Parallels image file can be corrupted this way: two guest memory areas
refer to the same host memory area (duplicated offsets in BAT). 
qemu-img check copies data from duplicated cluster to the new cluster and
writes new corresponding offset to BAT instead of duplicated one.

Test 314 uses sample corrupted image parallels-2-duplicated-cluster.bz2. 
Reading from duplicated offset and from original offset returns the same
data. After repairing changing either of these blocks of data
does not affect another one.

Natalia Kuzmina (3):
  qemu-img check: fixing duplicated clusters for parallels format
  iotests: 314 test on duplicated clusters (parallels format)
  docs: parallels image format supports consistency checks

 block/parallels.c                             |  66 ++++++++++++-
 docs/tools/qemu-img.rst                       |   2 +-
 tests/qemu-iotests/314                        |  88 ++++++++++++++++++
 tests/qemu-iotests/314.out                    |  36 +++++++
 .../parallels-2-duplicated-cluster.bz2        | Bin 0 -> 148 bytes
 5 files changed, 189 insertions(+), 3 deletions(-)
 create mode 100755 tests/qemu-iotests/314
 create mode 100644 tests/qemu-iotests/314.out
 create mode 100644 tests/qemu-iotests/sample_images/parallels-2-duplicated-cluster.bz2

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2.25.1