Bug#1085184: apt: Alternative approach?

[Tj <tj.iam.tj@proton.me>]

Package: apt
Followup-For: Bug #1085184
X-Debbugs-Cc: tj.iam.tj@proton.me

I helped diagnose this same issue last evening in the Debian Matrix room
and independently found the same causes as JAK reports.

To my mind the entire approach of trying to guess ahead of time how much
space will be required is brittle and prone to unexplained corner-case
failures such as this.

I've recently been working on a feature extension for the kernel
overlayfs and so it was a small step to wondering if apt could make use
of overlayfs (either the kernel module or fuse) here ?

On discovering a separate /boot/ :
  1. record its free space
	2. create an overlay (possibly using tmpfs for the upperdir)
	3. let packages install as normal (with any triggering of update-initramfs)
	4. after all changes are done unmount the overlay
	5. review upperdir and if files exist
	5a. check how much space they require
	5b. if they will fit in /boot/
	5b1. copy files to /boot/
	5b. else
	5b2. report difference, offer remedial actions (could be helper
	scripts)
	6. conditionally remove upperdir files, unmount any tmpfs

If not using a tmpfs the files could be left in place so the
administrator has the ability to manually fix up the issue.

The files being added aren't limited to the kernel image itself 
- for non x86 architectures there could be device-tree or other firmware
related files installed by other packages.

This space consideration  should not be limited to just /boot/ itself. 

One such example is the Raspberry Pi where, since Bookworm, the FAT
file-system containing the actual booted files is mounted at
/boot/firmware/, kernel packages and update-initramfs write to /boot/
as normal but there is an additional hook script that then copies and
renames the kernel image and initrd into /boot/firmware/ so there are
potentially two -ENOSPACE file-systems in play.

Rough PoC:

df /boot
mkdir --parents /var/tmp/apt/boot
mount -t tmpfs apt_tmp /var/tmp/apt/boot
mkdir /var/tmp/apt/boot/{upper,work}
mount -t overlay apt_boot -o lowerdir=/boot,upperdir=/var/tmp/apt/boot/upper,workdir=/var/tmp/apt/boot/work /boot
# all apt package operations would occur here
update-initramfs -u
# all packages are now installed/upgraded
umount /boot
find /var/tmp/apt/boot/upper -type f -ls
cp -a /var/tmp/apt/boot/upper/* /boot/
rm -rf /var/tmp/apt/boot/*
umount /var/tmp/apt/boot