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Re: Bug#1000239: Rescue system won't find root partition, but insists on /usr

Control: severity -1 serious
Control: tags = confirmed

CCing the release team, and CTTE because I don't know who else is
tracking issues related to the usrmerge effort.  I've consciously chosen
not to pour gasoline on the flame war by CCing anyone else (nor will I
contact anyone else about the existence of this bug).

Steps I used to try to reproduce:

1. Downloaded debian-testing-amd64-netinst.iso	2021-12-03 16:21 408M
2. Installed to EFI-enabled qemu eg:
   kvm -bios /usr/share/ovmf/bios.bin -m 2G \
   -hda debian-separate-usr-sda.raw \
   -cdrom debian-testing-amd64-netinst.iso
3. Guided partitioning with separate /home, then changed the mount point
   to /usr.  Partition layout is:
                                  sda1: ESP  fat32
                                  sda2: /    ext4
                                  sda3: swap swap
                                  sda4: /usr ext4
4. Selected only "Standard System Utilities"
5. Rebooted


Then, to test the rescue functionality:

1. I discovered qemu+OVMF boot order is broken without changing the
   command to: kvm -L /usr/share/ovmf/ -m 2G -boot menu=on \
                   -hda /scratch/debian-separate-usr-sda.raw \
                   -cdrom /scratch/debian-testing-amd64-netinst.iso
2. Selected sda2
3. Yes, mount /boot/efi
4. Execute a shell in /dev/sda2
5. No usable shell was found on your root file system (/dev/sda2)
6. Changed virtual terminal
7. cd /target && ls bin
   ls: bin: No such file or directory

Result: FAILED
Conclusion: This is a usrmerged system, and the rescue system does not
support usermerged systems.

The options are, as I see them, ranked from least to most work-hours:

1. Debian isn't yet ready for usrmerge.  Revert to normal system installation.
2. Reassign to src:rescue, and fix the rescue system.
   a) before chrooting, test for the presence of /target/bin/sh
   b) if /bin/sh is not found, either emit error to the user and present a
      dialogue for selecting /usr partition, or
   c) parse /target/etc/fstab, and attempt to mount other partitions
   d) b & c will be difficult to implement when attempting to accommodate
      the heterogeneity of possible MD, LUKS, and LVM layouts, not to mention
      the complications introduced by the possibility of a user-configured
      btrfs subvolume name "@usr" on any valid device.  Fstab parsing might
      make the btrfs case easier with:
        i)   Display a dialogue selector if a btrfs partition is detected
             The dialogue will list all detected subvolumes
        ii)  If the user cannot find a subvolume for "@usr", then
        iii) Allow the user to escape to the partition selection screen, and
        iv)  Unmount the partition
3. Disallow configuring of a mount for "/usr", and *Prominently* declare that
   Debian no longer supports separating /usr from /, declare this in *many
   places*, and reply to bugs on this topic for many years.  I put this one
   last because I believe the cost to work-hours is unbounded, and
   because I believe there may be a negative social cost associated with
   this action.  Also, if Fedora/RHEL/SUSE/Ubuntu support a separate /usr
   partition, then this action could make Debian look inferior.


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