On 07/18/2017 08:41 AM, Markus Armbruster wrote: > BlockdevRef is an alternate of BlockdevOptions (inline definition) and > str (reference to an existing block device by name). BlockdevRef > value "" is special: "no block device should be referenced." It's > actually interpreted that way in just one place: optional member > @backing of COW formats. Semantics: > > * Present means "use this block device" as backing storage > > * Absent means "default to the one stored in the image" > > * Except "" means "don't use backing storage at all" > > The first two are perfectly normal: when the parameter is absent, it > defaults to an implied value, but the value's meaning is the same. > > The third one overloads the parameter with a second meaning. The > overloading is *implicit*, i.e. it's not visible in the types. Works > here, because "" is not a value block device ID. > > Pressing argument values the schema accepts, but are semantically > invalid, into service to mean "do something else entirely" is not > general, as suitable invalid values need not exist. I also find it > ugly. > > To clean this up, we could add a separate flag argument to suppress > @backing, or add a distinct value to @backing. This commit implements > the latter: add JSON null to the values of @backing, deprecate "". > > Because we're so close to the 2.10 freeze, implement it in the > stupidest way possible: have qmp_blockdev_add() rewrite null to "" > before anything else can see the null. Works, because BlockdevRef > occurs only within arguments of blockdev-add. The proper way to do it > would be rewriting "" to null, preferably in a cleaner way, but that > requires fixing up code to work with null. Add a TODO comment for > that. > > Signed-off-by: Markus Armbruster <arm...@redhat.com> > ---
Reviewed-by: Eric Blake <ebl...@redhat.com> -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
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