660 lines
23 KiB
Python
660 lines
23 KiB
Python
#
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# Copyright 2012 Facebook
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#
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# Licensed under the Apache License, Version 2.0 (the "License"); you may
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# not use this file except in compliance with the License. You may obtain
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# a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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# License for the specific language governing permissions and limitations
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# under the License.
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"""Utilities for working with ``Future`` objects.
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``Futures`` are a pattern for concurrent programming introduced in
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Python 3.2 in the `concurrent.futures` package, and also adopted (in a
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slightly different form) in Python 3.4's `asyncio` package. This
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package defines a ``Future`` class that is an alias for `asyncio.Future`
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when available, and a compatible implementation for older versions of
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Python. It also includes some utility functions for interacting with
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``Future`` objects.
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While this package is an important part of Tornado's internal
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implementation, applications rarely need to interact with it
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directly.
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"""
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from __future__ import absolute_import, division, print_function
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import functools
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import platform
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import textwrap
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import traceback
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import sys
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import warnings
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from tornado.log import app_log
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from tornado.stack_context import ExceptionStackContext, wrap
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from tornado.util import raise_exc_info, ArgReplacer, is_finalizing
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try:
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from concurrent import futures
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except ImportError:
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futures = None
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try:
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import asyncio
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except ImportError:
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asyncio = None
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try:
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import typing
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except ImportError:
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typing = None
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# Can the garbage collector handle cycles that include __del__ methods?
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# This is true in cpython beginning with version 3.4 (PEP 442).
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_GC_CYCLE_FINALIZERS = (platform.python_implementation() == 'CPython' and
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sys.version_info >= (3, 4))
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class ReturnValueIgnoredError(Exception):
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pass
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# This class and associated code in the future object is derived
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# from the Trollius project, a backport of asyncio to Python 2.x - 3.x
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class _TracebackLogger(object):
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"""Helper to log a traceback upon destruction if not cleared.
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This solves a nasty problem with Futures and Tasks that have an
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exception set: if nobody asks for the exception, the exception is
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never logged. This violates the Zen of Python: 'Errors should
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never pass silently. Unless explicitly silenced.'
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However, we don't want to log the exception as soon as
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set_exception() is called: if the calling code is written
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properly, it will get the exception and handle it properly. But
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we *do* want to log it if result() or exception() was never called
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-- otherwise developers waste a lot of time wondering why their
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buggy code fails silently.
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An earlier attempt added a __del__() method to the Future class
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itself, but this backfired because the presence of __del__()
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prevents garbage collection from breaking cycles. A way out of
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this catch-22 is to avoid having a __del__() method on the Future
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class itself, but instead to have a reference to a helper object
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with a __del__() method that logs the traceback, where we ensure
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that the helper object doesn't participate in cycles, and only the
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Future has a reference to it.
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The helper object is added when set_exception() is called. When
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the Future is collected, and the helper is present, the helper
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object is also collected, and its __del__() method will log the
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traceback. When the Future's result() or exception() method is
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called (and a helper object is present), it removes the the helper
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object, after calling its clear() method to prevent it from
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logging.
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One downside is that we do a fair amount of work to extract the
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traceback from the exception, even when it is never logged. It
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would seem cheaper to just store the exception object, but that
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references the traceback, which references stack frames, which may
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reference the Future, which references the _TracebackLogger, and
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then the _TracebackLogger would be included in a cycle, which is
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what we're trying to avoid! As an optimization, we don't
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immediately format the exception; we only do the work when
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activate() is called, which call is delayed until after all the
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Future's callbacks have run. Since usually a Future has at least
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one callback (typically set by 'yield From') and usually that
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callback extracts the callback, thereby removing the need to
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format the exception.
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PS. I don't claim credit for this solution. I first heard of it
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in a discussion about closing files when they are collected.
