frr-reload.py: removing all interface configuration fails for active interface

[hedrok]

Description

When removing all configuration of interface, frr-reload.py runs command no interface eth0 that tries to delete an interface, which gives error.

Version

FRRouting 10.6-dev (r1) on Linux(6.6.117-vyos).
Copyright 1996-2005 Kunihiro Ishiguro, et al.
configured with:
    '--build=x86_64-linux-gnu' '--prefix=/usr' '--includedir=${prefix}/include' '--mandir=${prefix}/share/man' '--infodir=${prefix}/share/info' '--sysconfdir=/etc' '--localstatedir=/var' '--disable-option-checking' '--disable-silent-rules' '--libdir=${prefix}/lib/x86_64-linux-gnu' '--libexecdir=${prefix}/lib/x86_64-linux-gnu' '--disable-maintainer-mode' '--sbindir=/usr/lib/frr' '--with-vtysh-pager=/usr/bin/pager' '--libdir=/usr/lib/x86_64-linux-gnu/frr' '--with-moduledir=/usr/lib/x86_64-linux-gnu/frr/modules' '--disable-dependency-tracking' '--enable-rpki' '--enable-scripting' '--enable-pim6d' '--disable-grpc' '--disable-address-sanitizer' '--with-libpam' '--enable-doc' '--enable-doc-html' '--enable-snmp' '--enable-fpm' '--disable-protobuf' '--disable-zeromq' '--enable-ospfapi' '--enable-bgp-vnc' '--enable-multipath=256' '--enable-user=frr' '--enable-group=frr' '--enable-vty-group=frrvty' '--enable-configfile-mask=0640' '--enable-logfile-mask=0640' '--enable-pcre2posix' 'build_alias=x86_64-linux-gnu' 'PYTHON=python3'

How to reproduce

Add configuration for interface, reload configuration. Remove all configuration for interface, try to reload configuration.

Commands:

sudo ip link add dum771 type dummy

cat > frr-clean.conf << EOF
frr version 10.6-dev
frr defaults traditional
hostname r1
log syslog notifications
log timestamp precision 3
no log unique-id
service integrated-vtysh-config
!
EOF

cat > frr-with-interfaces.conf << EOF
frr version 10.6-dev
frr defaults traditional
hostname r1
log syslog notifications
log timestamp precision 3
no log unique-id
service integrated-vtysh-config
!
interface dum771
 ip address 10.77.0.1/24
exit
!
EOF

sudo /usr/lib/frr/frr-reload.py --reload frr-with-interfaces.conf

sudo /usr/lib/frr/frr-reload.py --test frr-clean.conf

# shows
# Lines To Delete
# ===============
# no interface dum771

sudo /usr/lib/frr/frr-reload.py --reload frr-clean.conf
# fails

Expected behavior

Reload runs successfully.

Actual behavior

Last command outputs:

Failed to execute no interface dum771
Failed to execute no interface
"no interface" we failed to remove this command
% Configuration failed.

commit failed session-id 15 on Unknown-FD-16 req-id 3 source-ds: candidate target-ds: running validate-only: 0: reason: 'Failed to create cfgdata: only inactive interfaces can be deleted'

Additional context

The behaviour was changed in PR #16723 to fix #15081

This PR deletes comment:

# We cannot do 'no interface' or 'no vrf' in FRR, and so deal with it

Checklist

• I have searched the open issues for this bug.
• I have not included sensitive information in this report.

[hedrok]

I see the following options to fix the issue:

1. Just revert changes in tools: fix reload interface deletion #16723. I can't reproduce ip of deleted GRE interface remains in running config, removing it returns an error #15081 now (it was FRR 8.5.3). Fix any similar future issues by making no commands under interface do nothing without errors in case of deleted interface.
2. Take different action based on whether interface is still active or not. If interface is active, run no for everything inside interface, but not no interface. If inactive just run no interface. I've tried to fix the issue in this way by PR frr-reload: fix deleting active interface #20335, but it fails - in some tests vtysh is not available when frr-reload.py --test is run, so I'm not sure now that it is good idea to add dependency on current system status.
3. Make write output output empty interface section for each active interface. It will fix the issue totally, but will add these empty sections...

I'll be thankful for feedback - which path to follow?