# Copyright 2011 OpenStack Foundation. # Copyright 2012, Red Hat, Inc. # # 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. """ Exception related utilities. """ import logging import os import sys import time import traceback import six from oslo_utils._i18n import _LE from oslo_utils import encodeutils from oslo_utils import reflection from oslo_utils import timeutils class CausedByException(Exception): """Base class for exceptions which have associated causes. NOTE(harlowja): in later versions of python we can likely remove the need to have a ``cause`` here as PY3+ have implemented :pep:`3134` which handles chaining in a much more elegant manner. :param message: the exception message, typically some string that is useful for consumers to view when debugging or analyzing failures. :param cause: the cause of the exception being raised, when provided this should itself be an exception instance, this is useful for creating a chain of exceptions for versions of python where this is not yet implemented/supported natively. """ def __init__(self, message, cause=None): super(CausedByException, self).__init__(message) self.cause = cause def __bytes__(self): return self.pformat().encode("utf8") def __str__(self): return self.pformat() def _get_message(self): # We must *not* call into the ``__str__`` method as that will # reactivate the pformat method, which will end up badly (and doesn't # look pretty at all); so be careful... return self.args[0] def pformat(self, indent=2, indent_text=" ", show_root_class=False): """Pretty formats a caused exception + any connected causes.""" if indent < 0: raise ValueError("Provided 'indent' must be greater than" " or equal to zero instead of %s" % indent) buf = six.StringIO() if show_root_class: buf.write(reflection.get_class_name(self, fully_qualified=False)) buf.write(": ") buf.write(self._get_message()) active_indent = indent next_up = self.cause seen = [] while next_up is not None and next_up not in seen: seen.append(next_up) buf.write(os.linesep) if isinstance(next_up, CausedByException): buf.write(indent_text * active_indent) buf.write(reflection.get_class_name(next_up, fully_qualified=False)) buf.write(": ") buf.write(next_up._get_message()) else: lines = traceback.format_exception_only(type(next_up), next_up) for i, line in enumerate(lines): buf.write(indent_text * active_indent) if line.endswith("\n"): # We'll add our own newlines on... line = line[0:-1] buf.write(line) if i + 1 != len(lines): buf.write(os.linesep) if not isinstance(next_up, CausedByException): # Don't go deeper into non-caused-by exceptions... as we # don't know if there exception 'cause' attributes are even # useable objects... break active_indent += indent next_up = getattr(next_up, 'cause', None) return buf.getvalue() def raise_with_cause(exc_cls, message, *args, **kwargs): """Helper to raise + chain exceptions (when able) and associate a *cause*. NOTE(harlowja): Since in py3.x exceptions can be chained (due to :pep:`3134`) we should try to raise the desired exception with the given *cause* (or extract a *cause* from the current stack if able) so that the exception formats nicely in old and new versions of python. Since py2.x does **not** support exception chaining (or formatting) the exception class provided should take a ``cause`` keyword argument (which it may discard if it wants) to its constructor which can then be inspected/retained on py2.x to get *similar* information as would be automatically included/obtainable in py3.x. :param exc_cls: the exception class to raise (typically one derived from :py:class:`.CausedByException` or equivalent). :param message: the text/str message that will be passed to the exceptions constructor as its first positional argument. :param args: any additional positional arguments to pass to the exceptions constructor. :param kwargs: any additional keyword arguments to pass to the exceptions constructor. """ if 'cause' not in kwargs: exc_type, exc, exc_tb = sys.exc_info() try: if exc is not None: kwargs['cause'] = exc finally: # Leave no references around (especially with regards to # tracebacks and any variables that it retains internally). del(exc_type, exc, exc_tb) six.raise_from(exc_cls(message, *args, **kwargs), kwargs.get('cause')) class save_and_reraise_exception(object): """Save current exception, run some code and then re-raise. In some cases the exception context can be cleared, resulting in None being attempted to be re-raised after an exception handler is run. This can happen when eventlet switches greenthreads or when running an exception handler, code raises and catches an exception. In both cases the exception context will be cleared. To work around this, we save the exception state, run handler code, and then re-raise the original exception. If another exception occurs, the saved exception is logged and the new exception is re-raised. In some cases the caller may not want to re-raise the exception, and for those circumstances this context provides a reraise flag that can be used to suppress the exception. For example:: except Exception: with save_and_reraise_exception() as ctxt: decide_if_need_reraise() if not should_be_reraised: ctxt.reraise = False If another exception occurs and reraise flag is False, the saved exception will not be logged. If the caller wants to raise new exception during exception handling he/she sets reraise to False initially with an ability to set it back to True if needed:: except Exception: with save_and_reraise_exception(reraise=False) as ctxt: [if statements to determine whether to raise a new exception] # Not raising a new exception, so reraise ctxt.reraise = True """ def __init__(self, reraise=True, logger=None): self.reraise = reraise if logger is None: logger = logging.getLogger() self.logger = logger def __enter__(self): self.type_, self.value, self.tb, = sys.exc_info() return self def __exit__(self, exc_type, exc_val, exc_tb): if exc_type is not None: if self.reraise: self.logger.error(_LE('Original exception being dropped: %s'), traceback.format_exception(self.type_, self.value, self.tb)) return False if self.reraise: six.reraise(self.type_, self.value, self.tb) def forever_retry_uncaught_exceptions(infunc): """Decorates provided function with infinite retry behavior. If the original function fails it may trigger a logging output to be written. The function retry delay is **always** one second. """ def inner_func(*args, **kwargs): last_exc_message = None same_failure_count = 0 watch = timeutils.StopWatch(duration=60) while True: try: return infunc(*args, **kwargs) except Exception as exc: this_exc_message = encodeutils.exception_to_unicode(exc) if this_exc_message == last_exc_message: same_failure_count += 1 else: same_failure_count = 1 if this_exc_message != last_exc_message or watch.expired(): # The watch has expired or the exception message # changed, so time to log it again... logging.exception( _LE('Unexpected exception occurred %d time(s)... ' 'retrying.') % same_failure_count) if not watch.has_started(): watch.start() else: watch.restart() same_failure_count = 0 last_exc_message = this_exc_message # This should be a very rare event. In case it isn't, do # a sleep. # # TODO(harlowja): why this is hard coded as one second # really should be fixed (since I'm not really sure why # one over some other value was chosen). time.sleep(1) return inner_func