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"""
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__slots__ = ('exc_info', 'formatted_tb')
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def __init__(self, exc_info):
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self.exc_info = exc_info
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self.formatted_tb = None
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def activate(self):
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exc_info = self.exc_info
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if exc_info is not None:
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self.exc_info = None
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self.formatted_tb = traceback.format_exception(*exc_info)
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def clear(self):
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self.exc_info = None
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self.formatted_tb = None
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def __del__(self, is_finalizing=is_finalizing):
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if not is_finalizing() and self.formatted_tb:
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app_log.error('Future exception was never retrieved: %s',
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''.join(self.formatted_tb).rstrip())
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class Future(object):
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"""Placeholder for an asynchronous result.
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A ``Future`` encapsulates the result of an asynchronous
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operation. In synchronous applications ``Futures`` are used
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to wait for the result from a thread or process pool; in
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Tornado they are normally used with `.IOLoop.add_future` or by
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yielding them in a `.gen.coroutine`.
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`tornado.concurrent.Future` is an alias for `asyncio.Future` when
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that package is available (Python 3.4+). Unlike
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`concurrent.futures.Future`, the ``Futures`` used by Tornado and
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`asyncio` are not thread-safe (and therefore faster for use with
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single-threaded event loops).
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In addition to ``exception`` and ``set_exception``, Tornado's
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``Future`` implementation supports storing an ``exc_info`` triple
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to support better tracebacks on Python 2. To set an ``exc_info``
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triple, use `future_set_exc_info`, and to retrieve one, call
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`result()` (which will raise it).
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.. versionchanged:: 4.0
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`tornado.concurrent.Future` is always a thread-unsafe ``Future``
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with support for the ``exc_info`` methods. Previously it would
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be an alias for the thread-safe `concurrent.futures.Future`
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if that package was available and fall back to the thread-unsafe
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implementation if it was not.
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.. versionchanged:: 4.1
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If a `.Future` contains an error but that error is never observed
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(by calling ``result()``, ``exception()``, or ``exc_info()``),
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a stack trace will be logged when the `.Future` is garbage collected.
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This normally indicates an error in the application, but in cases
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where it results in undesired logging it may be necessary to
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suppress the logging by ensuring that the exception is observed:
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``f.add_done_callback(lambda f: f.exception())``.
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.. versionchanged:: 5.0
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This class was previoiusly available under the name
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``TracebackFuture``. This name, which was deprecated since
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version 4.0, has been removed. When `asyncio` is available
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``tornado.concurrent.Future`` is now an alias for
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`asyncio.Future`. Like `asyncio.Future`, callbacks are now
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always scheduled on the `.IOLoop` and are never run
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synchronously.
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"""
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def __init__(self):
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self._done = False
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self._result = None
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self._exc_info = None
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self._log_traceback = False # Used for Python >= 3.4
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self._tb_logger = None # Used for Python <= 3.3
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self._callbacks = []
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# Implement the Python 3.5 Awaitable protocol if possible
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# (we can't use return and yield together until py33).
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if sys.version_info >= (3, 3):
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exec(textwrap.dedent("""
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def __await__(self):
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return (yield self)
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"""))
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else:
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# Py2-compatible version for use with cython.
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def __await__(self):
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result = yield self
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# StopIteration doesn't take args before py33,
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# but Cython recognizes the args tuple.
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e = StopIteration()
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e.args = (result,)
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raise e
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def cancel(self):
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"""Cancel the operation, if possible.
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Tornado ``Futures`` do not support cancellation, so this method always
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returns False.
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"""
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return False
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def cancelled(self):
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"""Returns True if the operation has been cancelled.
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Tornado ``Futures`` do not support cancellation, so this method
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always returns False.
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"""
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return False
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def running(self):
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"""Returns True if this operation is currently running."""
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return not self._done
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def done(self):
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"""Returns True if the future has finished running."""
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return self._done
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def _clear_tb_log(self):
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self._log_traceback = False
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if self._tb_logger is not None:
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self._tb_logger.clear()
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self._tb_logger = None
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def result(self, timeout=None):
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"""If the operation succeeded, return its result. If it failed,
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re-raise its exception.
