Comment 0 for bug 1791959

[Impact]
If a dkms package is installed which has REMAKE_INITRD or the same setting has be manually configured by a user then when a kernel is removed its possible for an ".old-dkms" file to be left in /boot with no associated kernel.

bug 1515513 dealt with removing initrd.img-<version>.old-dkms files using the kernel's prerm hook, but that is only executed for the kernel version being removed: any other old-dkms file generated prior to that would not be removed by the hook, taking space in the /boot directory.

Note: Filling up the /boot partition causes updates to fail.

[Test Case]
As the fix for bug 1515513 is available on Xenial it is no longer possible to reproduce this by simply installing and updating kernels (dkms 2.2.0.3-2ubuntu11.3 would be required for that). In order to replicate it an old dkms file will be created by hand.

This assumes a new Xenial schroot.

1) create a file to work as a placeholder for the initrd.img old dkms file
sudo touch /boot/initrd.img-4.0.0-0-generic.old-dkms

2) install 3 old kernels, r8168-dkms, and the current initramfs-tools
sudo apt-get install -y linux-image-4.4.0-21-generic linux-image-4.4.0-22-generic linux-image-4.4.0-24-generic r8168-dkms initramfs-tools=0.122ubuntu8.12

3) install the headers for the old kernels (forces dkms to run)
sudo apt-get install -y linux-headers-4.4.0-21-generic linux-headers-4.4.0-22-generic linux-headers-4.4.0-24-generic

4) verify that there are 4 old-dkms
ls /boot/*.old-dkms

5) install the proposed initramfs-tools with this fix
sudo apt-get install -y initramfs-tools

6) verify that the manually created old-dkms file was removed (only 3 files now)
ls /boot/*.old-dkms

7) autoremove the older kernel
sudo apt-get autoremove -y

8) verify that there are now only 2 old-dkms
ls /boot/*.old-dkms

[Regression Potential]
Somebody out there might expect the .old-dkms file to be kept, but that seems like an odd expectation.

One notices *.old-dkms files being left behind still sitting on the disk after purging the related kernel. This can cause /boot to become full, and when it gets really bad, even sudo apt-get autoremove won't fix the problem - only deleting the old-dkms files manually solves the problem.