Re: Let's remove some deprecated stuff

If you're cc'ed, you added a section to docs/system/deprecated.rst that is old enough to permit removal. This is *not* a demand to remove, it's a polite request to consider whether the time for removal has come. Extra points for telling us in a reply. "We should remove, but I can't do it myself right now" is a valid answer. Let's review the file:

Eric Blake: qemu-img amend to adjust backing file (since 5.1) ''''''''''''''''''''''''''''''''''''''''''''''''' The use of ``qemu-img amend`` to modify the name or format of a qcow2 backing image is deprecated; this functionality was never fully documented or tested, and interferes with other amend operations that need access to the original backing image (such as deciding whether a v3 zero cluster may be left unallocated when converting to a v2 image). Rather, any changes to the backing chain should be performed with ``qemu-img rebase -u`` either before or after the remaining changes being performed by amend, as appropriate. qemu-img backing file without format (since 5.1) '''''''''''''''''''''''''''''''''''''''''''''''' The use of ``qemu-img create``, ``qemu-img rebase``, or ``qemu-img convert`` to create or modify an image that depends on a backing file now recommends that an explicit backing format be provided. This is for safety: if QEMU probes a different format than what you thought, the data presented to the guest will be corrupt; similarly, presenting a raw image to a guest allows a potential security exploit if a future probe sees a non-raw image based on guest writes. To avoid the warning message, or even future refusal to create an unsafe image, you must pass ``-o backing_fmt=`` (or the shorthand ``-F`` during create) to specify the intended backing format. You may use ``qemu-img rebase -u`` to retroactively add a backing format to an existing image. However, be aware that there are already potential security risks to blindly using ``qemu-img info`` to probe the format of an untrusted backing image, when deciding what format to add into an existing image.

Kevin Wolf: ``nbd-server-add`` and ``nbd-server-remove`` (since 5.2) '''''''''''''''''''''''''''''''''''''''''''''''''''''''' Use the more generic commands ``block-export-add`` and ``block-export-del`` instead. As part of this deprecation, where ``nbd-server-add`` used a single ``bitmap``, the new ``block-export-add`` uses a list of ``bitmaps``.