[Bug 1872463] Re: Duplicate marker files will make chainload fail to launch subiquity

[dann frazier]

@Dimitri - is the significant difference there that $prefix can and
should be trusted to uniquely point to the boot disk and only fallback
to the .disk/info search?

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https://bugs.launchpad.net/bugs/1872463

Title:
  Duplicate marker files will make chainload fail to launch subiquity

Status in subiquity:
  New
Status in Ubuntu CD Images:
  New
Status in debian-installer package in Ubuntu:
  New

Bug description:
  Focal beta iso efi.img manipulates grub when searching for the target
  device to boot. The current searching method uses /.disk/info as a
  marker file to identify the target live usb device. efi.img assumes
  there is only one live bootable device inserted with the marker file
  /.disk/info and searches in this way:

   search --file --set=root /.disk/info
   set prefix=($root)/boot/grub
   source $prefix/arm64-efi/grub.cfg

  In some use cases, people pre-insert many different distributions so
  they could easy to boot to and re-install different distributions. The
  marker file method will block this kind of multiple bootable setup.

  [Steps to Reproduce]
  1. Pre-insert one or more Ubuntu Bionic (or above) live usb sticks to your target machine
  2. Boot from focal beta iso live usb stick

  [Expected Result]
  We boot to the subiquity and start to install the system.

  [Actual Result]
  We will boot to the grub menu provided by the other live usb sticks. The unexpected grub menu may lead you to load incorrect installer files and fail on installation.

  [More Information]
  The issue is reproducible 100% by a combination of Marvell Cavium ThunderX2 Sabre with Bionic live usb pre-inserted and intended to install the system via focal beta iso live usb. Use dd to destroys the bionic live usb will make the chainload boot to the expected focal one (and completed the installation with subiquity).

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