About
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Convert a C7, epel-7-sclo-x86_64 chroot, rpmdb created in EL9 as sqlite to bdb.

When the epel-7-sclo-x86_64 chroot is initialized from an EL9 host, the rpmdb will be
created by the host, for the host, or as sqlite. If you intend to shell into the chroot or
run `mock -r epel-7-sclo-x86_64 --chroot "<>;"` your rpm database is broken from the C7
point of view.

If our mock host was AL-8 or any EL8, its possible to run:
rpm --rebuilddb -D '_db_backend bdb' --root /var/lib/mock/epel-7-sclo-x86_64/root

in order to convert the database, since rpm in EL8 understands the difference between
bdb and sqlite, via the macro. Its not possible to use mock chroot and do it, since EL7 has
no support for sqlite, or its conversion.

Are we stuck with a broken epel-7 chroot when our mock host is EL9? No we can use a bindmount
trick and do the conversion of the epel-7 database from inside an AL-8 mock chroot.

To start, create the epel-7 chroot with no bootstrap support: 
config_opts['use_bootstrap'] = False
config_opts['use_bootstrap_image'] = False

It will now have the undesireable sqlite rpmdb.
mkdir /var/tmp/bm && cd $_
sudo tar -xf /var/cache/mock/epel-7-sclo-x86_64/root_cache/cache.tar.gz.sqlite

now we have a local 2nd chroot which we will bindmount into the AL-8 chroot.

[mockbuild@cs9min:~/.config/mock]$ !887
 ls -al /var/lib/mock/epel-7-sclo-x86_64/root/var/lib/rpm
total 35780
drwxr-xr-x  2 root root     4096 Nov 24  2021 .
drwxr-xr-x 14 root root     4096 Jul 16 18:26 ..
-rw-r--r--  1 root mock 36597760 Jul 16 18:27 rpmdb.sqlite
-rw-r--r--  1 root mock    32768 Jul 16 18:27 rpmdb.sqlite-shm
-rw-r--r--  1 root mock        0 Jul 16 18:27 rpmdb.sqlite-wal
-rw-r--r--  1 root mock        0 Jul 16 18:26 .rpm.lock

[mockbuild@cs9min:~/.config/mock]$ mock -r epel-7-sclo-x86_64 --init
INFO: mock.py version 3.0 starting (python version = 3.9.13, NVR = mock-3.0-1.el9)...
Start: init plugins
INFO: selinux disabled
Finish: init plugins
INFO: Signal handler active
Start: run
INFO: Setting hostname: c7mock
Start: clean chroot
Finish: clean chroot
Start: chroot init
INFO: calling preinit hooks
INFO: enabled root cache
Start: unpacking root cache
Finish: unpacking root cache
INFO: enabled package manager cache
Start: cleaning package manager metadata
Finish: cleaning package manager metadata
INFO: enabled HW Info plugin
Mock Version: 3.0
INFO: Mock Version: 3.0
Finish: chroot init
Finish: run
[mockbuild@cs9min:~/.config/mock]$  ls -al /var/lib/mock/epel-7-sclo-x86_64/root/var/lib/rpm
total 34412
drwxr-xr-x  2 root root     4096 Jul 16 18:35 .
drwxr-xr-x 14 root root     4096 Jul 16 18:34 ..
-rw-r--r--  1 root root  1589248 Jul 16 18:34 Basenames
-rw-r--r--  1 root root     8192 Jul 16 18:34 Conflictname
-rw-r--r--  1 root root        0 Jul 16 18:35 .dbenv.lock
-rw-r--r--  1 root root  1011712 Jul 16 18:34 Dirnames
-rw-r--r--  1 root root     8192 Jul 16 18:34 Enhancename
-rw-r--r--  1 root root     8192 Jul 16 18:34 Filetriggername
-rw-r--r--  1 root root     8192 Jul 16 18:34 Group
-rw-r--r--  1 root root    12288 Jul 16 18:34 Installtid
-rw-r--r--  1 root root    20480 Jul 16 18:34 Name
-rw-r--r--  1 root root     8192 Jul 16 18:34 Obsoletename
-rw-r--r--  1 root root 32190464 Jul 16 18:34 Packages
-rw-r--r--  1 root root   143360 Jul 16 18:34 Providename
-rw-r--r--  1 root root     8192 Jul 16 18:34 Recommendname
-rw-r--r--  1 root root   122880 Jul 16 18:34 Requirename
-rw-r--r--  1 root root    32768 Jul 16 18:34 Sha1header
-rw-r--r--  1 root root    24576 Jul 16 18:34 Sigmd5
-rw-r--r--  1 root root     8192 Jul 16 18:34 Suggestname
-rw-r--r--  1 root root     8192 Jul 16 18:34 Supplementname
-rw-r--r--  1 root root     8192 Jul 16 18:34 Transfiletriggername
-rw-r--r--  1 root root     8192 Jul 16 18:34 Triggername

Add these into the AL-8 chroot config, remember to touch the CFG into the past when done.
config_opts['plugin_conf']['bind_mount_enable'] = True
config_opts['plugin_conf']['bind_mount_opts']['dirs'].append(('/var/tmp/bm', '/mnt/' ))

Next, launch the AL-8 shell:
mock -r almalinux-dev-8-x86_64 --shell

now your epel-7 chroot is at /mnt!
while inside, we can run
rpm --rebuilddb -D '_db_backend bdb' --root /mnt

Exit the AL-8 chroot, then archive the fixed chroot back into /var/cache/mock:
cd /var/tmp/bm
sudo tar -Ipigz -cf /var/cache/mock/epel-7-sclo-x86_64/root_cache/cache.tar.gz ./

The next time you do:
mock -r epel-7-sclo-x86_64 --init

your chroot is OK for shell.
Even below works, although the DB is bdb.
mock -r epel-7-sclo-x86_64 --dnf-cmd replist

These warnings will appear:
warning: Found bdb_ro Packages database while attempting sqlite backend: using bdb_ro backend

They are harmless.

Note:
any attempt to rebuild the epel-7 rpmdb from the EL9 host will get:
[mockbuild@hatvm ~]$ sudo rpm --rebuilddb -D '_db_backend bdb' --root /var/tmp/bm
warning: invalid %_db_backend: bdb
warning: using dummy database, installs not possible
warning: invalid %_db_backend: bdb
warning: using dummy database, installs not possible

This is why the trick to use the AL-8 chroot was tried.

Author
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PryMar56
July 2022