Re: [PULL 20/21] python/qemu/qmp.py: re-raise OSError when encountered

[Nir Soffer]

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.

>              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

> +        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