The block-device hotplug work (#NNNNN) needs a number of cross-cutting
changes to the QMP plumbing that aren't hotplug-specific in
themselves: a refcounted DriveInfo so async stage callbacks can keep
slot refs, a counter-based naming scheme so multiple drives can share
the same backing path, pipelined remove-fd so QEMU's fdsets get
released at blockdev-del time, and a generic-completion callback that
doesn't tear the VM down on a runtime QMP error. None of these change
behaviour from a user's point of view, none of them depend on the
varlink hotplug methods landing, and several are wins on their own
(the fdset leak in particular is observable today with --extra-drive
under long-running VMs). Pulling them out of the hotplug PR keeps
that PR focused on the IDL + server-side method handlers, and lets
this preparatory work land on its own merits without waiting for the
larger feature review.
Cleanup pieces that fall out for free:
qmp-client: widen next_fdset_id to uint64_t
vmspawn: move VMSPAWN_PCIE_HOTPLUG_SPARES to vmspawn-qmp.h
vmspawn-varlink: use error < 0 in async QMP completion callbacks
vmspawn-varlink: simplify on_qmp_describe_complete result extraction
vmspawn-varlink: extract notify_event_subscribers from on_qmp_event
vmspawn-varlink: treat empty event subscription filter as catch-all
vmspawn-qmp: pass bridge to on_cont_complete via invoke userdata
Infrastructure the hotplug add path will sit on top of:
vmspawn-qmp: convert DriveInfo to a refcounted object
vmspawn-qmp: derive QMP node and device ids from a bridge counter
vmspawn-qmp: pipeline remove-fd after each blockdev-add
vmspawn-qmp: keep the event loop running on post-setup QMP failures
vmspawn-qmp: add the hotplug-capable block-device add machinery
vmspawn-qmp: add vmspawn_qmp_remove_block_device
The two final commits introduce vmspawn_qmp_add_block_device() and
vmspawn_qmp_remove_block_device() but leave them without varlink
callers — the io.systemd.VirtualMachineInstance method handlers that
forward into them land with the rest of the hotplug PR. Boot-time
drive setup is rewritten on top of vmspawn_qmp_add_block_device() so
the hotplug and boot paths share a single staged-add pipeline from
day one.
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