[Bug 1847924] [NEW] Introduce broken state parsing to mdadm

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[Impact]

* Currently, mounted raid0/md-linear arrays have no indication/warning
when one or more members are removed or suffer from some non-recoverable
error condition. The mdadm tool shows "clean" state regardless if a
member was removed.

* The patch proposed in this SRU addresses this issue by introducing a
new state "broken", which is analog to "clean" but indicates that array
is not in a good/correct state. The commit, available upstream as
43ebc910 ("mdadm: Introduce new array state 'broken' for raid0/linear")
[0], was extensively discussed and received a good amount of
reviews/analysis by both the current mdadm maintainer as well as an old
maintainer.

* One important note here is that this patch requires a counter-part in the kernel to be fully functional, which was SRUed in LP: #1847773.
It works fine/transparently without this kernel counter-part though.

[Test case]

* To test this patch, create a raid0 or linear md array on Linux using
mdadm, like: "mdadm --create md0 --level=0 --raid-devices=2 /dev/nvme0n1
/dev/nvme1n1";

* Format the array using a FS of your choice (for example ext4) and
mount the array;

* Remove one member of the array, for example using sysfs interface (for
nvme: echo 1 > /sys/block/nvme0n1/device/device/remove, for scsi: echo 1
> /sys/block/sdX/device/delete);

* Without this patch, the array state shown by "mdadm --detail" is
"clean", regardless a member is missing/failed.

[Regression potential]

* There's not much potential regression here; we just exhibit arrays'
state as "broken" if they have one or more missing/failed members; we
believe the most common "issue" that could be reported from this patch
is if an userspace tool rely on the array status as being always "clean"
even for broken devices, then such tool may behave differently with this
patch.

* Note that we *proactively* skipped Xenial SRU here, in order to
prevent potential regressions - Xenial mdadm tool lacks code
infrastructure used by this patch, so the decision was for
safety/stability, by only SRUing Bionic / Disco / Eoan mdadm versions.

[0]
https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=43ebc910

** Affects: mdadm (Ubuntu)
     Importance: Medium
     Assignee: Guilherme G. Piccoli (gpiccoli)
         Status: Confirmed

** Affects: mdadm (Ubuntu Bionic)
     Importance: Medium
     Assignee: Guilherme G. Piccoli (gpiccoli)
         Status: Confirmed

** Affects: mdadm (Ubuntu Disco)
     Importance: Medium
     Assignee: Guilherme G. Piccoli (gpiccoli)
         Status: Confirmed

** Affects: mdadm (Ubuntu Eoan)
     Importance: Medium
     Assignee: Guilherme G. Piccoli (gpiccoli)
         Status: Confirmed


** Tags: patch sts
-- 
Introduce broken state parsing to mdadm
https://bugs.launchpad.net/bugs/1847924
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