Design: improvements for integration with different init systems

[jk-ozlabs]

Currently, kexec -e just does a:

		execlp("shutdown", "shutdown", "-r", "now", NULL);

This makes a couple of assumptions about userspace though:

• that shutdown is present
• that the init system will handle a fairly normal reboot request in a way that kexec expects (ie, ending with a reboot(LINUX_REBOOT_CMD_KEXEC)). This means that the init system needs to (somehow) detect, out-of-band, that we want to do a kexec reboot rather than a normal one.

Neither of these is true for busybox init, hence @mikey 's problems running upstream kexec-lite on petitboot. We have some glue in petitboot to handle this, but that's specific to busybox, and probably not something we want to integrate into kexec-lite as-is.

So, it would be great if we can design a better, lightweight way to tell init to shutdown & kexec. We'd probably need to coordinate with kexec-tools for this too.

[jk-ozlabs]

Initially, I think we could create a convention of a specific binary for kexec & kexec-lite to call when it wants to start a graceful shutdown & kexec. The system would provide that binary to do the right thing for that userspace.

And we'd fall-back to shutdown -r now, of course.

[mpe]

I just use -f?

[jk-ozlabs]

@mpe: we still want a graceful shutdown of userspace, which -f prevents. In that case, (for example) you'll still have DHCP leases pending.