Fix ENOSPC empty media on large ISOs; harden stage 3 build

[isss802]

Problem

Large-ISO installs (e.g. English Windows 11) hang at boot. The console sits at

Booting from Hard Disk...

indefinitely — CPU pinned at 100%, disk I/O zero — and never enters Windows Setup.

Root cause: stage3 runs distrobuilder repack-windows on the ~30 GB build root. For the English Windows 11 ISO that root cannot hold the source (~8.5 GB) plus distrobuilder's extraction overlay plus the output ISO (~8.5 GB) at once, so ISO generation fails with ENOSPC (genisoimage exit 28) and leaves a truncated windows_virtio.iso. The only guard is [ -f windows_virtio.iso ] (existence), which the truncated file passes, so WoeUSB is handed a corrupt ISO, writes an empty media partition, and the script reboots into an unbootable disk. Smaller ISOs (Windows Server 2022, ~5 GB) fit in 30 GB, which is why only large ISOs are affected.

Confirmed by rescue-mounting a hung instance's disk: the 8 GB media partition held only autounattend.xml — no bootmgr, no sources/install.wim. SeaBIOS finds the boot-flagged partition, prints "Booting from Hard Disk…", finds no bootmgr, and spins.

Fix

Run the heavy repack on the 160 GB target disk (/dev/sdb), which is unused until it is partitioned later in the same function:

• Mount /dev/sdb at /mnt/work; put the source ISO, distrobuilder --cache-dir, the output ISO, and TMPDIR there.
• Validate the output, copy it back to the root, then hard-gate the unmount before repartitioning sdb.
• Enlarge the media partition 8 GB → 12 GB (the repacked English media is ~8.5 GB).

Harden stage3 so a broken build fails loudly instead of rebooting into a dead disk:

• Halt (no reboot) on any failure, leaving /root/BUILD_FAILED and the build environment alive for inspection.
• Validate windows_virtio.iso size and contents (sources/install.wim, bootmgr).
• Check WoeUSB's exit code; after WoeUSB, verify the media partition actually contains bootmgr + install.wim.
• Verify the MBR bootstrap is non-empty — an empty MBR reproduces the exact "Booting from Hard Disk…" hang even when the files are correct.
• Move partprobe / udevadm settle before formatting (it was racing device-node creation); size-check the staged ISO copy; install libarchive-tools for a reliable ISO content listing.

Testing

Deployed on Linode (jp-tyo-3, g6-standard-4).

• Before: English Windows 11 25H2 → SeaBIOS "Booting from Hard Disk…" hang; media partition empty.
• After: English Windows 11 25H2 installs unattended and reaches RDP — raw X.224 Connection Confirm 03 00 00 13 0e d0 ….
• The same instrumentation is what surfaced the original ENOSPC (exit 28) during diagnosis (the build halted with a clear message instead of silently shipping empty media).
• Control: Windows Server 2022 (small ISO) still builds end-to-end.

🤖 Generated with Claude Code