properly, it will get the exception and handle it properly. But
we *do* want to log it if result() or exception() was never called
-- otherwise developers waste a lot of time wondering why their
buggy code fails silently.
An earlier attempt added a __del__() method to the Future class
itself, but this backfired because the presence of __del__()
prevents garbage collection from breaking cycles. A way out of
this catch-22 is to avoid having a __del__() method on the Future
class itself, but instead to have a reference to a helper object
with a __del__() method that logs the traceback, where we ensure
that the helper object doesn't participate in cycles, and only the
Future has a reference to it.
The helper object is added when set_exception() is called. When
the Future is collected, and the helper is present, the helper
object is also collected, and its __del__() method will log the
traceback. When the Future's result() or exception() method is
called (and a helper object is present), it removes the the helper
object, after calling its clear() method to prevent it from
logging.
One downside is that we do a fair amount of work to extract the
traceback from the exception, even when it is never logged. It
would seem cheaper to just store the exception object, but that
references the traceback, which references stack frames, which may
reference the Future, which references the _TracebackLogger, and
then the _TracebackLogger would be included in a cycle, which is
what we're trying to avoid! As an optimization, we don't
immediately format the exception; we only do the work when
activate() is called, which call is delayed until after all the
Future's callbacks have run. Since usually a Future has at least
one callback (typically set by 'yield From') and usually that
callback extracts the callback, thereby removing the need to
format the exception.
PS. I don't claim credit for this solution. I first heard of it
in a discussion about closing files when they are collected.
"""
__slots__ = ('exc_info', 'formatted_tb')
def __init__(self, exc_info):
self.exc_info = exc_info
self.formatted_tb = None
def activate(self):
exc_info = self.exc_info
if exc_info is not None:
self.exc_info = None
self.formatted_tb = traceback.format_exception(*exc_info)
def clear(self):
self.exc_info = None
self.formatted_tb = None
def __del__(self):
if self.formatted_tb:
app_log.error('Future exception was never retrieved: %s',
''.join(self.formatted_tb).rstrip())
class Future(object):
"""Placeholder for an asynchronous result.
A ``Future`` encapsulates the result of an asynchronous
operation. In synchronous applications ``Futures`` are used
to wait for the result from a thread or process pool; in
Tornado they are normally used with `.IOLoop.add_future` or by
yielding them in a `.gen.coroutine`.
`tornado.concurrent.Future` is similar to
`concurrent.futures.Future`, but not thread-safe (and therefore
faster for use with single-threaded event loops).
In addition to ``exception`` and ``set_exception``, methods ``exc_info``
and ``set_exc_info`` are supported to capture tracebacks in Python 2.
The traceback is automatically available in Python 3, but in the
Python 2 futures backport this information is discarded.
This functionality was previously available in a separate class
``TracebackFuture``, which is now a deprecated alias for this class.
.. versionchanged:: 4.0
`tornado.concurrent.Future` is always a thread-unsafe ``Future``
with support for the ``exc_info`` methods. Previously it would
be an alias for the thread-safe `concurrent.futures.Future`
if that package was available and fall back to the thread-unsafe
implementation if it was not.
.. versionchanged:: 4.1
If a `.Future` contains an error but that error is never observed
(by calling ``result()``, ``exception()``, or ``exc_info()``),
a stack trace will be logged when the `.Future` is garbage collected.
This normally indicates an error in the application, but in cases
where it results in undesired logging it may be necessary to
suppress the logging by ensuring that the exception is observed:
``f.add_done_callback(lambda f: f.exception())``.
"""
def __init__(self):
self._done = False
self._result = None
self._exc_info = None
self._log_traceback = False # Used for Python >= 3.4
self._tb_logger = None # Used for Python <= 3.3
self._callbacks = []
# Implement the Python 3.5 Awaitable protocol if possible
# (we can't use return and yield together until py33).
if sys.version_info >= (3, 3):
exec(textwrap.dedent("""
def __await__(self):
return (yield self)
"""))
else:
# Py2-compatible version for use with cython.
def __await__(self):
result = yield self
# StopIteration doesn't take args before py33,
# but Cython recognizes the args tuple.
e = StopIteration()
e.args = (result,)
raise e
def cancel(self):
"""Cancel the operation, if possible.
