[Borgbackup] borg check errors after upgrade to 1.2.0

[Dan Christensen]

I upgraded from borg 1.1.7 to 1.2.0 on several different systems about a
month ago.  I use the fat binaries.  This morning, my monthly "borg
check" runs gave errors on three repositories on two systems.  Note that
I used the setup where several clients do their backups into the same
repositories.

System 1 runs Ubuntu 20.04.  Two of the three repos on this machine now
have errors:

# borg check /Backups/borg/home.borg
Index object count mismatch.
committed index: 1166413 objects
rebuilt index:   1166414 objects
ID: 8a158ba7fdfae9b1373063a5bb5ea8ea6698c93ed7feff89ca6ff0a3c8842ebd rebuilt index: (18596, 199132336) committed index: <not found>     
Finished full repository check, errors found.

# borg check /Backups/borg/system.borg
Index object count mismatch.
committed index: 2324200 objects
rebuilt index:   2324202 objects
ID: 1e20354918f4fdeb9cc0d677c28dffe1a383dd1b0db11ebcbc5ffb809d3c2b8a rebuilt index: (24666, 60168)   committed index: <not found>     
ID: d9c516b5bf53f661a1a9d2ada08c8db7c33a331713f23e058cd6969982728157 rebuilt index: (3516, 138963001) committed index: <not found>     
Finished full repository check, errors found.

The last monthly run was on April 10, just after I upgraded to 1.2.0,
and there were no errors then.

System 2 runs Debian buster.  One of the three repos on this machine now
has errors:

# borg check /Backups/borg/system.borg
Index object count mismatch.
committed index: 2052187 objects
rebuilt index:   2052188 objects
ID: 6b734ed388e7e086af7107847c6b6d3d34a29c20e7e539ded71b32606cb857bd rebuilt index: (946, 15871355)  committed index: <not found>     
Finished full repository check, errors found.

On this system, I also did a clean borg check run on around April 10,
using borg 1.2.0.

I have used borg on these systems for years, and no hardware has changed
recently.  System 1 has the repos on a RAID 1 mdadm device with two SATA
spinning disks.  System 2 also has the repos on RAID 1 mdadm devices
with two SATA disks, with lvm as a middle layer.  In both cases,
smartctl shows no issues for any of the drives, and memtester also shows
no errors.

Since the errors have happened on different machines within a month of
upgrading to 1.2.0, I am concerned that this is a borg issue rather than
a hardware issue.  It is also suspicious to me that the error is the
same in all cases, with a committed index not found.  Hardware errors
tend to produce garbage.

I have not run repair yet.  Is there anything I should do before running
repair to try to figure out the issue?

Each of the respositories is around 100GB, so I am reluctant to make
copies.

Thanks for any tips.  I'm happy to file an issue if that would help.

Dan