Re: [PATCH v2 1/2] Implement -run-with exit-with-parent=on

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Oct 09, 2025 at 05:12:46PM +0100, Richard W.M. Jones wrote:
> Libguestfs wants to use qemu to run a captive appliance.  When the
> program linked to libguestfs exits, we want qemu to be cleaned up.
> Libguestfs goes to great lengths to do this at the moment: it either
> forks a separate process to ensure clean-up is done, or it asks
> libvirt to clean up the qemu process.  However this is complicated and
> not totally reliable.
> 
> On Linux, FreeBSD and macOS, there are mechanisms to ensure a signal
> or message is delivered to a process when its parent process goes
> away.  The qemu test suite even uses this mechanism on Linux (see
> PR_SET_PDEATHSIG in tests/qtest/libqtest.c).
> 
> In nbdkit we have long had the concept of running nbdkit captively,
> and we have the nbdkit --exit-with-parent flag to help
> (https://libguestfs.org/nbdkit-captive.1.html#EXIT-WITH-PARENT)
> 
> This commit adds the same mechanism.  The syntax is:
> 
>   qemu -run-with exit-with-parent=on [...]
> 
> This is not a feature that most typical users of qemu (for running
> general purpose, long-lived VMs) should use, so it defaults to off.
> 
> The exit-with-parent.[ch] files are copied from nbdkit, where they
> have a 3-clause BSD license which is compatible with qemu:
> 
> https://gitlab.com/nbdkit/nbdkit/-/tree/master/common/utils?ref_type=heads
> 
> Thanks: Daniel P. Berrangé <berrange@redhat.com>
> Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
> ---
>  include/qemu/exit-with-parent.h |  57 +++++++++++++
>  qemu-options.hx                 |  13 ++-
>  system/exit-with-parent.c       | 140 ++++++++++++++++++++++++++++++++
>  system/meson.build              |   1 +
>  system/vl.c                     |  13 +++
>  5 files changed, 222 insertions(+), 2 deletions(-)
> 
> diff --git a/include/qemu/exit-with-parent.h b/include/qemu/exit-with-parent.h
> new file mode 100644
> index 0000000000..c00b863fe9
> --- /dev/null
> +++ b/include/qemu/exit-with-parent.h
> @@ -0,0 +1,57 @@

snip

> +
> +#ifndef NBDKIT_EXIT_WITH_PARENT_H
> +#define NBDKIT_EXIT_WITH_PARENT_H

I'd suggest s/NBDKIT/QEMU/ here & at the end

> +
> +/* Test if the feature is available on the platform. */
> +static inline bool can_exit_with_parent(void)
> +{
> +#if defined(__linux__) || defined(__FreeBSD__) || defined(__APPLE__)
> +    return true;
> +#else
> +    return false;
> +#endif
> +}
> +
> +/*
> + * --exit-with-parent: kill the current process if the parent exits.
> + * This may return -1 on error.
> + *
> + * Note this will abort on platforms where can_exit_with_parent()
> + * returned false.
> + */
> +extern int set_exit_with_parent(void);
> +
> +#endif /* NBDKIT_EXIT_WITH_PARENT_H */



With that minor change

 Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
 

With regards,
Daniel
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