Re: [PATCH v5 10/12] python/machine.py: split shutdown into hard and soft flavors

[Cleber Rosa <crosa@redhat.com>]

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On Fri, Jul 10, 2020 at 01:06:47AM -0400, John Snow wrote:
> This is done primarily to avoid the 'bare except' pattern, which
> suppresses all exceptions during shutdown and can obscure errors.
> 
> Replace this with a pattern that isolates the different kind of shutdown
> paradigms (_hard_shutdown and _soft_shutdown), and a new fallback shutdown
> handler (_do_shutdown) that gracefully attempts one before the other.
> 
> This split now also ensures that no matter what happens,
> _post_shutdown() is always invoked.
> 
> shutdown() changes in behavior such that if it attempts to do a graceful
> shutdown and is unable to, it will now always raise an exception to
> indicate this. This can be avoided by the test writer in three ways:
> 
> 1. If the VM is expected to have already exited or is in the process of
> exiting, wait() can be used instead of shutdown() to clean up resources
> instead. This helps avoid race conditions in shutdown.
> 
> 2. If a test writer is expecting graceful shutdown to fail, shutdown
> should be called in a try...except block.
> 
> 3. If the test writer has no interest in performing a graceful shutdown
> at all, kill() can be used instead.
> 
> 
> Handling shutdown in this way makes it much more explicit which type of
> shutdown we want and allows the library to report problems with this
> process.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  python/qemu/machine.py | 95 +++++++++++++++++++++++++++++++++++-------
>  1 file changed, 80 insertions(+), 15 deletions(-)
> 
> diff --git a/python/qemu/machine.py b/python/qemu/machine.py
> index aaa173f046..b24ce8a268 100644
> --- a/python/qemu/machine.py
> +++ b/python/qemu/machine.py
> @@ -48,6 +48,12 @@ class QEMUMachineAddDeviceError(QEMUMachineError):
>      """
>  
>  
> +class AbnormalShutdown(QEMUMachineError):
> +    """
> +    Exception raised when a graceful shutdown was requested, but not performed.
> +    """
> +
> +
>  class MonitorResponseError(qmp.QMPError):
>      """
>      Represents erroneous QMP monitor reply
> @@ -365,6 +371,7 @@ def _early_cleanup(self) -> None:
>          """
>          Perform any cleanup that needs to happen before the VM exits.
>  
> +        May be invoked by both soft and hard shutdown in failover scenarios.
>          Called additionally by _post_shutdown for comprehensive cleanup.
>          """
>          # If we keep the console socket open, we may deadlock waiting
> @@ -374,32 +381,90 @@ def _early_cleanup(self) -> None:
>              self._console_socket.close()
>              self._console_socket = None
>  
> +    def _hard_shutdown(self) -> None:
> +        """
> +        Perform early cleanup, kill the VM, and wait for it to terminate.
> +
> +        :raise subprocess.Timeout: When timeout is exceeds 60 seconds
> +            waiting for the QEMU process to terminate.
> +        """
> +        self._early_cleanup()

Like I commented on patch 5, I don't think the *current* type of
cleanup done is needed on a scenario like this...

> +        self._popen.kill()

... as I don't remember QEMU's SIGKILL handler to be susceptible to
the race condition that motivated the closing of the console file in
the first place.  But, I also can not prove it's not susceptible at
this time.

Note: I have some old patches that added tests for QEMUMachine itself.
I intend to respin them on top of your work, so we may have a clearer
understanding of the QEMU behaviors we need to handle.  So, feel free
to take the prudent route here, and keep the early cleanup.

Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>

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