Description of problem:

When tar exits with status greater than zero, rear copies the last 3 messages
from the backup.log to its own log. They may also be shown to the user if rear
is invoked with the "-v" option.

If the last 3 or more messages from tar are "socket ignored" (informational)
the actual error message (e.g. "file changed as we read it").

Version-Release number of selected component (if applicable):

rear-2.4-11.el7.x86_64

How reproducible:

Frequently

Steps to Reproduce:

1. Set-up /etc/rear/local.conf to save backups in a local filesystem, e.g.

BACKUP=NETFS
BACKUP_URL=file:///backup/
# remove /usr to speed-up the backup
BACKUP_PROG_EXCLUDE=("${BACKUP_PROG_EXCLUDE[@]}" '/usr' '/var/crash' '/var/tmp' '/dev' '/dev/shm' '/run' '/sys/fs/cgroup')

2. Ensure that a daemon that creates sockets is running (e.g. postfix)

   # yum install -y postfix
   # systemctl enable --now postfix

3. Make a backup of /etc/grub.d/README

   # mv /etc/grub.d/README /etc/grub.d/README~
   # cp -p /etc/grub.d/README~ /etc/grub.d/README

4. Run "rear -v mkbackup" while executing a command in parallel to append data
   to /etc/grub.d/README

   # ( while :; do echo .; done ) >> /etc/grub.d/README

5. Stop appending data to /etc/grub.d/README

Actual results:

# rear -v mkbackup
Relax-and-Recover 2.4 / Git
Using log file: /var/log/rear/rear-rhel-7-1.log
Using backup archive '/backup//rhel-7-1/backup.tar.gz'
Using UEFI Boot Loader for Linux (USING_UEFI_BOOTLOADER=1)
Creating disk layout
Excluding component fs:/backup
Using guessed bootloader 'EFI' (found in first bytes on /dev/vda)
Creating root filesystem layout
Trying to find what to use as UEFI bootloader...
Trying to find a 'well known file' to be used as UEFI bootloader...
Using '/boot/efi/EFI/redhat/grubx64.efi' as UEFI bootloader file
Copying logfile /var/log/rear/rear-rhel-7-1.log into initramfs as '/tmp/rear-rhel-7-1-partial-2020-08-03T15:28:45-0300.log'
Copying files and directories
Copying binaries and libraries
Copying kernel modules
Copying all files in /lib*/firmware/
Creating recovery/rescue system initramfs/initrd initrd.cgz with gzip default compression
Created initrd.cgz with gzip default compression (258832181 bytes) in 32 seconds
Making ISO image
Wrote ISO image: /var/lib/rear/output/rear-rhel-7-1.iso (291M)
Copying resulting files to file location
Saving /var/log/rear/rear-rhel-7-1.log as rear-rhel-7-1.log to file location
Creating tar archive '/backup//rhel-7-1/backup.tar.gz'
Archived 568 MiB [avg 10402 KiB/sec] OK
WARNING: tar ended with return code 1 and below output:
  ---snip---
  tar: /var/spool/postfix/public/qmgr: socket ignored
  tar: /var/spool/postfix/public/flush: socket ignored
  tar: /var/spool/postfix/public/showq: socket ignored
  ----------
This means that files have been modified during the archiving
process. As a result the backup may not be completely consistent
or may not be a perfect copy of the system. Relax-and-Recover
will continue, however it is highly advisable to verify the
backup in order to be sure to safely recover this system.

Archived 568 MiB in 57 seconds [avg 10220 KiB/sec]
Exiting rear mkbackup (PID 23976) and its descendant processes
Running exit tasks
(reverse-i-search)`.con': vim -O /etc/rear/local^Conf /usr/share/rear/backup/NETFS/default/500_make_backup.sh

Expected results:

  ---snip---
  tar: /etc/grub.d/README: file changed as we read it
  ----------

Additional info:

The output is generated in line 235 of

    /usr/share/rear/backup/NETFS/default/500_make_backup.sh:

    $(grep '^tar: ' "${TMP_DIR}/${BACKUP_PROG_ARCHIVE}.log" | sed -e 's/^/  /' | tail -n3)

Rear not only hides the real problem but also shows misleading information,
since ignored sockets are not a problem.