Re: [gentoo-portage-dev] [PATCH] bindbapi: add reentrant lock method (bug 685236)

[Zac Medico]

On 5/8/19 11:05 AM, Zac Medico wrote:
> On 5/7/19 1:01 PM, Zac Medico wrote:
>> On 5/7/19 7:55 AM, Alec Warner wrote:
>>
>>> Also curious why we are not implementing enter and exit so we can avoid
>>> unbalanced pairs by using context managers.
>>>
>>> e.g. in match(), we could likely write:
>>>
>>> with self.dbapi.lock():
>>>   # try to match
>>>   update_pkgindex = self._populate_local()
>>>   if update_pkgindex:
>>>     self._pkgindex_write(update_pkgindex)
>>>
>>> If the block raises an exception, __exit__ is still called, so we can
>>> eliminate the finally: blocks and replace them with a careful __exit__
>>> implementation?
>>>
>>> -A
>>
>> Yeah, the reference counted lock class could have a method that returns
>> a context manager which increments/decrements the lock count. The
>> context manager can be implemented with the contextlib.contextmanager
>> decorator like this:
>>
>> @contextlib.contextmanager
>> def contextmanager(self):
>>     self.lock()
>>     try:
>>         yield self
>>     finally:
>>         self.unlock()
>>
>
> Since we really don't want asynchronous code to block waiting for a
> lock, we really should use a (python3.5+ only) asynchronous context manager:
>
> https://www.python.org/dev/peps/pep-0492/#asynchronous-context-managers-and-async-with
>
> Given that python2.7 is scheduled for retirement in 2020
> (https://pythonclock.org/), maybe it's time to drop support for
> python3.4 and earlier.

Adapting the sample code provided in PEP 492, we can do something like
this for compatibility with older versions of python:

from portage.util.futures.compat_coroutine

@compat_coroutine.coroutine
def async_with(mgr, async_func):
  aexit = type(mgr).__aexit__
  aenter = type(mgr).__aenter__(mgr)

  aenter_result = (yield aenter)
  try:
    return_value = (yield async_func(aenter_result))
  except:
    if not (yield aexit(mgr, *sys.exc_info()):
      raise
  else:
    yield aexit(mgr, None, None, None)

  compat_coroutine.coroutine_return(return_value)


-- 
Thanks,
Zac