Re: [Qemu-devel] [PATCH 0/3] iotests: clean up resources using context managers

[Fam Zheng <famz@redhat.com>]

On Fri, 08/25 09:52, Stefan Hajnoczi wrote:
> On Fri, Aug 25, 2017 at 03:32:29PM +0800, Fam Zheng wrote:
> > On Thu, 08/24 19:04, Stefan Hajnoczi wrote:
> > > On Thu, Aug 24, 2017 at 04:38:43PM +0800, Fam Zheng wrote:
> > > > On Thu, 08/24 08:21, Stefan Hajnoczi wrote:
> > > > > Tests should declare resources upfront in a with statement.  Resources are
> > > > > automatically cleaned up whether the test passes or fails:
> > > > > 
> > > > >   with FilePath('test.img') as img_path,
> > > > >        VM() as vm:
> > > > >       ...test...
> > > > >   # img_path is deleted and vm is shut down automatically
> > > > 
> > > > Looks good but still requires test writers to learn and remember to use FilePath
> > > > and with.
> > > 
> > > You cannot forget to use FilePath() unless you love typing at
> > > os.path.join(iotests.test_dir, 'test.img').  It's much better than open
> > > coding filename generation!
> > > 
> > > > These are still boilerplates.  Here goes my personal oppinion, so may
> > > > not be plausible:
> > > > 
> > > > - For VM() maybe add an atexit in the launch() method also makes sure the VM is
> > > >   eventually terminated.
> > > > 
> > > >   This means vm.shutdown() is still needed in tearDown() if there are multiple
> > > >   test methods and each of them expects a clean state, but that is probably
> > > >   still less typing (and also indenting) than the with approach, and also easy
> > > >   to remember (otherwise a test will fail).
> > > 
> > > I looked into atexit before going this route.  atexit does not have an
> > > unregister() API in Python 2.  This makes it ugly to use because some
> > > tests do not want the resource to remain for the duration of the
> > > process.
> > > 
> > > A related point is that the Python objects used by atexit handlers live
> > > until the end of the process.  They cannot be garbage collected because
> > > the atexit handler still has a reference to them.
> > 
> > I think this shortcoming can be solved with a clean up list ("all problems in
> > computer science can be solved by another level of indirection"):
> > 
> > _clean_up_list = set()
> > def _clean_up_handler():
> >     for i in _clean_up_list:
> >         try:
> >             i()
> >         except:
> >             pass
> > 
> > atexit.register(_clean_up_handler)
> > 
> > class VM(...):
> > 
> >     def launch():
> >         ...
> >         _clean_up_list.add(self.launch)
> > 
> >     def shutdown():
> >         _clean_up_list.remove(self.launch)
> >         ...
> 
> atexit is still less powerful than context managers because its scope is
> fixed.  Handler functions are only called when the process terminates.
> Many test cases do not want resources (especially the VMs) around
> forever because they run several iterations or sub-tests.
> 
> The with statement can be used both for process-lifetime and for more
> fine-grained scoping.  That's why I chose it.
> 
> If you stick to atexit then sub-tests or iterations require manual
> vm.shutdown() - something that is not necessary using the with
> statement.

Sure!

I just think that if leftover VM instances are a concern and not all test code
are converted to "with", having the atexit handler in addition may provide more
robustness.

Fam