[action] [PR:4365] Fix migrate_sonic_packages() crash on symlink resolv.conf

[mssonicbld]

What I did

Fixed sonic-installer install failing during migrate_sonic_packages() when /etc/resolv.conf in the new image is a symlink to /run/resolvconf/resolv.conf.

The failure occurs because the cp command at main.py:386 follows the symlink through the overlay mount. Since the symlink target is an absolute path, it resolves to the host's /run/resolvconf/resolv.conf — the same file as the source. cp detects same source and destination inode and exits with:

cp: '/etc/resolv.conf' and '/tmp/image-<version>-fs/etc/resolv.conf' are the same file

This was introduced by the build_debian.sh change that replaced touch with ln -sf /run/resolvconf/resolv.conf for /etc/resolv.conf in the image filesystem.

How I did it

Check whether /etc/resolv.conf in the chroot is a symlink or a regular file, and handle each case appropriately:

• Symlink (images with resolvconf package installed): Read the symlink target via readlink (e.g. /run/resolvconf/resolv.conf), then create the target file inside the chroot with the host's DNS content. The symlink then resolves correctly inside the chroot. This avoids touching the symlink itself, so the overlay upper dir's etc/resolv.conf is never modified and the new image boots with the symlink intact. No cleanup is needed — the target lives under /run, which is a tmpfs recreated at every boot.

• Regular file (images without resolvconf, or where the build process explicitly creates a regular file via touch): Overwrite directly with the host's DNS content. No backup/restore is needed — the original file is empty (cleared during build), and after reboot the resolv-config service reconfigures DNS from CONFIG_DB.

The previous backup-overwrite-restore pattern has been removed since it is unnecessary in both cases.

How to verify it

1. Start with a switch running an image where /etc/resolv.conf is a symlink:

   # Confirm symlink exists
   ls -la /etc/resolv.conf
   # Expected: /etc/resolv.conf -> /run/resolvconf/resolv.conf
   
   # Ensure only one image is installed (clean state)
   sudo sonic-installer list
   # If the target image is already present, remove it:
   sudo sonic-installer remove <target-image> -y

2. Run sonic-installer install with an image that also has the symlink:

   sudo sonic-installer install <image-path> -y

3. Verify:

   • Installation completes without cp: ... are the same file error
   • sonic-installer list shows the new image as default
   • After reboot, /etc/resolv.conf is still a symlink to /run/resolvconf/resolv.conf
   • DNS resolution works

Signed-off-by: Sonic Build Admin sonicbld@microsoft.com

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[mssonicbld]

Original PR: #4365

[mssonicbld]

/azp run

[azure-pipelines]

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