Re: [PATCH v2 1/1] python/machine.py: refactor shutdown

[Kevin Wolf <kwolf@redhat.com>]

Am 02.06.2020 um 21:48 hat John Snow geschrieben:
> This is done primarily to avoid the 'bare except' pattern, which
> suppresses ALL exceptions and not just ones that we are anticipating to
> see.
> 
> Replace this with a pattern that isolates the different kind of shutdown
> paradigms and a new fallback shutdown handler that gracefully attempts
> one before the other.
> 
> Move all of the post-shutdown code into _post_shutdown, and adjust the
> main shutdown() function to ALWAYS call the post_shutdown logic no
> matter what kind of error we encountered: An expected one (subprocess
> wait timeout) or an unexpected one (Everything else).
> 
> In cases where we encounter an expected error in the graceful shutdown
> timeout, we will not re-raise an exception above shutdown(). Otherwise,
> after post_shutdown cleanup, we will.
> 
> I anticipate that this WILL lead to additional bug reports filed against
> this module, but that is unfortunately somewhat the point: This code
> shouldn't be hiding failures that exist elsewhere within the python
> code.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  python/qemu/machine.py | 93 +++++++++++++++++++++++++++++-------------
>  1 file changed, 65 insertions(+), 28 deletions(-)
> 
> diff --git a/python/qemu/machine.py b/python/qemu/machine.py
> index 041c615052e..a2abd2c35e3 100644
> --- a/python/qemu/machine.py
> +++ b/python/qemu/machine.py
> @@ -283,6 +283,8 @@ def _post_launch(self):
>              self._qmp.accept()
>  
>      def _post_shutdown(self):
> +        self._load_io_log()
> +
>          if self._qemu_log_file is not None:
>              self._qemu_log_file.close()
>              self._qemu_log_file = None
> @@ -296,6 +298,17 @@ def _post_shutdown(self):
>          while len(self._remove_files) > 0:
>              self._remove_if_exists(self._remove_files.pop())
>  
> +        exitcode = self.exitcode()
> +        if exitcode is not None and exitcode < 0:
> +            msg = 'qemu received signal %i; command: "%s"'
> +            if self._qemu_full_args:
> +                command = ' '.join(self._qemu_full_args)
> +            else:
> +                command = ''
> +            LOG.warning(msg, -int(exitcode), command)
> +
> +        self._launched = False

It would have been a little easier to review if this code motion to
_post_shutdown() were a separate patch. It looks mostly independent of
the other changes.

The big change here is that we now log warnings for exit due to a signal
for QEMUMachine.wait(), too. I assume this is okay.

>      def launch(self):
>          """
>          Launch the VM and make sure we cleanup and expose the
> @@ -344,12 +357,53 @@ def wait(self):
>          self._popen.wait()
>          if self._qmp:
>              self._qmp.close()
> -        self._load_io_log()
>          self._post_shutdown()
>  
> -    def shutdown(self, has_quit=False, hard=False):
> +    def _hard_shutdown(self) -> None:
>          """
> -        Terminate the VM and clean up
> +        Kill the VM if it is running.
> +        """
> +        if not self.is_running():
> +            return
> +
> +        self._popen.kill()
> +        self._popen.wait(timeout=60)
> +
> +    def _soft_shutdown(self, has_quit: bool = False, timeout: int = 3) -> None:
> +        """
> +        Attempt to shutdown the VM gracefully if it is running.
> +
> +        :param has_quit: When True, don't attempt to issue 'quit' QMP command
> +        :param timeout: Timeout for graceful shutdown. Default 3 seconds.
> +        """
> +        if not self.is_running():
> +            return
> +
> +        if self._qmp is not None:
> +            if not has_quit:
> +                self._qmp.cmd('quit')
> +            self._qmp.close()
> +
> +        self._popen.wait(timeout=timeout)
> +
> +    def _do_shutdown(self, has_quit: bool = False, timeout: int = 3) -> None:
> +        """
> +        Attempt to shutdown the VM gracefully; fallback to a hard shutdown.
> +
> +        :param has_quit: When True, don't attempt to issue 'quit' QMP command
> +        :param timeout: Timeout for graceful shutdown. Default 3 seconds.
> +        """
> +        try:
> +            self._soft_shutdown(has_quit, timeout)
> +        except subprocess.TimeoutExpired:
> +            self._hard_shutdown()
> +        except:
> +            self._hard_shutdown()
> +            raise
> +
> +    def shutdown(self, has_quit: bool = False, hard: bool = False) -> None:
> +        """
> +        Terminate the VM (gracefully if possible) and perform cleanup.
>          """
>          # If we keep the console socket open, we may deadlock waiting
>          # for QEMU to exit, while QEMU is waiting for the socket to
> @@ -358,35 +412,18 @@ def shutdown(self, has_quit=False, hard=False):
>              self._console_socket.close()
>              self._console_socket = None
>  
> -        if self.is_running():
> +        try:
>              if hard:
> -                self._popen.kill()
> -            elif self._qmp:
> -                try:
> -                    if not has_quit:
> -                        self._qmp.cmd('quit')
> -                    self._qmp.close()
> -                    self._popen.wait(timeout=3)
> -                except:
> -                    self._popen.kill()
> -            self._popen.wait()
> -
> -        self._load_io_log()
> -        self._post_shutdown()
> -
> -        exitcode = self.exitcode()
> -        if exitcode is not None and exitcode < 0 and \
> -                not (exitcode == -9 and hard):

The condition in the second line went away without a replacement. This
means that previously, intentionally killing a VM with hard=True would
not log a warning (which makes sense to me), but now it does.

On the other hand, I couldn't find a single user of hard=True or the
kill() method in the whole tree. So I guess it doesn't matter anyway?

> -            msg = 'qemu received signal %i: %s'
> -            if self._qemu_full_args:
> -                command = ' '.join(self._qemu_full_args)
> +                self._hard_shutdown()
>              else:
> -                command = ''
> -            LOG.warning(msg, -int(exitcode), command)
> -
> -        self._launched = False
> +                self._do_shutdown(has_quit)
> +        finally:
> +            self._post_shutdown()
>  
>      def kill(self):
> +        """
> +        Terminate the VM forcefully and perform cleanup.
> +        """
>          self.shutdown(hard=True)
>  
>      def set_qmp_monitor(self, enabled=True):

In the end, while the new behaviour is a bit different in details, it's
not unreasonable, and if it does cause any problems, we can still change
it back.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>