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This method takes a ``timeout`` argument for compatibility with
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`concurrent.futures.Future` but it is an error to call it
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before the `Future` is done, so the ``timeout`` is never used.
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"""
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self._clear_tb_log()
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if self._result is not None:
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return self._result
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if self._exc_info is not None:
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try:
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raise_exc_info(self._exc_info)
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finally:
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self = None
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self._check_done()
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return self._result
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def exception(self, timeout=None):
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"""If the operation raised an exception, return the `Exception`
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object. Otherwise returns None.
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This method takes a ``timeout`` argument for compatibility with
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`concurrent.futures.Future` but it is an error to call it
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before the `Future` is done, so the ``timeout`` is never used.
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"""
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self._clear_tb_log()
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if self._exc_info is not None:
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return self._exc_info[1]
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else:
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self._check_done()
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return None
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def add_done_callback(self, fn):
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"""Attaches the given callback to the `Future`.
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It will be invoked with the `Future` as its argument when the Future
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has finished running and its result is available. In Tornado
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consider using `.IOLoop.add_future` instead of calling
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`add_done_callback` directly.
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"""
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if self._done:
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from tornado.ioloop import IOLoop
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IOLoop.current().add_callback(fn, self)
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else:
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self._callbacks.append(fn)
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def set_result(self, result):
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"""Sets the result of a ``Future``.
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It is undefined to call any of the ``set`` methods more than once
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on the same object.
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"""
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self._result = result
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self._set_done()
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def set_exception(self, exception):
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"""Sets the exception of a ``Future.``"""
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self.set_exc_info(
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(exception.__class__,
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exception,
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getattr(exception, '__traceback__', None)))
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def exc_info(self):
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"""Returns a tuple in the same format as `sys.exc_info` or None.
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.. versionadded:: 4.0
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"""
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self._clear_tb_log()
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return self._exc_info
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def set_exc_info(self, exc_info):
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"""Sets the exception information of a ``Future.``
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Preserves tracebacks on Python 2.
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.. versionadded:: 4.0
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"""
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self._exc_info = exc_info
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self._log_traceback = True
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if not _GC_CYCLE_FINALIZERS:
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self._tb_logger = _TracebackLogger(exc_info)
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try:
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self._set_done()
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finally:
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# Activate the logger after all callbacks have had a
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# chance to call result() or exception().
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if self._log_traceback and self._tb_logger is not None:
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self._tb_logger.activate()
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self._exc_info = exc_info
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def _check_done(self):
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if not self._done:
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raise Exception("DummyFuture does not support blocking for results")
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def _set_done(self):
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self._done = True
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if self._callbacks:
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from tornado.ioloop import IOLoop
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loop = IOLoop.current()
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for cb in self._callbacks:
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loop.add_callback(cb, self)
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self._callbacks = None
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# On Python 3.3 or older, objects with a destructor part of a reference
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# cycle are never destroyed. It's no longer the case on Python 3.4 thanks to
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# the PEP 442.
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if _GC_CYCLE_FINALIZERS:
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def __del__(self, is_finalizing=is_finalizing):
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if is_finalizing() or not self._log_traceback:
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# set_exception() was not called, or result() or exception()
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# has consumed the exception
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return
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tb = traceback.format_exception(*self._exc_info)
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app_log.error('Future %r exception was never retrieved: %s',
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self, ''.join(tb).rstrip())
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if asyncio is not None:
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Future = asyncio.Future # noqa
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if futures is None:
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FUTURES = Future # type: typing.Union[type, typing.Tuple[type, ...]]
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else:
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FUTURES = (futures.Future, Future)
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def is_future(x):
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return isinstance(x, FUTURES)
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class DummyExecutor(object):
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def submit(self, fn, *args, **kwargs):
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future = Future()
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try:
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future_set_result_unless_cancelled(future, fn(*args, **kwargs))
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except Exception:
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future_set_exc_info(future, sys.exc_info())
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return future
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def shutdown(self, wait=True):
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pass
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dummy_executor = DummyExecutor()
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def run_on_executor(*args, **kwargs):
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"""Decorator to run a synchronous method asynchronously on an executor.
