TOOL-30798 track resolute's GA kernel for the generic flavor

[prakashsurya]

Problem

linux-kernel-generic is the one kernel flavor whose upstreams/os-upgrade has never been refreshed for resolute. It still sits on Ubuntu-6.8.0-59.61 from 2025-04-11, i.e. noble's GA kernel from the 24.04 cycle, while update-package refreshed the other four this week:

linux-kernel-aws      Ubuntu-aws-7.0.0-1010.10     pushed 2026-08-09
linux-kernel-azure    Ubuntu-azure-7.0.0-1011.11   pushed 2026-08-09
linux-kernel-oracle   Ubuntu-oracle-7.0.0-1009.9   pushed 2026-08-09
linux-kernel-gcp      Ubuntu-gcp-7.0.0-1008.8      pushed 2026-08-10
linux-kernel-generic  Ubuntu-6.8.0-59.61           never pushed

kernel_update_upstream() treats generic as an HWE flavor (tag_prefix_flavour="Ubuntu-hwe"), paired with get_kernel_version_for_platform_from_apt() asking apt for linux-image-generic-hwe-26.04. The apt half resolves, so the version and ABI it computes are correct; #411 confirmed 7.0.0-14.14 in resolute and 7.0.0-29.29 in resolute-updates. The tag name built from them is not: it searches the resolute tree for Ubuntu-hwe-7.0-7.0.0-29, falls back to Ubuntu-hwe-7.0.0-29, matches neither, and dies before reaching the point where anything would be pushed. That is why the branch is untouched rather than half-advanced the way makedumpfile and nfs-utils were.

The resolute tree carries no Ubuntu-hwe tag at all, and that is expected rather than a Canonical policy change. An Ubuntu-hwe-X.Y tag only exists in the tree of the release receiving a backport, and never for that release's own GA kernel:

jammy    (GA 5.15)   Ubuntu-hwe-{5.17,5.19,6.2,6.5,6.8}
noble    (GA 6.8)    Ubuntu-hwe-{6.11,6.14,6.17,7.0}   and zero Ubuntu-hwe-6.8
resolute (GA 7.0)    none

noble's own 6.8 appears one release down, in jammy, as Ubuntu-hwe-6.8-6.8.0-100.100_22.04.1. resolute has nothing newer than 7.0 to backport into it yet; per Canonical, new installs only default to the HWE kernel from 26.04.2.

This is the second cycle it has bitten. The 24.04 cutover collapsed the same case to an unconditional Ubuntu prefix (7e2bf0c, DLPX-94086, 2025-05-12), which is why generic's upstreams/os-upgrade sits on a GA tag rather than an HWE one, and DLPX-95001 / #364 restored Ubuntu-hwe three months later (98e143d, 2025-08-04) once noble's HWE track had rolled to 6.11.

Solution

The solution taken in this PR is to track the GA kernel of the suite being built for generic, the same way the other four flavors track theirs:

• tag_prefix_flavour="Ubuntu" in kernel_update_upstream(). The lookup lands via the existing fallback prefix, since Canonical tags a release's own kernel as Ubuntu-<version>-<abi>.<upload> with no series segment.
• linux-image-generic rather than linux-image-generic-hwe-26.04 in get_kernel_version_for_platform_from_apt(), which removes the last generic special case there.

This is deliberately the 24.04 shape rather than a general fix. Once resolute's HWE track starts rolling at 26.04.2, linux-image-generic-hwe-26.04 moves off the GA kernel and its backport does get tagged Ubuntu-hwe-* in the resolute tree, at which point this wants to go back the way DLPX-95001 took it. Arguably the better solution is to derive the prefix from the resolved version rather than hardcoding it (use Ubuntu when apt hands back the target suite's own GA kernel, Ubuntu-hwe otherwise), so that a third cycle does not repeat this. That is a larger change than the branch needs today, so I've opted for the narrow one for now; the follow-up that restores HWE after 26.04.2 can pick up the adaptive version.

The apt lookup has one other caller, fetch_kernel_from_apt_for_platform(), reached only by the archive kernel package source. DEFAULT_LINUX_KERNEL_PACKAGE_SOURCE is delphix, so that path is off by default, and on resolute both metapackages depend on the same kernel today, so what it would download is unchanged.

os-upgrade only. develop correctly uses the HWE kernel for generic, since noble's HWE track is live.

Testing Done

make check (shellcheck + shfmt) passes.

Replicated the prefix construction from kernel_update_upstream() and resolved it against the real resolute tree (246 tags), for both ABIs apt can hand back (7.0.0-14 from resolute, 7.0.0-29 from resolute-updates):

kver=7.0.0 abi=14 flavour=Ubuntu-hwe -> NO MATCH (tried Ubuntu-hwe-7.0-7.0.0-14 then Ubuntu-hwe-7.0.0-14) => die
kver=7.0.0 abi=14 flavour=Ubuntu     -> MATCH Ubuntu-7.0.0-14.14 (via Ubuntu-7.0.0-14)
kver=7.0.0 abi=29 flavour=Ubuntu-hwe -> NO MATCH (tried Ubuntu-hwe-7.0-7.0.0-29 then Ubuntu-hwe-7.0.0-29) => die
kver=7.0.0 abi=29 flavour=Ubuntu     -> MATCH Ubuntu-7.0.0-29.29 (via Ubuntu-7.0.0-29)

No update-package run yet. linux-pkg/os-upgrade/update-package/linux-kernel-generic is where this gets exercised for real, and the merge stage after it is expected to need hand conflict resolution the way makedumpfile and nfs-utils did.

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