If you have the chain 'base.qcow2 <- top.qcow2' and want to merge a
bitmap from top into base, qemu-img was failing with:
qemu-img: Could not open 'top.qcow2': Could not open backing file: Failed to get shared
"write" lock
Is another process using the image [base.qcow2]?
The easiest fix is to not open the entire backing chain of either
image (source or destination); after all, the point of 'qemu-img
bitmap' is solely to manipulate bitmaps directly within a single qcow2
image, and this is made more precise if we don't pay attention to
other images in the chain that may happen to have a bitmap by the same
name.
However, note that during normal usage, it is a feature that qemu will
allow a bitmap from a backing image to be exposed by an overlay BDS;
doing so makes it easier to perform incremental backup, where we have:
Base <- Active <- temporrary
\--block job ->/
with temporary being fed by a block-copy 'sync' job; when exposing
temporary over NBD, referring to a bitmap that lives only in Active is
less effort than having to copy a bitmap into temporary [1]. So the
testsuite additions in this patch check both where bitmaps get
allocated (the qemu-img info output), and, when NOT using 'qemu-img
bitmap', that bitmaps are indeed visible through a backing chain.
[1] Full disclosure: prior to the recent commit 374eedd1c4 and
friends, we were NOT able to see bitmaps through filters, which meant
that we actually did not have nice clean semantics for uniformly being
able to pick up bitmaps from anywhere in the backing chain (seen as a
change in behavior between qemu 4.1 and 4.2 at commit 00e30f05de, when
block-copy swapped from a one-off to a filter). Which means libvirt
was already coded to copy bitmaps around for the sake of older qemu,
even though modern qemu no longer needs it. Oh well.
Fixes: http://bugzilla.redhat.com/1877209
Reported-by: Eyal Shenitzky <eshenitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
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