Tornado ``Futures`` do not support cancellation, so this method always
returns False.
"""
return False
def cancelled(self):
"""Returns True if the operation has been cancelled.
Tornado ``Futures`` do not support cancellation, so this method
always returns False.
"""
return False
def running(self):
"""Returns True if this operation is currently running."""
return not self._done
def done(self):
"""Returns True if the future has finished running."""
return self._done
def _clear_tb_log(self):
self._log_traceback = False
if self._tb_logger is not None:
self._tb_logger.clear()
self._tb_logger = None
def result(self, timeout=None):
"""If the operation succeeded, return its result. If it failed,
re-raise its exception.
This method takes a ``timeout`` argument for compatibility with
`concurrent.futures.Future` but it is an error to call it
before the `Future` is done, so the ``timeout`` is never used.
"""
self._clear_tb_log()
if self._result is not None:
return self._result
if self._exc_info is not None:
raise_exc_info(self._exc_info)
self._check_done()
return self._result
def exception(self, timeout=None):
"""If the operation raised an exception, return the `Exception`
object. Otherwise returns None.
This method takes a ``timeout`` argument for compatibility with
`concurrent.futures.Future` but it is an error to call it
before the `Future` is done, so the ``timeout`` is never used.
"""
self._clear_tb_log()
if self._exc_info is not None:
return self._exc_info[1]
else:
self._check_done()
return None
def add_done_callback(self, fn):
"""Attaches the given callback to the `Future`.
It will be invoked with the `Future` as its argument when the Future
has finished running and its result is available. In Tornado
consider using `.IOLoop.add_future` instead of calling
`add_done_callback` directly.
"""
if self._done:
fn(self)
else:
self._callbacks.append(fn)
def set_result(self, result):
"""Sets the result of a ``Future``.
It is undefined to call any of the ``set`` methods more than once
on the same object.
"""
self._result = result
self._set_done()
def set_exception(self, exception):
"""Sets the exception of a ``Future.``"""
self.set_exc_info(
(exception.__class__,
exception,
getattr(exception, '__traceback__', None)))
def exc_info(self):
"""Returns a tuple in the same format as `sys.exc_info` or None.
.. versionadded:: 4.0
"""
self._clear_tb_log()
return self._exc_info
def set_exc_info(self, exc_info):
"""Sets the exception information of a ``Future.``
Preserves tracebacks on Python 2.
.. versionadded:: 4.0
"""
self._exc_info = exc_info
self._log_traceback = True
if not _GC_CYCLE_FINALIZERS:
self._tb_logger = _TracebackLogger(exc_info)
try:
self._set_done()
finally:
# Activate the logger after all callbacks have had a
# chance to call result() or exception().
if self._log_traceback and self._tb_logger is not None:
self._tb_logger.activate()
self._exc_info = exc_info
def _check_done(self):
if not self._done:
raise Exception("DummyFuture does not support blocking for results")
def _set_done(self):
self._done = True
for cb in self._callbacks:
try:
cb(self)
except Exception:
app_log.exception('Exception in callback %r for %r',
cb, self)
self._callbacks = None
# On Python 3.3 or older, objects with a destructor part of a reference
# cycle are never destroyed. It's no longer the case on Python 3.4 thanks to
# the PEP 442.
if _GC_CYCLE_FINALIZERS:
def __del__(self):
if not self._log_traceback:
# set_exception() was not called, or result() or exception()
# has consumed the exception
return
tb = traceback.format_exception(*self._exc_info)
app_log.error('Future %r exception was never retrieved: %s',
self, ''.join(tb).rstrip())
TracebackFuture = Future
if futures is None:
FUTURES = Future
else:
FUTURES = (futures.Future, Future)
def is_future(x):
return isinstance(x, FUTURES)
class DummyExecutor(object):
def submit(self, fn, *args, **kwargs):
future = TracebackFuture()
try:
future.set_result(fn(*args, **kwargs))
except Exception:
future.set_exc_info(sys.exc_info())
return future
def shutdown(self, wait=True):
pass
dummy_executor = DummyExecutor()
def run_on_executor(*args, **kwargs):
"""Decorator to run a synchronous method asynchronously on an executor.
