qemu-img: Make unallocated part of backing chain obvious in map

The recently-added NBD context qemu:allocation-depth is able to
distinguish between locally-present data (even when that data is
sparse) [shown as depth 1 over NBD], and data that could not be found
anywhere in the backing chain [shown as depth 0]; and the libnbd
project was recently patched to give the human-readable name "absent"
to an allocation-depth of 0.  But qemu-img map --output=json predates
that addition, and has the unfortunate behavior that all portions of
the backing chain that resolve without finding a hit in any backing
layer report the same depth as the final backing layer.  This makes it
harder to reconstruct a qcow2 backing chain using just 'qemu-img map'
output, especially when using "backing":null to artificially limit a
backing chain, because it is impossible to distinguish between a
QCOW2_CLUSTER_UNALLOCATED (which defers to a [missing] backing file)
and a QCOW2_CLUSTER_ZERO_PLAIN cluster (which would override any
backing file), since both types of clusters otherwise show as
"data":false,"zero":true" (but note that we can distinguish a
QCOW2_CLUSTER_ZERO_ALLOCATED, which would also have an "offset":
listing).

The task of reconstructing a qcow2 chain was made harder in commit
0da9856851 (nbd: server: Report holes for raw images), because prior
to that point, it was possible to abuse NBD's block status command to
see which portions of a qcow2 file resulted in BDRV_BLOCK_ALLOCATED
(showing up as NBD_STATE_ZERO in isolation) vs. missing from the chain
(showing up as NBD_STATE_ZERO|NBD_STATE_HOLE); but now qemu reports
more accurate sparseness information over NBD.

An obvious solution is to make 'qemu-img map --output=json' add an
additional "present":false designation to any cluster lacking an
allocation anywhere in the chain, without any change to the "depth"
parameter to avoid breaking existing clients.  The iotests have
several examples where this distinction demonstrates the additional
accuracy.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210701190655.2131223-3-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
[eblake: fix more iotest fallout]
Signed-off-by: Eric Blake <eblake@redhat.com>

1 files changed, 12 insertions, 11 deletions

diff --git a/tests/qemu-iotests/244.out b/tests/qemu-iotests/244.out
index 99f56ac18c..5e03add054 100644
--- a/tests/qemu-iotests/244.out
+++ b/tests/qemu-iotests/244.out
@@ -57,11 +57,12 @@ wrote 3145728/3145728 bytes at offset 3145728
 3 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 No errors were found on the image.
 
-[{ "start": 0, "length": 1048576, "depth": 0, "zero": true, "data": false},
-{ "start": 1048576, "length": 1048576, "depth": 0, "zero": false, "data": true, "offset": 1048576},
-{ "start": 2097152, "length": 2097152, "depth": 0, "zero": true, "data": false},
-{ "start": 4194304, "length": 1048576, "depth": 0, "zero": true, "data": false, "offset": 4194304},
-{ "start": 5242880, "length": 61865984, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 1048576, "depth": 0, "present": false, "zero": true, "data": false},
+{ "start": 1048576, "length": 1048576, "depth": 0, "present": true, "zero": false, "data": true, "offset": 1048576},
+{ "start": 2097152, "length": 2097152, "depth": 0, "present": true, "zero": true, "data": false},
+{ "start": 4194304, "length": 1048576, "depth": 0, "present": true, "zero": true, "data": false, "offset": 4194304},
+{ "start": 5242880, "length": 1048576, "depth": 0, "present": true, "zero": true, "data": false},
+{ "start": 6291456, "length": 60817408, "depth": 0, "present": false, "zero": true, "data": false}]
 
 read 1048576/1048576 bytes at offset 0
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
@@ -93,10 +94,10 @@ wrote 3145728/3145728 bytes at offset 3145728
 3 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 No errors were found on the image.
 
-[{ "start": 0, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": 0},
-{ "start": 2097152, "length": 2097152, "depth": 0, "zero": true, "data": false},
-{ "start": 4194304, "length": 2097152, "depth": 0, "zero": true, "data": false, "offset": 4194304},
-{ "start": 6291456, "length": 60817408, "depth": 0, "zero": false, "data": true, "offset": 6291456}]
+[{ "start": 0, "length": 2097152, "depth": 0, "present": true, "zero": false, "data": true, "offset": 0},
+{ "start": 2097152, "length": 2097152, "depth": 0, "present": true, "zero": true, "data": false},
+{ "start": 4194304, "length": 2097152, "depth": 0, "present": true, "zero": true, "data": false, "offset": 4194304},
+{ "start": 6291456, "length": 60817408, "depth": 0, "present": true, "zero": false, "data": true, "offset": 6291456}]
 
 read 1048576/1048576 bytes at offset 0
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
@@ -122,8 +123,8 @@ read 1048576/1048576 bytes at offset 0
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 Offset          Length          Mapped to       File
 0               0x100000        0               TEST_DIR/t.qcow2.data
-[{ "start": 0, "length": 1048576, "depth": 0, "zero": false, "data": true, "offset": 0},
-{ "start": 1048576, "length": 66060288, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 1048576, "depth": 0, "present": true, "zero": false, "data": true, "offset": 0},
+{ "start": 1048576, "length": 66060288, "depth": 0, "present": false, "zero": true, "data": false}]
 
 === Copy offloading ===