Re: [PATCH v2 26/39] tests/qtest: migration-test: Make sure QEMU process "to" exited after migration is canceled

[Daniel P . Berrangé]

On Wed, Sep 21, 2022 at 05:29:55PM +0100, Dr. David Alan Gilbert wrote:
> * Bin Meng (bmeng.cn@gmail.com) wrote:
> > From: Xuzhou Cheng <xuzhou.cheng@windriver.com>
> > 
> > Make sure QEMU process "to" exited before launching another target
> > for migration in the test_multifd_tcp_cancel case.
> > 
> > Signed-off-by: Xuzhou Cheng <xuzhou.cheng@windriver.com>
> > Signed-off-by: Bin Meng <bin.meng@windriver.com>
> > Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> 
> Hmm you might want to put a small usleep in that loop; otherwise
> it'll burn CPU.
> 
> There is a slim risk with this that another, entirely unrelated, process 
> will start up with the same PID between the end of migrate_cancel
> and then you'll be spinning on it rather than the 'to' qemu.
> 
> I wonder if there's a better way to check for it dieing; e.g. an error
> on it's qmp interface or something?

Both the qtest and qmp sockets should give EOF. So if there's an API
that can call g_poll() on the FD with POLL_HUP event, it would be the
reliable way to detect it, without busy-looping.

With regards,
Daniel
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