On 10/20/20 2:15 PM, Nir Soffer wrote:
> On Tue, Oct 20, 2020 at 8:52 PM John Snow <jsnow@redhat.com> wrote:
>>
>> Nested if conditions don't change when the exception block fires; we
>> need to explicitly re-raise the error if we didn't intend to capture and
>> suppress it.
>>
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> Message-id: 20201009175123.249009-3-jsnow@redhat.com
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> ---
>> python/qemu/qmp.py | 11 ++++++-----
>> 1 file changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/python/qemu/qmp.py b/python/qemu/qmp.py
>> index d911999da1..4969e5741c 100644
>> --- a/python/qemu/qmp.py
>> +++ b/python/qemu/qmp.py
>> @@ -165,14 +165,15 @@ def __get_events(self, wait: Union[bool, float] = False) -> None:
>> """
>>
>> # Check for new events regardless and pull them into the cache:
>> - self.__sock.setblocking(False)
>> try:
>> + self.__sock.setblocking(False)
>
> This change is not required. The idiom is:
>
> do stuff
> try:
> something
> finally:
> undo stuff
>
> If do stuff failed, there is no need to undo it.
>
> socket.setblocking() should not fail with EAGAIN, so it
> does not need to be inside the try block.
>
Squashing this change in, will send a new V2 cover letter.
>> self.__json_read()
>> except OSError as err:
>> - if err.errno == errno.EAGAIN:
>> - # No data available
>> - pass
>> - self.__sock.setblocking(True)
>> + # EAGAIN: No data available; not critical
>> + if err.errno != errno.EAGAIN:
>> + raise
>
> In python 3 this can be simplified to:
>
> try:
> self.__json_read()
> except BlockingIOError:
> pass
>
> https://docs.python.org/3.6/library/exceptions.html#BlockingIOError
>
I'm a lot less clear on this. We only check for EAGAIN, but that would
check for EAGAIN, EALREADY, EWOULDBLOCK and EINPROGRESS.
That's probably fine, really, but:
There is something worse lurking in the code here too, and I really
didn't want to get into it on this series, but we are making use of
undefined behavior (sockfile.readline() on a non-blocking socket) -- It
seems to work in practice so far, but it's begging to break.
For that reason (This code should never have worked anyway), I am
extremely reluctant to change the exception classes we catch here until
we fix the problem.
--js
>> + finally:
>> + self.__sock.setblocking(True)
>>
>> # Wait for new events, if needed.
>> # if wait is 0.0, this means "no wait" and is also implicitly false.
>> --
>> 2.26.2
>
> Nir
>