rbd: show the entire chain of parent images

Make /sys/bus/rbd/devices/<id>/parent show the entire chain of parent
images.  While at it, kernel sprintf() doesn't return negative values,
casting to unsigned long long is no longer necessary and there is no
good reason to split into multiple sprintf() calls.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
rbd: introduce rbd_dev_header_info()

A wrapper around rbd_dev_v{1,2}_header_info() to reduce duplication.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
rbd: remove unnecessary asserts in rbd_dev_image_probe()

spec->image_id assert doesn't buy us much and image_format is asserted
in rbd_dev_header_name() and rbd_dev_header_info() anyway.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
rbd: split rbd_dev_spec_update() into two functions

rbd_dev_spec_update() has two modes of operation, with nothing in
common between them.  Split it into two functions, one for each mode
and make our expectations more clear.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
rbd: harden rbd_dev_refresh() caller

Recently discovered watch/notify problems showed that we really can't
ignore errors in anything refresh related.  Alas, currently there is
not much we can do in response to those errors, except print warnings.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
rbd: update mapping size only on refresh

There is no sense in trying to update the mapping size before it's even
been set.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
rbd: do not read in parent info before snap context

Currently rbd_dev_v2_header_info() reads in parent info before the snap
context is read in.  This is wrong, because we may need to look at the
the parent_overlap value of the snapshot instead of that of the base
image, for example when mapping a snapshot - see next commit.  (When
mapping a snapshot, all we got is its name and we need the snap context
to translate that name into an id to know which parent info to look
for.)

The approach taken here is to make sure rbd_dev_v2_parent_info() is
called after the snap context has been read in.  The other approach
would be to add a parent_overlap field to struct rbd_mapping and
maintain it the same way rbd_mapping::size is maintained.  The reason
I chose the first approach is that the value of keeping around both
base image values and the actual mapping values is unclear to me.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
rbd: take snap_id into account when reading in parent info

If we are mapping a snapshot, we must read in the parent_overlap value
of that snapshot instead of that of the base image.  Not doing so may
in particular result in us returning zeros instead of user data:

    # cat overlap-snap.sh
    #!/bin/bash
    rbd create --size 10 --image-format 2 foo
    FOO_DEV=$(rbd map foo)
    dd if=/dev/urandom of=$FOO_DEV bs=1M &>/dev/null
    echo "Base image"
    dd if=$FOO_DEV bs=1 count=16 skip=$(((4 << 20) - 8)) 2>/dev/null | xxd
    rbd snap create foo@snap
    rbd snap protect foo@snap
    rbd clone foo@snap bar
    rbd snap create bar@snap
    BAR_DEV=$(rbd map bar@snap)
    echo "Snapshot"
    dd if=$BAR_DEV bs=1 count=16 skip=$(((4 << 20) - 8)) 2>/dev/null | xxd
    rbd resize --allow-shrink --size 4 bar
    echo "Snapshot after base image resize"
    dd if=$BAR_DEV bs=1 count=16 skip=$(((4 << 20) - 8)) 2>/dev/null | xxd

    # ./overlap-snap.sh
    Base image
    0000000: e781 e33b d34b 2225 6034 2845 a2e3 36ed  ...;.K"%`4(E..6.
    Snapshot
    0000000: e781 e33b d34b 2225 6034 2845 a2e3 36ed  ...;.K"%`4(E..6.
    Resizing image: 100% complete...done.
    Snapshot after base image resize
    0000000: e781 e33b d34b 2225 0000 0000 0000 0000  ...;.K"%........

Even though bar@snap was taken with the old bar parent_overlap (8M),
reads from bar@snap beyond the new bar parent_overlap (4M) return
zeroes.  Fix it.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>