exchange_behind_private_lock (drivers.rs) is the single write primitive for
the projection directory: one nonblocking compare-exchange behind a lock only
Idunn can hold. An absent lock is created O_EXCL 0600; a lock that is not a
regular file, ours, mode 0600 is replaced by a fresh 0600 file renamed over it
(a chmod does not revoke a descriptor another uid already holds); a private
lock is opened as is. The topology driver's mutate and the incident store both
go through it; the blocking compare_exchange_snapshot call, publish_projection_mode
(which published the lock 0644 on a false premise that readers open it) and
incident.rs secure_lock are deleted.
mutate now retries only a lost race; a held lock is Err "the topology
projection is locked by another holder" and a no-op, so no caller records a
digest, evidence or phase for a publish that did not land.
Test helpers: the lock holder is a separate flock process, and run_while_held
counts opens of the lock (inotify), so ANY wait on a held lock fails, not only
one past the 30 s ceiling. Tests reproduce the live default ACL and use
setpriv for the other-uid half.
This commit has not been built; the verify run follows.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
102c129891