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#!/usr/bin/env python
# Copyright 2010 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
# 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.
"""`StackContext` allows applications to maintain threadlocal-like state
that follows execution as it moves to other execution contexts.
The motivating examples are to eliminate the need for explicit
``async_callback`` wrappers (as in `tornado.web.RequestHandler`), and to
allow some additional context to be kept for logging.
This is slightly magic, but it's an extension of the idea that an
exception handler is a kind of stack-local state and when that stack
is suspended and resumed in a new context that state needs to be
preserved. `StackContext` shifts the burden of restoring that state
from each call site (e.g. wrapping each `.AsyncHTTPClient` callback
in ``async_callback``) to the mechanisms that transfer control from
one context to another (e.g. `.AsyncHTTPClient` itself, `.IOLoop`,
thread pools, etc).
Example usage::
def die_on_error():
except Exception:
logging.error("exception in asynchronous operation",exc_info=True)
with StackContext(die_on_error):
# Any exception thrown here *or in callback and its descendants*
# will cause the process to exit instead of spinning endlessly
# in the ioloop.
http_client.fetch(url, callback)
Most applications shouldn't have to work with `StackContext` directly.
Here are a few rules of thumb for when it's necessary:
* If you're writing an asynchronous library that doesn't rely on a
stack_context-aware library like `tornado.ioloop` or `tornado.iostream`
(for example, if you're writing a thread pool), use
`.stack_context.wrap()` before any asynchronous operations to capture the
stack context from where the operation was started.
* If you're writing an asynchronous library that has some shared
resources (such as a connection pool), create those shared resources
within a ``with stack_context.NullContext():`` block. This will prevent
``StackContexts`` from leaking from one request to another.
* If you want to write something like an exception handler that will
persist across asynchronous calls, create a new `StackContext` (or
`ExceptionStackContext`), and make your asynchronous calls in a ``with``
block that references your `StackContext`.
from __future__ import absolute_import, division, print_function, with_statement
import sys
import threading
from tornado.util import raise_exc_info
class StackContextInconsistentError(Exception):
class _State(threading.local):
def __init__(self):
self.contexts = (tuple(), None)
_state = _State()
class StackContext(object):
"""Establishes the given context as a StackContext that will be transferred.
Note that the parameter is a callable that returns a context
manager, not the context itself. That is, where for a
non-transferable context manager you would say::
with my_context():
StackContext takes the function itself rather than its result::
with StackContext(my_context):
The result of ``with StackContext() as cb:`` is a deactivation
callback. Run this callback when the StackContext is no longer
needed to ensure that it is not propagated any further (note that
deactivating a context does not affect any instances of that
context that are currently pending). This is an advanced feature
and not necessary in most applications.
def __init__(self, context_factory):
self.context_factory = context_factory
self.contexts = [] = True
def _deactivate(self): = False
# StackContext protocol
def enter(self):
context = self.context_factory()
def exit(self, type, value, traceback):
context = self.contexts.pop()
context.__exit__(type, value, traceback)
# Note that some of this code is duplicated in ExceptionStackContext
# below. ExceptionStackContext is more common and doesn't need
# the full generality of this class.
def __enter__(self):
self.old_contexts = _state.contexts
self.new_contexts = (self.old_contexts[0] + (self,), self)
_state.contexts = self.new_contexts
_state.contexts = self.old_contexts
return self._deactivate
def __exit__(self, type, value, traceback):
self.exit(type, value, traceback)
final_contexts = _state.contexts
_state.contexts = self.old_contexts
# Generator coroutines and with-statements with non-local
# effects interact badly. Check here for signs of
# the stack getting out of sync.
# Note that this check comes after restoring _state.context
# so that if it fails things are left in a (relatively)
# consistent state.
if final_contexts is not self.new_contexts:
raise StackContextInconsistentError(
'stack_context inconsistency (may be caused by yield '
'within a "with StackContext" block)')
# Break up a reference to itself to allow for faster GC on CPython.
self.new_contexts = None
class ExceptionStackContext(object):
"""Specialization of StackContext for exception handling.
The supplied ``exception_handler`` function will be called in the
event of an uncaught exception in this context. The semantics are
similar to a try/finally clause, and intended use cases are to log
an error, close a socket, or similar cleanup actions. The
``exc_info`` triple ``(type, value, traceback)`` will be passed to the
exception_handler function.
