This series introduces context managers for the two most commonly used
resources: files and VMs. Context managers eliminate the need to call a
cleanup function explicitly.
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
The final patch converts 194 to use context managers instead of
atexit.register(). This makes the code shorter and easier to read.
Stefan Hajnoczi (3):
qemu.py: make VM() a context manager
iotests.py: add FilePath context manager
qemu-iotests: use context managers for resource cleanup in 194
scripts/qemu.py | 16 ++++++++-
tests/qemu-iotests/194 | 83 +++++++++++++++++++++----------------------
tests/qemu-iotests/iotests.py | 26 ++++++++++++++
3 files changed, 82 insertions(+), 43 deletions(-)
--
2.13.5
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. 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).
- For scratch how about adding atexit in iotests.main to clean up everything in
the scratch directory? The rationale is similar to above.
Fam
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.
The with statement's identation is annoying for straightforward scripts.
More complex tests use functions anyway, so the indentation doesn't
matter there - it can be hidden by a parent function or even a
decorator.
> - For scratch how about adding atexit in iotests.main to clean up everything in
> the scratch directory? The rationale is similar to above.
If we decide to clear out TEST_DIR then it should be done in ./check,
not by iotests.py, so that all tests get the same file cleanup behavior,
regardless of the language they are written in.
Also, we can then chdir(iotests.test_dir) so filename generation isn't
necessary at all. Tests can simply use 'test.img'. I guess there may
be some cases where absolute paths are necessary, but for the most part
this would be a win.
Kevin may have an opinion on whether TEST_DIR should be cleared out or
not.
Stefan
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)
...
>
> The with statement's identation is annoying for straightforward scripts.
> More complex tests use functions anyway, so the indentation doesn't
> matter there - it can be hidden by a parent function or even a
> decorator.
>
> > - For scratch how about adding atexit in iotests.main to clean up everything in
> > the scratch directory? The rationale is similar to above.
>
> If we decide to clear out TEST_DIR then it should be done in ./check,
> not by iotests.py, so that all tests get the same file cleanup behavior,
> regardless of the language they are written in.
>
> Also, we can then chdir(iotests.test_dir) so filename generation isn't
> necessary at all. Tests can simply use 'test.img'. I guess there may
> be some cases where absolute paths are necessary, but for the most part
> this would be a win.
Good point, ./check can even invoke scripts with scratch as their working
directory.
Fam
>
> Kevin may have an opinion on whether TEST_DIR should be cleared out or
> not.
>
> Stefan
>
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.
Stefan
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
On Fri, Aug 25, 2017 at 05:29:14PM +0800, Fam Zheng wrote:
> 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.
Okay, I checked this. Existing code doesn't need to be changed (yet)
because:
1. Most existing code uses unittest's setUp()/tearDown() and already
correctly handles cleanup when the test fails.
2. The LUKS crypto test doesn't use unittest but also doesn't use VM(),
so it doesn't need.
Are you happy for me to merge this series?
Stefan
On Wed, 08/30 13:44, Stefan Hajnoczi wrote:
> On Fri, Aug 25, 2017 at 05:29:14PM +0800, Fam Zheng wrote:
> > 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.
>
> Okay, I checked this. Existing code doesn't need to be changed (yet)
> because:
>
> 1. Most existing code uses unittest's setUp()/tearDown() and already
> correctly handles cleanup when the test fails.
>
> 2. The LUKS crypto test doesn't use unittest but also doesn't use VM(),
> so it doesn't need.
>
> Are you happy for me to merge this series?
Yes. Sounds good! Thanks.
Fam
Stefan Hajnoczi <stefanha@redhat.com> writes:
> 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.
Moreover, context managers are Pythonic.
> If you stick to atexit then sub-tests or iterations require manual
> vm.shutdown() - something that is not necessary using the with
> statement.
>
> Stefan
On Thu, Aug 24, 2017 at 08:21:59AM +0100, Stefan Hajnoczi wrote:
> This series introduces context managers for the two most commonly used
> resources: files and VMs. Context managers eliminate the need to call a
> cleanup function explicitly.
>
> 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
>
> The final patch converts 194 to use context managers instead of
> atexit.register(). This makes the code shorter and easier to read.
>
> Stefan Hajnoczi (3):
> qemu.py: make VM() a context manager
> iotests.py: add FilePath context manager
> qemu-iotests: use context managers for resource cleanup in 194
>
> scripts/qemu.py | 16 ++++++++-
> tests/qemu-iotests/194 | 83 +++++++++++++++++++++----------------------
> tests/qemu-iotests/iotests.py | 26 ++++++++++++++
> 3 files changed, 82 insertions(+), 43 deletions(-)
>
> --
> 2.13.5
>
Thanks, applied to my block-next tree:
https://github.com/stefanha/qemu/commits/block-next
Stefan
© 2016 - 2024 Red Hat, Inc.