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The decorated method may be called with a ``callback`` keyword
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argument and returns a future.
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The executor to be used is determined by the ``executor``
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attributes of ``self``. To use a different attribute name, pass a
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keyword argument to the decorator::
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@run_on_executor(executor='_thread_pool')
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def foo(self):
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pass
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This decorator should not be confused with the similarly-named
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`.IOLoop.run_in_executor`. In general, using ``run_in_executor``
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when *calling* a blocking method is recommended instead of using
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this decorator when *defining* a method. If compatibility with older
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versions of Tornado is required, consider defining an executor
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and using ``executor.submit()`` at the call site.
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.. versionchanged:: 4.2
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Added keyword arguments to use alternative attributes.
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.. versionchanged:: 5.0
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Always uses the current IOLoop instead of ``self.io_loop``.
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.. versionchanged:: 5.1
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Returns a `.Future` compatible with ``await`` instead of a
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`concurrent.futures.Future`.
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.. deprecated:: 5.1
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The ``callback`` argument is deprecated and will be removed in
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6.0. The decorator itself is discouraged in new code but will
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not be removed in 6.0.
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"""
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def run_on_executor_decorator(fn):
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executor = kwargs.get("executor", "executor")
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@functools.wraps(fn)
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def wrapper(self, *args, **kwargs):
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callback = kwargs.pop("callback", None)
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async_future = Future()
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conc_future = getattr(self, executor).submit(fn, self, *args, **kwargs)
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chain_future(conc_future, async_future)
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if callback:
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warnings.warn("callback arguments are deprecated, use the returned Future instead",
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DeprecationWarning)
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from tornado.ioloop import IOLoop
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IOLoop.current().add_future(
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async_future, lambda future: callback(future.result()))
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return async_future
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return wrapper
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if args and kwargs:
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raise ValueError("cannot combine positional and keyword args")
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if len(args) == 1:
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return run_on_executor_decorator(args[0])
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elif len(args) != 0:
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raise ValueError("expected 1 argument, got %d", len(args))
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return run_on_executor_decorator
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_NO_RESULT = object()
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def return_future(f):
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"""Decorator to make a function that returns via callback return a
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`Future`.
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This decorator was provided to ease the transition from
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callback-oriented code to coroutines. It is not recommended for
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new code.
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The wrapped function should take a ``callback`` keyword argument
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and invoke it with one argument when it has finished. To signal failure,
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the function can simply raise an exception (which will be
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captured by the `.StackContext` and passed along to the ``Future``).
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From the caller's perspective, the callback argument is optional.
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If one is given, it will be invoked when the function is complete
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with ``Future.result()`` as an argument. If the function fails, the
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callback will not be run and an exception will be raised into the
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surrounding `.StackContext`.
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If no callback is given, the caller should use the ``Future`` to
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wait for the function to complete (perhaps by yielding it in a
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coroutine, or passing it to `.IOLoop.add_future`).
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Usage:
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.. testcode::
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@return_future
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def future_func(arg1, arg2, callback):
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# Do stuff (possibly asynchronous)
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callback(result)
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async def caller():
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await future_func(arg1, arg2)
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..
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Note that ``@return_future`` and ``@gen.engine`` can be applied to the
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same function, provided ``@return_future`` appears first. However,
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consider using ``@gen.coroutine`` instead of this combination.
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.. versionchanged:: 5.1
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Now raises a `.DeprecationWarning` if a callback argument is passed to
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the decorated function and deprecation warnings are enabled.
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.. deprecated:: 5.1
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This decorator will be removed in Tornado 6.0. New code should
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use coroutines directly instead of wrapping callback-based code
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with this decorator. Interactions with non-Tornado
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callback-based code should be managed explicitly to avoid
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relying on the `.ExceptionStackContext` built into this
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decorator.
