redbear_qt_ensure_dep_sysroots repairs OTHER recipes' sysroots, so with
COOKBOOK_COOK_JOBS>1 it runs while a parallel cook is compiling against the
very directory it is relinking. It did `rm -f` then `ln -sf`, leaving a window
where the path did not exist. kirigami died in that window:
cstdio:47: fatal error:
.../qtdeclarative/target/.../sysroot/include/stdio.h: No such file
with another cook's `rm -f`/`ln -sf` on qtdeclarative's sysroot interleaved in
the same log.
Now: skip entirely when the link already points at the right target (the
common case, and the cheapest way to stop the churn), otherwise replace via
ln -s to a temp name + `mv -T`, a single rename(2) -- readers see the old link
or the new one, never nothing.
Verified with 300 concurrent relinks against 3000 probes: 0 missing
observations, where the old sequence reproduced the gap.
vasilito
7ae9646968