The decorated method may be called with a ``callback`` keyword
argument and returns a future.
The `.IOLoop` and executor to be used are determined by the ``io_loop``
and ``executor`` attributes of ``self``. To use different attributes,
pass keyword arguments to the decorator::
@run_on_executor(executor='_thread_pool')
def foo(self):
pass
.. versionchanged:: 4.2
Added keyword arguments to use alternative attributes.
"""
def run_on_executor_decorator(fn):
executor = kwargs.get("executor", "executor")
io_loop = kwargs.get("io_loop", "io_loop")
@functools.wraps(fn)
def wrapper(self, *args, **kwargs):
callback = kwargs.pop("callback", None)
future = getattr(self, executor).submit(fn, self, *args, **kwargs)
if callback:
getattr(self, io_loop).add_future(
future, lambda future: callback(future.result()))
return future
return wrapper
if args and kwargs:
raise ValueError("cannot combine positional and keyword args")
if len(args) == 1:
return run_on_executor_decorator(args[0])
elif len(args) != 0:
raise ValueError("expected 1 argument, got %d", len(args))
return run_on_executor_decorator
_NO_RESULT = object()
def return_future(f):
"""Decorator to make a function that returns via callback return a
`Future`.
The wrapped function should take a ``callback`` keyword argument
and invoke it with one argument when it has finished. To signal failure,
the function can simply raise an exception (which will be
captured by the `.StackContext` and passed along to the ``Future``).
From the caller's perspective, the callback argument is optional.
If one is given, it will be invoked when the function is complete
with `Future.result()` as an argument. If the function fails, the
callback will not be run and an exception will be raised into the
surrounding `.StackContext`.
If no callback is given, the caller should use the ``Future`` to
wait for the function to complete (perhaps by yielding it in a
`.gen.engine` function, or passing it to `.IOLoop.add_future`).
Usage:
.. testcode::
@return_future
def future_func(arg1, arg2, callback):
# Do stuff (possibly asynchronous)
callback(result)
@gen.engine
def caller(callback):
yield future_func(arg1, arg2)
callback()
..
Note that ``@return_future`` and ``@gen.engine`` can be applied to the
same function, provided ``@return_future`` appears first. However,
consider using ``@gen.coroutine`` instead of this combination.
"""
replacer = ArgReplacer(f, 'callback')
@functools.wraps(f)
def wrapper(*args, **kwargs):
future = TracebackFuture()
callback, args, kwargs = replacer.replace(
lambda value=_NO_RESULT: future.set_result(value),
args, kwargs)
def handle_error(typ, value, tb):
future.set_exc_info((typ, value, tb))
return True
exc_info = None
with ExceptionStackContext(handle_error):
try:
result = f(*args, **kwargs)
if result is not None:
raise ReturnValueIgnoredError(
"@return_future should not be used with functions "
"that return values")
except:
exc_info = sys.exc_info()
raise
if exc_info is not None:
# If the initial synchronous part of f() raised an exception,
# go ahead and raise it to the caller directly without waiting
# for them to inspect the Future.
future.result()
# If the caller passed in a callback, schedule it to be called
# when the future resolves. It is important that this happens
# just before we return the future, or else we risk confusing
# stack contexts with multiple exceptions (one here with the
# immediate exception, and again when the future resolves and
# the callback triggers its exception by calling future.result()).
if callback is not None:
def run_callback(future):
result = future.result()
if result is _NO_RESULT:
callback()
else:
callback(future.result())
future.add_done_callback(wrap(run_callback))
return future
return wrapper
def chain_future(a, b):
"""Chain two futures together so that when one completes, so does the other.
The result (success or failure) of ``a`` will be copied to ``b``, unless
``b`` has already been completed or cancelled by the time ``a`` finishes.
"""
def copy(future):
assert future is a
if b.done():
return
if (isinstance(a, TracebackFuture) and isinstance(b, TracebackFuture)
and a.exc_info() is not None):
b.set_exc_info(a.exc_info())
elif a.exception() is not None:
b.set_exception(a.exception())
else:
b.set_result(a.result())
a.add_done_callback(copy)
