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Find the bootstrap slot by its sector headers: - #77
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mergify[bot] merged 2 commits intoOct 15, 2026
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Tinkerbell needed a layout file published with every ISO to find the slot. Fetching, caching and keeping that file in step with the ISO, across redirects and ISOs replaced in place, was where most of Tinkerbell's complexity and failure modes came from. Every sector of the slot now starts with a header identifying it, so Tinkerbell finds the slot in the bytes it serves, as it already does the kernel arguments, and no layout file is published. Each header is an empty cpio archive, so an ISO served unmodified still boots. The build checks that the slot is stored intact and sector aligned, and that nothing else in the ISO looks like one of its headers. An ISO that fails the check is removed, and an existing ISO is only reused when it passes. Signed-off-by: Jacob Weinstock <jakobweinstock@gmail.com>
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🟡 Changes recommended
Interrupted builds can leave incomplete ISOs that pass slot validation and are subsequently reused.
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What changed in this PR
Replaces bootstrap-slot layout sidecars with sector headers so Tinkerbell can locate the slot directly in ISO bytes.
Changes:
- Adds sector headers and validates slot integrity, alignment, and uniqueness.
- Validates existing ISOs before reuse.
- Removes layout sidecars from artifact collection and CI uploads.
| File | Description |
|---|---|
| captain/iso.py | Generates sector headers and validates bootstrap slots. |
| captain/flavor.py | Removes layout sidecars from artifact listings. |
| captain/cli/stages.py | Checks slots before reusing existing ISOs. |
| captain/artifacts.py | Stops collecting layout sidecars. |
| .github/workflows/ci.yml | Uploads only the ISO artifact. |
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grub-mkrescue wrote straight to the final path, so a build that failed or was interrupted after writing the bootstrap slot left a truncated ISO whose slot still passes the check, and a later build reused it. Build and check the ISO under a .partial name and move it into place only when both succeed. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Signed-off-by: Jacob Weinstock <jakobweinstock@gmail.com>
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This pull request spent 26 minutes 2 seconds in the queue, including 25 minutes 15 seconds running CI. Required conditions to merge
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Description
Tinkerbell needed a layout file published with every ISO to find the slot. Fetching, caching and keeping that file in step with the ISO, across redirects and ISOs replaced in place, was where most of Tinkerbell's complexity and failure modes came from.
Every sector of the slot now starts with a header identifying it, so Tinkerbell finds the slot in the bytes it serves, as it already does the kernel arguments, and no layout file is published. Each header is an empty cpio archive, so an ISO served unmodified still boots.
The build checks that the slot is stored intact and sector aligned, and that nothing else in the ISO looks like one of its headers. An ISO that fails the check is removed, and an existing ISO is only reused when it passes.
Fixes: #
How Has This Been Tested?
How are existing users impacted? What migration steps/scripts do we need?
Checklist:
I have: