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461 lines
16 KiB
Python
461 lines
16 KiB
Python
#!/usr/bin/env python
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#
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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 threads and ``Futures``.
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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 (this package has also
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been backported to older versions of Python and can be installed with
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``pip install futures``). Tornado will use `concurrent.futures.Future` if
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it is available; otherwise it will use a compatible class defined in this
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module.
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"""
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from __future__ import absolute_import, division, print_function, with_statement
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import functools
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import platform
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import traceback
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import sys
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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
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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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# 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):
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if 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 similar to
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`concurrent.futures.Future`, but not thread-safe (and therefore
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faster for use with single-threaded event loops).
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In addition to ``exception`` and ``set_exception``, methods ``exc_info``
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and ``set_exc_info`` are supported to capture tracebacks in Python 2.
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The traceback is automatically available in Python 3, but in the
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Python 2 futures backport this information is discarded.
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This functionality was previously available in a separate class
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``TracebackFuture``, which is now a deprecated alias for this class.
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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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"""
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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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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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"""
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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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raise_exc_info(self._exc_info)
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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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"""
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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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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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for cb in self._callbacks:
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try:
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cb(self)
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except Exception:
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app_log.exception('exception calling callback %r for %r',
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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):
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if 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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TracebackFuture = Future
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if futures is None:
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FUTURES = Future
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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 = TracebackFuture()
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try:
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future.set_result(fn(*args, **kwargs))
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except Exception:
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future.set_exc_info(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(fn):
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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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This decorator should be used only on methods of objects with attributes
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``executor`` and ``io_loop``.
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"""
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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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future = self.executor.submit(fn, self, *args, **kwargs)
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if callback:
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self.io_loop.add_future(future,
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lambda future: callback(future.result()))
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return future
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return wrapper
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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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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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`.gen.engine` function, or passing it to `.IOLoop.add_future`).
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Usage::
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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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@gen.engine
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def caller(callback):
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yield future_func(arg1, arg2)
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callback()
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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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"""
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replacer = ArgReplacer(f, 'callback')
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@functools.wraps(f)
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def wrapper(*args, **kwargs):
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future = TracebackFuture()
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callback, args, kwargs = replacer.replace(
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lambda value=_NO_RESULT: future.set_result(value),
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args, kwargs)
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def handle_error(typ, value, tb):
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future.set_exc_info((typ, value, tb))
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return True
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exc_info = None
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with ExceptionStackContext(handle_error):
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try:
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result = f(*args, **kwargs)
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if result is not None:
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raise ReturnValueIgnoredError(
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"@return_future should not be used with functions "
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"that return values")
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except:
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exc_info = sys.exc_info()
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raise
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if exc_info is not None:
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# If the initial synchronous part of f() raised an exception,
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# go ahead and raise it to the caller directly without waiting
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# for them to inspect the Future.
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future.result()
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# If the caller passed in a callback, schedule it to be called
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# when the future resolves. It is important that this happens
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# just before we return the future, or else we risk confusing
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# stack contexts with multiple exceptions (one here with the
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# immediate exception, and again when the future resolves and
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# the callback triggers its exception by calling future.result()).
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if callback is not None:
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def run_callback(future):
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result = future.result()
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if result is _NO_RESULT:
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callback()
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else:
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callback(future.result())
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future.add_done_callback(wrap(run_callback))
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return future
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return wrapper
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def chain_future(a, b):
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"""Chain two futures together so that when one completes, so does the other.
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The result (success or failure) of ``a`` will be copied to ``b``, unless
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``b`` has already been completed or cancelled by the time ``a`` finishes.
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"""
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def copy(future):
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assert future is a
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if b.done():
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return
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if (isinstance(a, TracebackFuture) and isinstance(b, TracebackFuture)
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and a.exc_info() is not None):
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b.set_exc_info(a.exc_info())
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elif a.exception() is not None:
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b.set_exception(a.exception())
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else:
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b.set_result(a.result())
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a.add_done_callback(copy)
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