[backup] Restore lost some amount of values

[pilshchikov]

Jira Link: DB-3065

Description

Case:

1. Load data - sample-apps workload SqlDataLoad 8 min load, 10 tables. All varchar
2. At the last minute of load restart the random VM.
3. Create a backup
4. Restore on different namespace
5. Check restored tables with old namespace
6. Drop old namespace
7. Repeat from No.1 4 times

Found that one table has different amount of values

> /home/yugabyte/tserver/bin/ysqlsh --c "select count(*) from backup_2460ec_wsqldataload_c11" -h <ip> -d backup_68562e
< count
 12675798
 (1 row) 
> /home/yugabyte/tserver/bin/ysqlsh --c "select count(*) from backup_2460ec_wsqldataload_c11" -h <ip> -d backup_fd2843
< count
 12675711
 (1 row)

Logs:
logs.tar.gz

Version: 2.15.2.0-b40

[def-]

Is it possible some of the last values are not flushed yet and thus don't get backed up properly? Flushing them manually to check if that fixes it could work to verify if it's a cause.
I had another bug with flushing and missing rows here: #12684

[pilshchikov]

@def- no, this one is different. I checked manually several times after an hour from restore and the results are still the same. You can do it by yourself, this universe is still running.

[druzac]

@pilshchikov do you have a script that repros this, or can you easily extract one? I'd like to take a look at the sst files in the snapshot, the sst files in the backup, and the sst files in the cluster for the source table to sanity check the theory that the last few writes aren't making it to the backup. I can't confirm that through the logs.