[Bug 2147288] Re: os-prober fails to detect Linux distributions on Btrfs subvolumes (e.g. Fedora /root)

[Serhii Cherneha]

I've also found that linux-boot-prober needs a fix to correctly mount
Btrfs subvolumes to detect kernels. Attached is the second patch for
/usr/lib/linux-boot-probes/50mounted-tests.

** Patch added: "linux-boot-prober-btrfs-subvol.patch"
   https://bugs.launchpad.net/ubuntu/+source/os-prober/+bug/2147288/+attachment/5958706/+files/linux-boot-prober-btrfs-subvol.patch

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Title:
  os-prober fails to detect Linux distributions on Btrfs subvolumes
  (e.g. Fedora /root)

Status in os-prober package in Ubuntu:
  New

Bug description:
  1) The release of Ubuntu you are using:
  Ubuntu 24.04.4 LTS (Noble)

  2) The version of the package you are using:
  os-prober 1.81ubuntu4

  3) What you expected to happen:
  When running `sudo os-prober` or `sudo update-grub`, other Linux distributions installed on Btrfs partitions (like Fedora) should be automatically detected and added to the GRUB menu.

  4) What happened instead:
  os-prober fails to detect Linux installations when the root filesystem is located within a Btrfs subvolume (e.g., the "root" subvolume used by Fedora default installs). It only detects the partition but fails to identify the OS because it only checks the top-level directory of the partition's mount point.

  Bug Description:
  The issue is located in the /usr/lib/os-probes/mounted/90linux-distro script. When os-prober mounts a Btrfs partition via grub-mount, it only looks for /etc/os-release and other distribution identifiers in the root of the mount point. However, many distributions (Fedora, openSUSE, etc.) place their root filesystem inside a subvolume (like /root, /@, or /@rootfs).

  As a result, os-prober doesn't see the /etc/os-release file and
  ignores the partition, leaving the user with no boot entry for the
  other OS.

  Steps to reproduce:
  1. Install Fedora on a disk using default Btrfs partitioning (which creates a "root" subvolume for /).
  2. Boot into Ubuntu.
  3. Run `sudo os-prober`. Fedora is not listed in the output.

  I have attached a patch that adds a fallback check for common Btrfs
  subvolume names to 90linux-distro. This fix has been tested and
  successfully detects Fedora 43.

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