If the exception handler returns true, the exception will be
consumed and will not be propagated to other exception handlers.
def __init__(self, exception_handler):
self.exception_handler = exception_handler = True
def _deactivate(self): = False
def exit(self, type, value, traceback):
if type is not None:
return self.exception_handler(type, value, traceback)
def __enter__(self):
self.old_contexts = _state.contexts
self.new_contexts = (self.old_contexts[0], self)
_state.contexts = self.new_contexts
return self._deactivate
def __exit__(self, type, value, traceback):
if type is not None:
return self.exception_handler(type, value, traceback)
final_contexts = _state.contexts
_state.contexts = self.old_contexts
if final_contexts is not self.new_contexts:
raise StackContextInconsistentError(
'stack_context inconsistency (may be caused by yield '
'within a "with StackContext" block)')
# Break up a reference to itself to allow for faster GC on CPython.
self.new_contexts = None
class NullContext(object):
"""Resets the `StackContext`.
Useful when creating a shared resource on demand (e.g. an
`.AsyncHTTPClient`) where the stack that caused the creating is
not relevant to future operations.
def __enter__(self):
self.old_contexts = _state.contexts
_state.contexts = (tuple(), None)
def __exit__(self, type, value, traceback):
_state.contexts = self.old_contexts
def _remove_deactivated(contexts):
"""Remove deactivated handlers from the chain"""
# Clean ctx handlers
stack_contexts = tuple([h for h in contexts[0] if])
# Find new head
head = contexts[1]
while head is not None and not
head = head.old_contexts[1]
# Process chain
ctx = head
while ctx is not None:
parent = ctx.old_contexts[1]
while parent is not None:
ctx.old_contexts = parent.old_contexts
parent = parent.old_contexts[1]
ctx = parent
return (stack_contexts, head)
def wrap(fn):
"""Returns a callable object that will restore the current `StackContext`
when executed.
Use this whenever saving a callback to be executed later in a
different execution context (either in a different thread or
asynchronously in the same thread).
# Check if function is already wrapped
if fn is None or hasattr(fn, '_wrapped'):
return fn
# Capture current stack head
# TODO: Any other better way to store contexts and update them in wrapped function?
cap_contexts = [_state.contexts]
if not cap_contexts[0][0] and not cap_contexts[0][1]:
# Fast path when there are no active contexts.
def null_wrapper(*args, **kwargs):
current_state = _state.contexts
_state.contexts = cap_contexts[0]
return fn(*args, **kwargs)
_state.contexts = current_state
null_wrapper._wrapped = True
return null_wrapper
def wrapped(*args, **kwargs):
ret = None
# Capture old state
current_state = _state.contexts
# Remove deactivated items
cap_contexts[0] = contexts = _remove_deactivated(cap_contexts[0])
# Force new state
_state.contexts = contexts
# Current exception
exc = (None, None, None)
top = None
# Apply stack contexts
last_ctx = 0
stack = contexts[0]
# Apply state
for n in stack:
last_ctx += 1
# Exception happened. Record exception info and store top-most handler
exc = sys.exc_info()
top = n.old_contexts[1]
# Execute callback if no exception happened while restoring state
if top is None:
ret = fn(*args, **kwargs)
exc = sys.exc_info()
top = contexts[1]
# If there was exception, try to handle it by going through the exception chain
if top is not None:
exc = _handle_exception(top, exc)
# Otherwise take shorter path and run stack contexts in reverse order
while last_ctx > 0:
last_ctx -= 1
c = stack[last_ctx]
exc = sys.exc_info()
top = c.old_contexts[1]
top = None
# If if exception happened while unrolling, take longer exception handler path
if top is not None:
exc = _handle_exception(top, exc)
# If exception was not handled, raise it
if exc != (None, None, None):
_state.contexts = current_state
return ret
wrapped._wrapped = True
return wrapped
def _handle_exception(tail, exc):
while tail is not None:
if tail.exit(*exc):
exc = (None, None, None)
exc = sys.exc_info()
tail = tail.old_contexts[1]
return exc
def run_with_stack_context(context, func):
"""Run a coroutine ``func`` in the given `StackContext`.
It is not safe to have a ``yield`` statement within a ``with StackContext``
block, so it is difficult to use stack context with `.gen.coroutine`.
This helper function runs the function in the correct context while
keeping the ``yield`` and ``with`` statements syntactically separate.
def incorrect():
with StackContext(ctx):
# ERROR: this will raise StackContextInconsistentError
yield other_coroutine()
def correct():
yield run_with_stack_context(StackContext(ctx), other_coroutine)
.. versionadded:: 3.1
with context:
return func()
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