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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> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Signed-off-by: Richard W.M. Jones <rjones@redhat.com> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>