Looking at the postinst script, it calls update-initramfs -u and expects it to do somethin, i.e. create the initramfs.

However, it can choose to not do so, and deffer the update. And thus not create an initramfs. However, when we are upgrading to a new kernel image, we expect to generate an initramfs cause we will be fixing symlinks and so on right after.

I have linux-image-4.3.0-6.17 installed, via linux-virtual metapackage on an old cloud image (see my private fileshare). And upgrading that to xenial/release 4.3.0-7.18 fails like so:

ubuntu@ubuntu:~$ dpkg -l | grep linux
ii  console-setup-linux              1.108ubuntu10                           all          Linux specific part of console-setup
ii  libselinux1:s390x                2.4-3                                   s390x        SELinux runtime shared libraries
ii  linux-headers-4.3.0-6            4.3.0-6.17                              all          Header files related to Linux kernel version 4.3.0
ii  linux-headers-4.3.0-6-generic    4.3.0-6.17                              s390x        Linux kernel headers for version 4.3.0 on System 390x SMP
ii  linux-headers-generic            4.3.0.6.7                               s390x        Generic Linux kernel headers
ii  linux-headers-virtual            4.3.0.6.7                               s390x        Transitional package.
ii  linux-image-4.3.0-6-generic      4.3.0-6.17                              s390x        Linux kernel image for version 4.3.0 on System 390x SMP
ii  linux-image-virtual              4.3.0.6.7                               s390x        This package will always depend on the latest minimal generic kernel image.
ii  linux-virtual                    4.3.0.6.7                               s390x        Minimal Generic Linux kernel and headers
ii  util-linux                       2.27.1-1ubuntu3                         s390x        Miscellaneous system utilities
ubuntu@ubuntu:~$

# upgrading to release (to 4.3.0-7.18)

...

Setting up linux-image-4.3.0-7-generic (4.3.0-7.18) ...
Progress: [ 76%] [#############################################.............]
update-initramfs: deferring update (hook will be called later)
cp: cannot stat ‘/boot/initrd.img-4.3.0-7-generic’: No such file or directory
Failed to copy /boot/initrd.img-4.3.0-7-generic to /boot/initrd.img at /var/lib/dpkg/info/linux-imageProgress: [ 77%] [#############################################.............]
dpkg: error processing package linux-image-4.3.0-7-generic (--configure):
 subprocess installed post-installation script returned error exit status 1

...

dpkg: dependency problems prevent configuration of linux-image-virtual:
 linux-image-virtual depends on linux-image-4.3.0-7-generic; however:
   Package linux-image-4.3.0-7-generic is not configured yet.

dpkg: error processing package linux-image-virtual (--configure):
 dependency problems - leaving unconfigured
 Setting up linux-headers-generic (4.3.0.7.8) ...No apport report written because the error message indicates its a followup error from a previous failure.

Setting up linux-headers-virtual (4.3.0.7.8) ...
dpkg: dependency problems prevent configuration of linux-virtual:
 linux-virtual depends on linux-image-virtual (= 4.3.0.7.8); however:
   Package linux-image-virtual is not configured yet.

dpkg: error processing package linux-virtual (--configure):
 dependency problems - leaving unconfigured
 Setting up lxcfs (0.17-0ubuntu2) ...No apport report written because the error message indicates its a followup error from a previous failure.


...

My solution to the problem is to force update-initramfs to create an initrd when the linux-image upgrade scripts expect to have that to happen. See the attached patch.