qcow2: Expose bitmaps' size during measure

It's useful to know how much space can be occupied by qcow2 persistent
bitmaps, even though such metadata is unrelated to the guest-visible
data.  Report this value as an additional QMP field, present when
measuring an existing image and output format that both support
bitmaps.  Update iotest 178 and 190 to updated output, as well as new
coverage in 190 demonstrating non-zero values made possible with the
recently-added qemu-img bitmap command (see 3b51ab4b).

The new 'bitmaps size:' field is displayed automatically as part of
'qemu-img measure' any time it is present in QMP (that is, any time
both the source image being measured and destination format support
bitmaps, even if the measurement is 0 because there are no bitmaps
present).  If the field is absent, it means that no bitmaps can be
copied (source, destination, or both lack bitmaps, including when
measuring based on size rather than on a source image).  This behavior
is compatible with an upcoming patch adding 'qemu-img convert
--bitmaps': that command will fail in the same situations where this
patch omits the field.

The addition of a new field demonstrates why we should always
zero-initialize qapi C structs; while the qcow2 driver still fully
populates all fields, the raw and crypto drivers had to be tweaked to
avoid uninitialized data.

Consideration was also given towards having a 'qemu-img measure
--bitmaps' which errors out when bitmaps are not possible, and
otherwise sums the bitmaps into the existing allocation totals rather
than displaying as a separate field, as a potential convenience
factor.  But this was ultimately decided to be more complexity than
necessary when the QMP interface was sufficient enough with bitmaps
remaining a separate field.

See also: https://bugzilla.redhat.com/1779904

Reported-by: Nir Soffer <nsoffer@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200521192137.1120211-3-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
index 38d464e..320cb52 100644
--- a/docs/tools/qemu-img.rst
+++ b/docs/tools/qemu-img.rst
@@ -616,6 +616,7 @@ Command description:
 
     required size: 524288
     fully allocated size: 1074069504
+    bitmaps size: 0
 
   The ``required size`` is the file size of the new image.  It may be smaller
   than the virtual disk size if the image format supports compact representation.
@@ -625,6 +626,12 @@ Command description:
   occupy with the exception of internal snapshots, dirty bitmaps, vmstate data,
   and other advanced image format features.
 
+  The ``bitmaps size`` is the additional size required in order to
+  copy bitmaps from a source image in addition to the guest-visible
+  data; the line is omitted if either source or destination lacks
+  bitmap support, or 0 if bitmaps are supported but there is nothing
+  to copy.
+
 .. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
 
   List, apply, create or delete snapshots in image *FILENAME*.