iso-patch: fail loudly when the Ubuntu ISO's minimal.squashfs has no kernel (refs #66)

[adihanifsdr]

Refs #66 (Ubuntu 24.04 ISO hangs at "Installing Linux"). @jamesstringer90 — this doesn't add 24.04 support, it just turns the silent hang into an immediate, explained failure.

What happens today

do_ubuntu_to_vhdx ingests casper/minimal.squashfs and takes the kernel version from the first /boot/vmlinuz-* seen during the walk. On the 26.04 desktop ISO that layer carries the kernel. On 24.04 it does not — the kernel lives only in the minimal.standard.live overlay — so kernel_ver stays empty and the bootstrap grub.cfg is written as:

linux  /boot/vmlinuz- root=UUID=... ro console=tty0 console=ttyS0,115200
initrd /boot/initrd.img-

GRUB can't load that. The guest sits at the GRUB prompt (1 vCPU at ~30 %, the other 7 at 0 %, ~24 MB of guest RAM touched, nothing on the COM1 serial pipe because the kernel never starts), the agent never connects, and the UI shows "Installing Linux" forever. Nothing in the log says why, because run_iso_patch_ubuntu only parses PROGRESS: / ERROR: / DONE: and drops iso-patch's STATUS: lines.

Full-ingest evidence from ubuntu-24.04.4-desktop-amd64.iso with this patch applied:

STATUS:Source squashfs: E:\casper\minimal.squashfs
STATUS:ingest: walk=0 files=85075 dirs=9287 syms=29728 special=8 errors=0 bytes=4901.4 MiB
ERROR:no kernel (/boot/vmlinuz-*) found in E:\casper\minimal.squashfs - the disk would not boot. Only Ubuntu Desktop 26.04 LTS ISOs are supported.
EXITCODE:1        (VHDX deleted by cleanup)

Changes

• tools/iso-patch/ubuntu_vhdx.c — after the squashfs ingest, an empty kernel version is a hard error: log_err() names the image that lacked /boot/vmlinuz-* and states that only Ubuntu Desktop 26.04 LTS ISOs are supported; exit_code stays 1 so cleanup deletes the VHDX. On success the detected kernel is logged as a STATUS: line.
• src/backend_win/asb_core.c (run_iso_patch_ubuntu) — forward iso-patch STATUS: and ERROR: lines into the app log as iso-patch: .... The Windows --to-vhdx path intentionally ignores STATUS: because the in-guest installer logs are available later; the Linux build has no such fallback, so these lines are the only host-side record of how the disk was built.

With both, a 24.04 ISO now fails the create step in ~70 s with the error surfaced in the UI log and as the create error, instead of leaving a VM that never finishes.

Considered and dropped

A cheap pre-flight on casper/minimal.manifest (refuse before the ingest if no linux-image-* line) would have saved the ~70 s, but the manifest lists no linux-image-* package on the 26.04 and 26.04.1 ISOs either (checked via range reads against releases.ubuntu.com), so it can't discriminate. The post-ingest check reads the real squashfs contents and only rejects builds that would have been unbootable anyway — there is no path where a currently-working 26.04 build is affected.

Testing

• Built iso-patch.vcxproj and AppSandbox.sln Release|x64 (VS 2026 Community). Only pre-existing warnings (xz_crc64_init in prefetch_build_deps.c, DwmSetWindowAttribute in vm_display_idd.c).
• iso-patch.exe --ubuntu-to-vhdx ubuntu-24.04.4-desktop-amd64.iso --output ... --size-gb 16 (elevated): full ingest, then the new error, exit code 1, no VHDX left behind (log excerpt above).
• Reproduced the original hang on Windows 11 26200 / RTX 3070 with the same ISO before the patch: VM stuck 1.5 h at "Installing Linux" with the vCPU/memory/serial signature described above.

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