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"""
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warnings.warn("@return_future is deprecated, use coroutines instead",
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DeprecationWarning)
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return _non_deprecated_return_future(f, warn=True)
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def _non_deprecated_return_future(f, warn=False):
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# Allow auth.py to use this decorator without triggering
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# deprecation warnings. This will go away once auth.py has removed
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|
# its legacy interfaces in 6.0.
|
|
replacer = ArgReplacer(f, 'callback')
|
|
|
|
@functools.wraps(f)
|
|
def wrapper(*args, **kwargs):
|
|
future = Future()
|
|
callback, args, kwargs = replacer.replace(
|
|
lambda value=_NO_RESULT: future_set_result_unless_cancelled(future, value),
|
|
args, kwargs)
|
|
|
|
def handle_error(typ, value, tb):
|
|
future_set_exc_info(future, (typ, value, tb))
|
|
return True
|
|
exc_info = None
|
|
esc = ExceptionStackContext(handle_error, delay_warning=True)
|
|
with esc:
|
|
if not warn:
|
|
# HACK: In non-deprecated mode (only used in auth.py),
|
|
# suppress the warning entirely. Since this is added
|
|
# in a 5.1 patch release and already removed in 6.0
|
|
# I'm prioritizing a minimial change instead of a
|
|
# clean solution.
|
|
esc.delay_warning = False
|
|
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:
|
|
warnings.warn("callback arguments are deprecated, use the returned Future instead",
|
|
DeprecationWarning)
|
|
|
|
def run_callback(future):
|
|
result = future.result()
|
|
if result is _NO_RESULT:
|
|
callback()
|
|
else:
|
|
callback(future.result())
|
|
future_add_done_callback(future, 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.
|
|
|
|
.. versionchanged:: 5.0
|
|
|
|
Now accepts both Tornado/asyncio `Future` objects and
|
|
`concurrent.futures.Future`.
|
|
|
|
"""
|
|
def copy(future):
|
|
assert future is a
|
|
if b.done():
|
|
return
|
|
if (hasattr(a, 'exc_info') and
|
|
a.exc_info() is not None):
|
|
future_set_exc_info(b, a.exc_info())
|
|
elif a.exception() is not None:
|
|
b.set_exception(a.exception())
|
|
else:
|
|
b.set_result(a.result())
|
|
if isinstance(a, Future):
|
|
future_add_done_callback(a, copy)
|
|
else:
|
|
# concurrent.futures.Future
|
|
from tornado.ioloop import IOLoop
|
|
IOLoop.current().add_future(a, copy)
|
|
|
|
|
|
def future_set_result_unless_cancelled(future, value):
|
|
"""Set the given ``value`` as the `Future`'s result, if not cancelled.
|
|
|
|
Avoids asyncio.InvalidStateError when calling set_result() on
|
|
a cancelled `asyncio.Future`.
|
|
|
|
.. versionadded:: 5.0
|
|
"""
|
|
if not future.cancelled():
|
|
future.set_result(value)
|
|
|
|
|
|
def future_set_exc_info(future, exc_info):
|
|
"""Set the given ``exc_info`` as the `Future`'s exception.
|
|
|
|
Understands both `asyncio.Future` and Tornado's extensions to
|
|
enable better tracebacks on Python 2.
|
|
|
|
.. versionadded:: 5.0
|
|
"""
|
|
if hasattr(future, 'set_exc_info'):
|
|
# Tornado's Future
|
|
future.set_exc_info(exc_info)
|
|
else:
|
|
# asyncio.Future
|
|
future.set_exception(exc_info[1])
|
|
|
|
|
|
def future_add_done_callback(future, callback):
|
|
"""Arrange to call ``callback`` when ``future`` is complete.
|
|
|
|
``callback`` is invoked with one argument, the ``future``.
|
|
|
|
If ``future`` is already done, ``callback`` is invoked immediately.
|
|
This may differ from the behavior of ``Future.add_done_callback``,
|
|
which makes no such guarantee.
|
|
|
|
.. versionadded:: 5.0
|
|
"""
|
|
if future.done():
|
|
callback(future)
|
|
else:
|
|
future.add_done_callback(callback)
|