This is a follow-up to 5d2036b5f3, and also makes
the "machinectl clean" verb asynchronous, after all it's little more than a
series of image removals.
The changes required to make this happen are a bit more comprehensive as we
need to pass information about deleted images back to the client, as well as
information about the image we failed on if we failed on one. Hence, create a
temporary file in /tmp, serialize that data into, and read it from the parent
after the operation is complete.
Let's make sigkill_wait() take a normal pid_t, and add sigkill_waitp() that
takes a pointer (which is useful for usage in _cleanup_), following the usual
logic we have for this.
Cloning an image can be slow, if the image is not on a btrfs subvolume, hence
let's make sure we do this asynchronously in a child process, so that machined
isn't blocked as long as we process the client request.
This adds a new, generic "Operation" object to machined, that is used to track
these kind of background processes.
This is inspired by the MachineOperation object that already exists to make
copy operations asynchronous. A later patch will rework the MachineOperation
logic to use the generic Operation instead.
Torstein Husebø