diff options
Diffstat (limited to 'Lib/asyncio/futures.py')
| -rw-r--r-- | Lib/asyncio/futures.py | 122 |
1 files changed, 16 insertions, 106 deletions
diff --git a/Lib/asyncio/futures.py b/Lib/asyncio/futures.py index d11d289..39721ea 100644 --- a/Lib/asyncio/futures.py +++ b/Lib/asyncio/futures.py @@ -27,86 +27,6 @@ _FINISHED = base_futures._FINISHED STACK_DEBUG = logging.DEBUG - 1 # heavy-duty debugging -class _TracebackLogger: - """Helper to log a traceback upon destruction if not cleared. - - This solves a nasty problem with Futures and Tasks that have an - exception set: if nobody asks for the exception, the exception is - never logged. This violates the Zen of Python: 'Errors should - never pass silently. Unless explicitly silenced.' - - However, we don't want to log the exception as soon as - set_exception() is called: if the calling code is written - properly, it will get the exception and handle it properly. But - we *do* want to log it if result() or exception() was never called - -- otherwise developers waste a lot of time wondering why their - buggy code fails silently. - - An earlier attempt added a __del__() method to the Future class - itself, but this backfired because the presence of __del__() - prevents garbage collection from breaking cycles. A way out of - this catch-22 is to avoid having a __del__() method on the Future - class itself, but instead to have a reference to a helper object - with a __del__() method that logs the traceback, where we ensure - that the helper object doesn't participate in cycles, and only the - Future has a reference to it. - - The helper object is added when set_exception() is called. When - the Future is collected, and the helper is present, the helper - object is also collected, and its __del__() method will log the - traceback. When the Future's result() or exception() method is - called (and a helper object is present), it removes the helper - object, after calling its clear() method to prevent it from - logging. - - One downside is that we do a fair amount of work to extract the - traceback from the exception, even when it is never logged. It - would seem cheaper to just store the exception object, but that - references the traceback, which references stack frames, which may - reference the Future, which references the _TracebackLogger, and - then the _TracebackLogger would be included in a cycle, which is - what we're trying to avoid! As an optimization, we don't - immediately format the exception; we only do the work when - activate() is called, which call is delayed until after all the - Future's callbacks have run. Since usually a Future has at least - one callback (typically set by 'yield from') and usually that - callback extracts the callback, thereby removing the need to - format the exception. - - PS. I don't claim credit for this solution. I first heard of it - in a discussion about closing files when they are collected. - """ - - __slots__ = ('loop', 'source_traceback', 'exc', 'tb') - - def __init__(self, future, exc): - self.loop = future._loop - self.source_traceback = future._source_traceback - self.exc = exc - self.tb = None - - def activate(self): - exc = self.exc - if exc is not None: - self.exc = None - self.tb = traceback.format_exception(exc.__class__, exc, - exc.__traceback__) - - def clear(self): - self.exc = None - self.tb = None - - def __del__(self): - if self.tb: - msg = 'Future/Task exception was never retrieved\n' - if self.source_traceback: - src = ''.join(traceback.format_list(self.source_traceback)) - msg += 'Future/Task created at (most recent call last):\n' - msg += '%s\n' % src.rstrip() - msg += ''.join(self.tb).rstrip() - self.loop.call_exception_handler({'message': msg}) - - class Future: """This class is *almost* compatible with concurrent.futures.Future. @@ -164,25 +84,21 @@ class Future: def __repr__(self): return '<%s %s>' % (self.__class__.__name__, ' '.join(self._repr_info())) - # On Python 3.3 and older, objects with a destructor part of a reference - # cycle are never destroyed. It's not more the case on Python 3.4 thanks - # to the PEP 442. - if compat.PY34: - def __del__(self): - if not self._log_traceback: - # set_exception() was not called, or result() or exception() - # has consumed the exception - return - exc = self._exception - context = { - 'message': ('%s exception was never retrieved' - % self.__class__.__name__), - 'exception': exc, - 'future': self, - } - if self._source_traceback: - context['source_traceback'] = self._source_traceback - self._loop.call_exception_handler(context) + def __del__(self): + if not self._log_traceback: + # set_exception() was not called, or result() or exception() + # has consumed the exception + return + exc = self._exception + context = { + 'message': ('%s exception was never retrieved' + % self.__class__.__name__), + 'exception': exc, + 'future': self, + } + if self._source_traceback: + context['source_traceback'] = self._source_traceback + self._loop.call_exception_handler(context) def cancel(self): """Cancel the future and schedule callbacks. @@ -317,13 +233,7 @@ class Future: self._exception = exception self._state = _FINISHED self._schedule_callbacks() - if compat.PY34: - self._log_traceback = True - else: - self._tb_logger = _TracebackLogger(self, exception) - # Arrange for the logger to be activated after all callbacks - # have had a chance to call result() or exception(). - self._loop.call_soon(self._tb_logger.activate) + self._log_traceback = True def __iter__(self): if not self.done(): |