The handover chowned the whole build root with a serial 'chown -R'.
Through the overlay staging every lower-layer file it touched is
copy-up'd — the full data movement of the staged tree — which the
profile of a kernel build showed as 15m53s in 'Preparing the build
user', 19% of the whole run.
The handover is now a 'find ! -uid | xargs -P chown' pipeline: only
entries the build user does not own yet are chowned (staging and quilt
re-application flip entries back to root; a resumed tree is already
owned and the phase collapses to a metadata-only walk), and the
copy-ups of independent files parallelize across the chroot's cores.
On a 200k-file tree the handover drops from 9.9s to 0.9s.
The ccache handover ran only when the build user was fresh, which is
every build in practice (the chroot is per-run scratch); it now rides
the same selective pipeline on every run, still best-effort — a failed
handover costs cache hits, never the build.
vhaudiquet
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