logcontext.py

# Copyright 2014, 2015 OpenMarket Ltd
#
# 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.

from twisted.internet import defer

import threading
import logging

logger = logging.getLogger(__name__)

try:
    import resource

    # Python doesn't ship with a definition of RUSAGE_THREAD but it's defined
    # to be 1 on linux so we hard code it.
    RUSAGE_THREAD = 1

    # If the system doesn't support RUSAGE_THREAD then this should throw an
    # exception.
    resource.getrusage(RUSAGE_THREAD)

    def get_thread_resource_usage():
        return resource.getrusage(RUSAGE_THREAD)
except:
    # If the system doesn't support resource.getrusage(RUSAGE_THREAD) then we
    # won't track resource usage by returning None.
    def get_thread_resource_usage():
        return None


class LoggingContext(object):
    """Additional context for log formatting. Contexts are scoped within a
    "with" block. Contexts inherit the state of their parent contexts.
    Args:
        name (str): Name for the context for debugging.
    """

    __slots__ = [
        "parent_context", "name", "usage_start", "usage_end", "main_thread", "__dict__"
    ]

    thread_local = threading.local()

    class Sentinel(object):
        """Sentinel to represent the root context"""

        __slots__ = []

        def __str__(self):
            return "sentinel"

        def copy_to(self, record):
            pass

        def start(self):
            pass

        def stop(self):
            pass

    sentinel = Sentinel()

    def __init__(self, name=None):
        self.parent_context = None
        self.name = name
        self.ru_stime = 0.
        self.ru_utime = 0.
        self.usage_start = None
        self.main_thread = threading.current_thread()

    def __str__(self):
        return "%s@%x" % (self.name, id(self))

    @classmethod
    def current_context(cls):
        """Get the current logging context from thread local storage"""
        return getattr(cls.thread_local, "current_context", cls.sentinel)

    @classmethod
    def set_current_context(cls, context):
        """Set the current logging context in thread local storage
        Args:
            context(LoggingContext): The context to activate.
        Returns:
            The context that was previously active
        """
        current = cls.current_context()
        if current is not context:
            current.stop()
            cls.thread_local.current_context = context
            context.start()
        return current

    def __enter__(self):
        """Enters this logging context into thread local storage"""
        if self.parent_context is not None:
            raise Exception("Attempt to enter logging context multiple times")
        self.parent_context = self.set_current_context(self)
        return self

    def __exit__(self, type, value, traceback):
        """Restore the logging context in thread local storage to the state it
        was before this context was entered.
        Returns:
            None to avoid suppressing any exeptions that were thrown.
        """
        current = self.set_current_context(self.parent_context)
        if current is not self:
            if current is self.sentinel:
                logger.debug("Expected logging context %s has been lost", self)
            else:
                logger.warn(
                    "Current logging context %s is not expected context %s",
                    current,
                    self
                )
        self.parent_context = None

    def __getattr__(self, name):
        """Delegate member lookup to parent context"""
        return getattr(self.parent_context, name)

    def copy_to(self, record):
        """Copy fields from this context and its parents to the record"""
        if self.parent_context is not None:
            self.parent_context.copy_to(record)
        for key, value in self.__dict__.items():
            setattr(record, key, value)

        record.ru_utime, record.ru_stime = self.get_resource_usage()

    def start(self):
        if threading.current_thread() is not self.main_thread:
            return

        if self.usage_start and self.usage_end:
            self.ru_utime += self.usage_end.ru_utime - self.usage_start.ru_utime
            self.ru_stime += self.usage_end.ru_stime - self.usage_start.ru_stime
            self.usage_start = None
            self.usage_end = None

        if not self.usage_start:
            self.usage_start = get_thread_resource_usage()

    def stop(self):
        if threading.current_thread() is not self.main_thread:
            return

        if self.usage_start:
            self.usage_end = get_thread_resource_usage()

    def get_resource_usage(self):
        ru_utime = self.ru_utime
        ru_stime = self.ru_stime

        if self.usage_start and threading.current_thread() is self.main_thread:
            current = get_thread_resource_usage()
            ru_utime += current.ru_utime - self.usage_start.ru_utime
            ru_stime += current.ru_stime - self.usage_start.ru_stime

        return ru_utime, ru_stime


class LoggingContextFilter(logging.Filter):
    """Logging filter that adds values from the current logging context to each
    record.
    Args:
        **defaults: Default values to avoid formatters complaining about
            missing fields
    """
    def __init__(self, **defaults):
        self.defaults = defaults

    def filter(self, record):
        """Add each fields from the logging contexts to the record.
        Returns:
            True to include the record in the log output.
        """
        context = LoggingContext.current_context()
        for key, value in self.defaults.items():
            setattr(record, key, value)
        context.copy_to(record)
        return True


class PreserveLoggingContext(object):
    """Captures the current logging context and restores it when the scope is
    exited. Used to restore the context after a function using
    @defer.inlineCallbacks is resumed by a callback from the reactor."""

    __slots__ = ["current_context", "new_context"]

    def __init__(self, new_context=LoggingContext.sentinel):
        self.new_context = new_context

    def __enter__(self):
        """Captures the current logging context"""
        self.current_context = LoggingContext.set_current_context(
            self.new_context
        )

    def __exit__(self, type, value, traceback):
        """Restores the current logging context"""
        LoggingContext.set_current_context(self.current_context)
        if self.current_context is not LoggingContext.sentinel:
            if self.current_context.parent_context is None:
                logger.warn(
                    "Restoring dead context: %s",
                    self.current_context,
                )


class _PreservingContextDeferred(defer.Deferred):
    """A deferred that ensures that all callbacks and errbacks are called with
    the given logging context.
    """
    def __init__(self, context):
        self._log_context = context
        defer.Deferred.__init__(self)

    def addCallbacks(self, callback, errback=None,
                     callbackArgs=None, callbackKeywords=None,
                     errbackArgs=None, errbackKeywords=None):
        callback = self._wrap_callback(callback)
        errback = self._wrap_callback(errback)
        return defer.Deferred.addCallbacks(
            self, callback,
            errback=errback,
            callbackArgs=callbackArgs,
            callbackKeywords=callbackKeywords,
            errbackArgs=errbackArgs,
            errbackKeywords=errbackKeywords,
        )

    def _wrap_callback(self, f):
        def g(res, *args, **kwargs):
            with PreserveLoggingContext(self._log_context):
                res = f(res, *args, **kwargs)
            return res
        return g


def preserve_context_over_fn(fn, *args, **kwargs):
    """Takes a function and invokes it with the given arguments, but removes
    and restores the current logging context while doing so.

    If the result is a deferred, call preserve_context_over_deferred before
    returning it.
    """
    with PreserveLoggingContext():
        res = fn(*args, **kwargs)

    if isinstance(res, defer.Deferred):
        return preserve_context_over_deferred(res)
    else:
        return res


def preserve_context_over_deferred(deferred):
    """Given a deferred wrap it such that any callbacks added later to it will
    be invoked with the current context.
    """
    current_context = LoggingContext.current_context()
    d = _PreservingContextDeferred(current_context)
    deferred.chainDeferred(d)
    return d