Introduce `linux-headers` package (transition plan from glibc)

[sighook]

Problem

Currently, Linux kernel headers are bundled inside the glibc and glibc-32 packages. This creates tight coupling: glibc owns files under /usr/include that really belong to the kernel. Updating headers or glibc independently is messy, and file conflicts are inevitable if we try to split them abruptly.

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Goal

Create a dedicated linux-headers package to provide kernel headers separately, while ensuring a smooth transition that does not break builds or cause file conflicts.

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Transition Strategy

1. Install Path Separation

   • New linux-headers package installs headers under:
     /usr/include/linux-headers-<version>/
   • Glibc is configured with --with-headers=/usr/include/linux-headers-<version> so it no longer needs to ship headers itself.
   • This avoids direct overlap with glibc’s current /usr/include ownership.

2. Temporary Duplication

   • During the transition, headers remain in glibc packages and are provided by linux-headers.
   • This ensures existing builds don’t break while we test the new separation.
   • Once validated, headers are dropped from glibc, leaving linux-headers as the sole owner.

3. Dependency Update

   • After duplication phase, glibc and glibc-32 gain Depends on: linux-headers.
   • Builders install headers first, then glibc builds against them.
   • No file conflicts, since glibc no longer installs headers itself.

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Pitfalls & Safeguards

• Coupling: glibc must always be built against the same header set provided by linux-headers. Updating headers independently can break ABI expectations.
• Conflict avoidance: path separation ensures no overlapping ownership of /usr/include.
• Release timing: the split should be staged across one release cycle, with duplication in place before headers are removed from glibc.

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Next Steps

• Prototype linux-headers package with path separation.
• Test glibc build configured against external headers.
• Validate duplication phase (headers in both glibc and linux-headers).
• Remove headers from glibc once stable.
• Document coordination rules for updating headers vs. glibc baseline:
  • Codebook: Toolchain Update Order.
  • glibc/linux-headers Pkgfiles (briefly).